Ugrás a tartalomhoz

Plant Physiology

Ördög Vince, Molnár Zoltán (2011)

Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem

Nutrient supply of plant

Nutrient supply of plant

Unlike heterotrophic organisms, which depend for their existence on energy-rich organic molecules previously synthesized by other organisms, plants must survive in an entirely inorganic environment. As autotrophic organisms, plants must take in carbon dioxide from the atmosphere and water and mineral nutrients from the soil, and from these simple, inorganic components, make all of the complex molecules of a living organism. Since plants stand at the bottom of the food chain, mineral nutrients assimilated by plants eventually find their way into the matter that makes up all animals, including humans.

Plant nutrition is traditionally treated as two separate topics: organic nutrition and inorganic nutrition. Organic nutrition focuses on the production of carbon compounds, specifically the incorporation of carbon, hydrogen, and oxygen via photosynthesis, while inorganic nutrition is concerned primarily with the acquisition of mineral elements from the soil. Photosynthesis and the acquisition of mineral ions from the soil are so interdependent, however, that this distinction between organic and inorganic nutrition is more a matter of convenience than real.

Essential nutrients

Special techniques are used in nutritional studies

To demonstrate that an element is essential requires that plants be grown under experimental conditions in which only the element under investigation is absent. Such conditions are extremely difficult to achieve with plants grown in a complex medium such as soil. In the nineteenth century, several researchers, including Nicolas-Theodore de Saussure, Julius von Sachs, Jean-Baptiste-Joseph-Dieudonne Boussingault, and Wilhelm Knop, approached this problem by growing plants with their roots immersed in a nutrient solution containing only inorganic salts. Their demonstration that plants could grow normally with no soil or organic matter proved unequivocally that plants can fulfill all their needs from only inorganic elements, water, and sunlight.

The technique of growing plants with their roots immersed in a nutrient solution without soil is called solution culture or hydroponics (Figure 1.11). Successful hydroponic culture requires a large volume of nutrient solution or frequent adjustment of the nutrient solution to prevent nutrient uptake by roots from producing large changes in the nutrient concentrations and pH of the solution. A sufficient supply of oxygen to the root system – also critical – may be achieved by vigorous bubbling of air through the solution. Hydroponics is used in the commercial production of many greenhouse crops, such as tomatoes (Lycopersicon esculentum). In another form of hydroponic culture, plant roots lie on the surface of a trough, and nutrient solutions flow in a thin layer along the trough over the roots. This nutrient film growth system ensures that the roots receive an ample supply of oxygen.

Figure 1.11 Hydroponic growth system: plants are grown in nutrient solution fully saturated with oxygen (source: Taiz L., Zeiger E., 2010)

Another alternative, which has sometimes been heralded as the medium of the future for scientific investigations, is to grow the plants aeroponically. In this technique plants are grown with their roots suspended in air while being sprayed continuously with a nutrient solution. This approach provides easy manipulation of the gaseous environment around the roots, but it requires higher levels of nutrients than hydroponic culture does to sustain rapid plant growth. For this reason and other technical difficulties, the use of aeroponics is not widespread. An ebb-and-flow system is yet another approach to solution culture. In such systems, the nutrient solution periodically rises to immerse plant roots and then recedes, exposing the roots to a moist atmosphere. Like aeroponics, ebb-and-flow systems require higher levels of nutrients than hydroponics or nutrient films.

Nutrient solutions containing only inorganic salts have been used in nutritional studies

Over the years, many formulations have been used for nutrient solutions. Early formulations developed by Knop in Germany included only KNO3, Ca(NO3)2, KH2PO4, MgSO4, and an iron salt. At the time, this nutrient solution was believed to contain all the minerals required by plants, but these experiments were carried out with chemicals that were contaminated with other elements that are now known to be essential (such as boron or molybdenum).

A modified Hoagland solution contains all the known mineral elements needed for rapid plant growth. The concentrations of these elements are set at the highest possible levels without producing toxicity symptoms or salinity stress, and thus may be several orders of magnitude higher than those found in the soil around plant roots. For example, whereas phosphorus is present in the soil solution at concentrations normally less than 0.06 ppm, here it is offered at 62 ppm. Another important property of the modified Hoagland formulation is that nitrogen is supplied as both ammonium (NH4+) and nitrate (NO3-). Supplying nitrogen in a balanced mixture of cations and anions tends to reduce the rapid rise in the pH of the medium that is commonly observed when the nitrogen is supplied solely as nitrate anion. Even when the pH of the medium is kept neutral, most plants grow better if they have access to both NH4+ and NO3- because absorption and assimilation of the two nitrogen forms promotes cation-anion balances within the plant.

Essential nutrients

Only certain elements have been determined to be essential for plants. An essential element is defined as:

  • one that is intrinsic component in the structure or metabolism,

  • whose absence causes several abnormalities in plant growth, development, or reproduction.

If plants are given these essential elements, as well as water and energy from sunlight, they can synthesize all the compounds they need for normal growth. Hydrogen, carbon, and oxygen are not considered mineral nutrients because they are obtained primarily from water or carbon dioxide.

Essential mineral elements are usually classified as macronutrients or micronutrients according to their relative concentrations in plant tissue. In some cases the differences in tissue content between macronutrients and micronutrients are not as great as indicated in the literature. For example, some plant tissues, such as leaf mesophyll, have almost as much iron or manganese as they do sulfur or magnesium. Often elements are present in concentrations greater than the plant's minimum requirements.

The essential elements be classified instead according to their biochemical role and physiological function. Plant nutrients have been divided into four basic groups:

  1. Nitrogen and sulfur constitute the first group of essential elements. Plants assimilate these nutrients via biochemical reactions involving oxidation and reduction to form covalent bonds with carbon and create organic compounds.

  2. The second group is important in energy storage reactions or in maintaining structural integrity. Elements in this group are often present in plant tissues as phosphate, borate, and silicate esters in which the elemental group is covalently bound to an organic molecule (e.g., sugar phosphate).

  3. The third group is present in plant tissue as either free ions dissolved in the plant water or ions electrostatically bound to substances such as the pectic acids present in the plant cell wall. Elements in this group have important roles as enzyme cofactors and in the regulation of osmotic potentials.

  4. The fourth group, comprising metals such as iron, has important roles in reactions involving electron transfer.

Some naturally occurring elements, such as aluminum, selenium, and cobalt, that are not essential elements can also accumulate in plant tissues. Aluminum, for example, is not considered to be an essential element, but plants commonly contain from 0.1 to 500 ppm aluminum, and addition of low levels of aluminum to a nutrient solution may stimulate plant growth. Many species in the genera Astragalus, Xylorhiza, and Stanleya accumulate selenium, although plants have not been shown to have a specific requirement for this element. Cobalt is part of cobalamin (vitamin B12 and its derivatives), a component of several enzymes in nitrogen-fixing microorganisms. Crop plants normally contain only relatively small amounts of such nonessential elements.

Nutrient uptake

Soil, roots, and microbes

Soil is complex physically, chemically, and biologically. It is a heterogeneous substance containing solid, liquid, and gaseous phases. All of these phases interact with mineral elements. The inorganic particles of the solid phase provide a reservoir of potassium, calcium, magnesium, and iron. Also associated with this solid phase are organic compounds containing nitrogen, phosphorus, and sulfur, among other elements. The liquid phase of soil constitutes the soil solution, which contains dissolved mineral ions and serves as the medium for ion movement to the root surface. Gases such as oxygen, carbon dioxide, and nitrogen are dissolved in the soil solution, but roots exchange gases with soils predominantly through the air gaps between soil particles.

Negatively charged soil particles affect the adsorption of mineral nutrients

Soil particles, both inorganic and organic, have predominantly negative charges on their surfaces. Many inorganic soil particles are crystal lattices that are tetrahedral arrangements of the cationic forms of aluminum (Al3+) and silicon (Si4+) bound to oxygen atoms, thus forming aluminates and silicates. When cations of lesser charge replace Al3+ and Si4+ within the crystal lattice, these inorganic soil particles become negatively charged. The negative surface charges of organic particles result from the dissociation of hydrogen ions from the carboxylic acid and phenolic groups present in this component of the soil. Most of the world's soil particles, however, are inorganic.

Mineral cations such as ammonium (NH4+) and potassium (K+) adsorb to the negative surface charges of inorganic and organic soil particles. This cation adsorption is an important factor in soil fertility. Mineral cations adsorbed on the surface of soil particles, which are not easily lost when the soil is leached by water, provide a nutrient reserve available to plant roots. Mineral nutrients adsorbed in this way can be replaced by other cations in a process known as cation exchange. The degree to which a soil can adsorb and exchange ions is termed its cation exchange capacity (CEC) and is highly dependent on the soil type.

Mineral anions such as nitrate (NO3-) and chloride (Cl-) tend to be repelled by the negative charge on the surface of soil particles and remain dissolved in the soil solution. Thus the anion exchange capacity of most agricultural soils is small compared with the cation exchange capacity. Nitrate, in particular, remains mobile in the soil solution, where it is susceptible to leaching by water moving through the soil.

Soil pH affects nutrient availability, excess mineral ions in the soil limit plant growth

Hydrogen ion concentration (pH) is an important property of soils because it affects the growth of plant roots and soil microorganisms. Root growth is generally favored in slightly acidic soils, at pH values between 5.5 and 6.5. Fungi generally predominate in acidic (pH below 7) soils; bacteria become more prevalent in alkaline (pH above 7) soils. Soil pH determines the availability of soil nutrients (Figure 1.12). Acidity promotes the weathering of rocks that releases K+, Mg2+, Ca2+, and Mn2+ and increases the solubility of carbonates, sulfates, and phosphates.

Figure 1.12 Influence of soil pH on the availability of nutrient elements in organic soils (source: Taiz L., Zeiger E., 2010)

When excess mineral ions are present in soil, the soil is said to be saline, and plant growth may be restricted if these mineral ions reach levels that limit water availability or exceed the adequate zone for a particular nutrient. Sodium chloride and sodium sulfate are the most common salts in saline soils. Excess mineral ions in soils can be a major problem in arid and semiarid regions because rainfall is insufficient to leach them from the soil layers near the surface. Irrigated agriculture fosters soil salinization if the amount of water applied is insufficient to leach the salt below the root zone. Irrigation water can contain 100 to 1000 g of mineral ions per cubic meter, and over a number of growing seasons, high levels of mineral ions may accumulate in the soil. Another important problem with excess mineral ions is the accumulation of heavy metals, e.g., zinc, copper, cobalt, nickel, mercury, lead, cadmium, in the soil, which can cause severe toxicity in plants as well as humans.

Plants develop extensive root system

The ability of plants to obtain both water and mineral nutrients from the soil is related to their capacity to develop an extensive root system. Nonetheless, making observations on root systems is difficult and usually requires special techniques. Plant roots may grow continuously throughout the year. Their proliferation, however, depends on the availability of water and minerals in the immediate microenvironment surrounding the root, the so-called rhizosphere. If fertilization and irrigation provide abundant nutrients and water, root growth may not keep pace with shoot growth. Plant growth under such conditions becomes carbohydrate-limited, and a relatively small root system meets the nutrient needs of the whole plant. Indeed, crops under fertilization and irrigation allocate more resources to the shoot and reproductive structures than to roots, and this shift in allocation patterns often results in higher yields.

Within the soil, nutrients can move to the root surface both by bulk flow and by diffusion. In bulk flow, nutrients are carried by water moving through the soil toward the root. The amounts of nutrients provided to the root by bulk flow depend on the rate of water flow through the soil toward the plant, which depends on transpiration rates and on nutrient levels in the soil solution. When both the rate of water flow and the concentrations of nutrients in the soil solution are high, bulk flow can play an important role in nutrient supply. In diffusion, mineral nutrients move from a region of higher concentration to a region of lower concentration. Nutrient uptake by roots lowers the concentrations of nutrients at the root surface, generating concentration gradients in the soil solution surrounding the root.

Roots sense the below ground environment – through gravitropism, thigmotropism, chemotropism, and hydrotropism – to guide their growth toward soil resources. Some of these responses involve auxin. The extent to which roots proliferate within a soil patch varies with nutrient levels (Figure 1.13). Root growth is minimal in poor soils because the roots become nutrient-limited. As soil nutrient availability increases, roots proliferate.

Figure 1.13 Root biomass as a function of extractable soil NH4+ and NO3- (source: Taiz L., Zeiger E., 2010)

Mycorrhizal fungi facilitate nutrient uptake by roots

Mycorrhizae (singular mycorrhiza) are not unusual; in fact, they are widespread under natural conditions. Much of the world's vegetation appears to have roots associated with mycorrhizal fungi: 83% of dicots, 79% of monocots, and all gymnosperms regularly form mycorrhizal associations. Mycorrhizae are absent from roots in very dry, saline, or flooded soils, or where soil fertility is extreme, either high or low. The host plant provides its associated mycorrhizae with carbohydrates. Mycorrhizal fungi are composed of fine tubular filaments called hyphae (singular hypha). The mass of hyphae that forms the body of the fungus is called the mycelium (plural mycelia). There are two major classes of mycorrhizal fungi that are important in terms of mineral nutrient uptake by plants: ectotrophic mycorrhizae and arbuscular mycorrhizae.

Ectotrophic mycorrhizal fungi typically form a thick sheath, or mantle, of mycelium around roots, and some of the mycelium penetrates between the cortical cells (Figure 1.14). The cortical cells themselves are not penetrated by the fungal hyphae, but instead are surrounded by a network of hyphae called the Hartig net. Often the amount of fungal mycelium is so extensive that its total mass is comparable to that of the roots themselves. The fungal mycelium also extends into the soil. The capacity of the root system to absorb nutrients is improved by the presence of external fungal hyphae because they are much finer than plant roots and can reach beyond the nutrient depletion zone near the roots.

Figure 1.14 Root infected with ectotrophic mycorrhizal fungi (source: Taiz L., Zeiger E., 2010)

Unlike the ectotrophic mycorrhizal fungi, arbuscular mycorrhizal fungi (previously called vesicular-arbuscular mycorrhizae) do not produce a compact mantle of fungal mycelium around the root. Instead, the hyphae grow in a less dense arrangement, both within the root itself and extending outward from the root into the surrounding soil. After entering the root through either the epidermis or a root hair via a mechanism that has commonalities with the entry of the bacteria responsible for the nitrogen-fixing symbiosis, the hyphae not only extend through the regions between cells, but also penetrate individual cells of the cortex. Within these cells, the hyphae can form oval structures called vesicles and branched structures called arbuscules. The arbuscules appear to be sites of nutrient transfer between the fungus and the host plant.

The association of arbuscular mycorrhizae with plant roots facilitates the uptake of phosphorus, trace metals such as zinc and copper, and water. By extending beyond the depletion zone for phosphorus around the root, the external mycelium improves phosphorus absorption. The external mycelium of ectotrophic mycorrhizae can also absorb phosphate and make it available to plants. Little is known about the mechanism by which the mineral nutrients absorbed by mycorrhizal fungi are transferred to the cells of plant roots.

Symbiotic nitrogen fixation

Biological nitrogen fixation accounts for most of the conversion of atmospheric N2 into ammonium, and thus serves as the key entry point of molecular nitrogen into the biogeochemical cycle of nitrogen. Some bacteria can convert atmospheric nitrogen into ammonium. Most of these nitrogen-fixing prokaryotes live in the soil, generally independent of other organisms. A few form symbiotic associations with higher plants in which the prokaryote directly provides the host plant with fixed nitrogen in exchange for other nutrients and carbohydrates. Such symbioses occur in nodules that form on the roots of the plant and contain the nitrogen-fixing bacteria. The most common type of symbiosis occurs between members of the plant family Fabaceae (Leguminosae) and soil bacteria of the genera Azorhizobium, Bradyrhizobium, Photorhizobium, Rhizobium, and Sinorhizobium (collectively called rhizobia).

Because nitrogen fixation involves the expenditure of large amounts of energy, the nitrogenase enzymes that catalyze these reactions have sites that facilitate the high-energy exchange of electrons. Oxygen, being a strong electron acceptor, can damage these sites and irreversibly inactivate nitrogenase, so nitrogen must be fixed under anaerobic conditions. Each of the nitrogen-fixing organisms either functions under natural anaerobic conditions or creates an internal, local anaerobic environment in the presence of oxygen.

Symbiotic nitrogen-fixing prokaryotes dwell within nodules, the special organs of the plant host that enclose the nitrogen-fixing bacteria (Figure 1.15). In the case of legumes and actinorhizal plants, the nitrogen-fixing bacteria induce the plant to form root nodules. Grasses can also develop symbiotic relationships with nitrogen-fixing organisms, but in these associations root nodules are not produced. Legumes and actinorhizal plants regulate gas permeability in their nodules, maintaining a level of oxygen within the nodule that can support respiration but is sufficiently low to avoid inactivation of the nitrogenase. Nodules contain an oxygen-binding heme protein called leghemoglobin. Leghemoglobin is present in the cytoplasm of infected nodule cells at high concentrations (700 µM in soybean nodules) and gives the nodules a pink color.

Figure 1.15 Root nodules on a common bean (Phaseolus vulgaris) (source: Taiz L., Zeiger E., 2010)

The symbiosis between legumes and rhizobia is not obligatory. Legume seedlings germinate without any association with rhizobia, and they may remain unassociated throughout their life cycle. Rhizobia also occur as free-living organisms in the soil. Under nitrogen-limited conditions, however, the symbionts seek each other out through an elaborate exchange of signals. This signaling, the subsequent infection process, and the development of nitrogen-fixing nodules involve specific genes in both the host and the symbionts. Plant genes specific to nodules are called nodulin (Nod) genes; rhizobial genes that participate in nodule formation are called nodulation (nod) genes. The first stage in the formation of the symbiotic relationship between the nitrogen-fixing bacteria and their host is migration of the bacteria toward the roots of the host plant. This migration is a chemotactic response mediated by chemical attractants, especially (iso)flavonoids and betaines, secreted by the roots. These attractants activate the rhizobial NodD protein, which then induces transcription of the other nod genes.

Two processes – infection and nodule organogenesis – occur simultaneously during root nodule formation. During the infection process, rhizobia attached to the root hairs release Nod factors that induce a pronounced curling of the root hair cells. The rhizobia become enclosed in the small compartment formed by the curling. The cell wall of the root hair degrades in these regions, also in response to Nod factors, allowing the bacterial cells direct access to the outer surface of the plant plasma membrane. The next step is formation of the infection thread, an internal tubular extension of the plasma membrane that is produced by the fusion of Golgi-derived membrane vesicles at the site of infection. The thread grows at its tip by the fusion of secretory vesicles to the end of the tube. Deeper into the root cortex, near the xylem, cortical cells dedifferentiate and start dividing, forming a distinct area within the cortex, called a nodule primordium, from which the nodule will develop. The infection thread filled with proliferating rhizobia elongates through the root hair and cortical cell layers, in the direction of the nodule primordium. When the infection thread reaches specialized cells within the nodule, its tip fuses with the plasma membrane of the host cell, releasing bacterial cells that are packaged in a membrane derived from the host cell plasma membrane. At first the bacteria continue to divide, and the surrounding membrane increases in surface area to accommodate this growth by fusing with smaller vesicles. Soon thereafter, upon an undetermined signal from the plant, the bacteria stop dividing and begin to enlarge and to differentiate into nitrogen-fixing endosymbiotic organelles called bacteroids. The membrane surrounding the bacteroids is called the peribacteroid membrane.

Biological nitrogen fixation produces ammonia from molecular nitrogen. The nitrogenase enzyme complex – the Fe protein and the MoFe protein – catalyzes this reaction. The symbiotic nitrogen-fixing prokaryotes release ammonia that, to avoid toxicity, must be rapidly converted into organic forms in the root nodules before being transported to the shoot via the xylem. Nitrogen-fixing legumes can be classified as amide exporters or ureide exporters, depending on the composition of the xylem sap. Amides (principally the amino acids asparagine or glutamine) are exported by temperate-region legumes, such as pea (Pisum), clover (Trifolium), broad bean (Vicia), and lentil (Lens). Ureides are exported by legumes of tropical origin, such as soybean (Glycine), common bean (Phaseolus). The three major ureides are allantoin, allantoic acid, and citrulline. All three compounds are ultimately released into the xylem and transported to the shoot, where they are rapidly catabolized to ammonium.

Ion transport in roots

Mineral nutrients absorbed by the root are carried to the shoot by the transpiration stream moving through the xylem. Both the initial uptake of nutrients and water and the subsequent movement of these substances from the root surface across the cortex and into the xylem are highly specific, well-regulated processes. Ion transport across the root obeys the same biophysical laws that govern cellular transport.

Solutes move through both apoplast and symplast

In terms of the transport of small molecules, the cell wall is an open lattice of polysaccharides through which mineral nutrients diffuse readily. Because all plant cells are separated by cell walls, ions can diffuse across a tissue (or be carried passively by water flow) entirely through the cell wall space without ever entering a living cell. This continuum of cell walls is called the extracellular space, or apoplast. Typically, 5 to 20% of the plant tissue volume is occupied by cell walls. Just as the cell walls form a continuous phase, so do the cytoplasms of neighboring cells, collectively referred to as the symplast. Plant cells are interconnected by cytoplasmic bridges called plasmodesmata, cylindrical pores 20 to 60 nm in diameter (Figure 1.16). Each plasmodesma is lined with plasma membrane and contains a narrow tubule, the desmotubule, that is a continuation of the endoplasmic reticulum.

Figure 1.16 Plasmodesmata connect the cytoplasms of neighbouring cells facilitating cell-to-cell communication and solute transport (source: Taiz L., Zeiger E., 2010)

Ion absorption by the root is more pronounced in the root hair zone than in the meristem and elongation zones. Cells in the root hair zone have completed their elongation but have not yet begun secondary growth. The root hairs are simply extensions of specific epidermal cells that greatly increase the surface area available for ion absorption. An ion that enters a root may immediately enter the symplast by crossing the plasma membrane of an epidermal cell, or it may enter the apoplast and diffuse between the epidermal cells through the cell walls. From the apoplast of the cortex, an ion (or other solute) may either be transported across the plasma membrane of a cortical cell, thus entering the symplast, or diffuse radially all the way to the endodermis via the apoplast. The apoplast forms a continuous phase from the root surface through the cortex. However, in all cases, ions must enter the symplast before they can enter the stele, because of the presence of the Casparian strip. The Casparian strip is a suberized layer that forms rings around cells of the specialized endodermis and effectively blocks the entry of water and solutes into the stele via the apoplast. Once an ion has entered the stele through the symplastic connections across the endodermis, it continues to diffuse from cell to cell into the xylem. Finally, the ion is released into the apoplast and diffuses into a xylem tracheid or vessel element. The presence of the Casparian strip allows the plant to maintain a higher ion concentration in the xylem than exists in the soil water surrounding the roots.

Xylem parenchima cells participate in xylem loading

Once ions have been taken up into the symplast of the root at the epidermis or cortex, they must be loaded into the tracheids or vessel elements of the stele to be translocated to the shoot. The stele consists of dead tracheary elements and living xylem parenchyma. Because the xylem tracheary elements are dead cells, they lack cytoplasmic continuity with the surrounding xylem parenchyma. To enter the tracheary elements, the ions must exit the symplast by crossing a plasma membrane a second time.

The process whereby ions exit the symplast and enter the conducting cells of the xylem is called xylem loading. Xylem parenchyma cells, like other living plant cells, maintain plasma membrane H+-ATPase activity and a negative membrane potential. The plasma membranes of xylem parenchyma cells contain proton pumps, aquaporins, and a variety of ion channels and carriers specialized for influx or efflux. Several types of anion-selective channels have also been identified that participate in unloading of Cl- and NO3- from the xylem parenchyma. Other, less selective ion channels found in the plasma membrane of xylem parenchyma cells are permeable to K+, Na+, and anions.

Passive and active transport

Molecular and ionic movement from one location to another is known as transport. Local transport of solutes into or within cells is regulated mainly by membranes. Larger-scale transport between plant organs, or between plant and environment, is also controlled by membrane transport at the cellular level. For example, the transport of sucrose from leaf to root through the phloem, referred to as translocation, is driven and regulated by membrane transport into the phloem cells of the leaf and from the phloem to the storage cells of the root.

According to Fick's first law, the movement of molecules by diffusion always proceeds spontaneously, down a gradient of free energy or chemical potential, until equilibrium is reached. The spontaneous "downhill" movement of molecules is termed passive transport. At equilibrium, no further net movements of solutes can occur without the application of a driving force. The movement of substances against a gradient of chemical potential, or "uphill", is termed active transport. It is not spontaneous, and it requires that work be done on the system by the application of cellular energy. One common way (but not the only way) of accomplishing this task is to couple transport to the hydrolysis of ATP.

The chemical potential for any solute is defined as the sum of the concentration, electric, and hydrostatic potentials (and the chemical potential under standard conditions). The importance of the concept of chemical potential is that it sums all the forces that may act on a molecule to drive net transport. In general, diffusion (passive transport) always moves molecules energetically downhill from areas of higher chemical potential to areas of lower chemical potential. Movement against a chemical-potential gradient is indicative of active transport (Figure 1.17). If we take the diffusion of sucrose across a cell membrane as an example, we can accurately approximate the chemical potential of sucrose in any compartment by the concentration term alone.

Figure 1.17 Relationship between chemical potential and transport (passive, active) processes (source: Taiz L., Zeiger E., 2010)

If the solute carries an electric charge (as does, for example, the potassium ion), the electrical component of the chemical potential must also be considered. Suppose the membrane is permeable to K+ and Cl- rather than to sucrose. K+ ions diffuse in response to both their concentration gradients and any electrical potential difference between the two compartments. Ions can be driven passively against their concentration gradients if an appropriate voltage (electric field) is applied between the two compartments. Because of the importance of electric fields in the biological transport of any charged molecule, an electrochemical potential is exists, and a difference in electrochemical potential between the two compartments as well.

If the two KCl solutions in the previous example are separated by a biological membrane, diffusion is complicated by the fact that the ions must move through the membrane as well as across the open solutions. The extent to which a membrane permits the movement of a substance is called membrane permeability. Permeability depends on the composition of the membrane as well as on the chemical nature of the solute. When salts diffuse across a membrane, an electrical membrane potential (voltage) can develop. The K+ and Cl- ions will permeate the membrane independently as they diffuse down their respective gradients of electrochemical potential. And unless the membrane is very porous, its permeability to the two ions will differ. As a consequence of these different permeabilities, K+ and Cl- will initially diffuse across the membrane at different rates (Figure 1.18). The result is a slight separation of charge, which instantly creates an electrical potential across the membrane. In biological systems, membranes are usually more permeable to K+ than to Cl-. Therefore, K+ will diffuse out of the cell faster than Cl-, causing the cell to develop a negative electric charge with respect to the extracellular medium. A potential that develops as a result of diffusion is called a diffusion potential.

Figure 1.18 Development of a diffusion potential and a charge separation between two compartments separated by a membrane (source: Taiz L., Zeiger E., 2010)

The Nernst equation distinguishes between active and passive transport

Because the membrane in the preceding example is permeable to both K+ and Cl- ions, equilibrium will not be reached for either ion until the concentration gradients decrease to zero. However, if the membrane were permeable only to K+, diffusion of K+ would carry charges across the membrane until the membrane potential balanced the concentration gradient. The Nernst equation states that at equilibrium, the difference in concentration of an ion between two compartments is balanced by the voltage difference between the compartments. A membrane potential of 59 mV would maintain a tenfold concentration gradient of an ion whose movement across the membrane is driven by passive diffusion.

The concentration of each ion in the external solution bathing the pea root tissue and the measured membrane potential were substituted into the Nernst equation, and a predicted internal concentration was calculated for that ion. The anions NO3-, Cl-, H2PO4-, and SO42- all have higher internal concentrations than predicted, indicating that their uptake is active. The cations Na+, Mg2+, and Ca2+ have lower internal concentrations than predicted; therefore, these ions enter the cell by diffusion down their electrochemical-potential gradients and are then actively exported.

A change in membrane potential caused by an electrogenic pump will change the driving forces for diffusion of all ions that cross the membrane. For example, the outward transport of H+ can create an electrical driving force for the passive diffusion of K+ into the cell.

Membrane transport processes

Artificial membranes made of pure phospholipids have been used extensively to study membrane permeability. Biological and artificial membranes have similar permeabilities to nonpolar molecules and many small polar molecules. On the other hand, biological membranes are much more permeable to ions, to some large polar molecules, such as sugars, and to water than artificial bilayers are. The reason is that, unlike artificial bilayers, biological membranes contain transport proteins that facilitate the passage of selected ions and other molecules. The general term transport proteins encompasses three main categories of proteins: channels, carriers, and pumps (Figure 1.19). Although a particular transport protein is usually highly specific for the kinds of substances it will transport, its specificity is often not absolute. In plants, for example, a K+ transporter in the plasma membrane may transport K+, Rb+, and Na+ with different preferences. In contrast, most K+ transporters are completely ineffective in transporting anions such as Cl- or uncharged solutes such as sucrose.

Figure 1.19 Three classes of membrane transport proteins: channels, carriers, and pumps (source: Taiz L., Zeiger E., 2010)

Channels enhance diffusion across membranes

Channels are transmembrane proteins that function as selective pores through which molecules or ions can diffuse across the membrane. The size of a pore and the density and nature of the surface charges on its interior lining determine its transport specificity. Transport through channels is always passive, and because the specificity of transport depends on pore size and electric charge more than on selective binding, channel transport is limited mainly to ions or water. As long as the channel pore is open, solutes that can penetrate the pore diffuse through it extremely rapidly: about 108 ions per second through each channel protein. Channel pores are not open all the time, however. Channel proteins have structures called gates that open and close the pore in response to external signals. Signals that can regulate channel activity include membrane potential changes, ligands, hormones, light, and posttranslational modifications such as phosphorylation. Individual ion channels can be studied in detail by an electrophysiological technique called patch clamping, which can detect the electrical current carried by ions diffusing through a single open channel or a collection of channels.

Carriers bind and transport specific substances

Unlike channels, carrier proteins do not have pores that extend completely across the membrane. In transport mediated by a carrier, the substance being transported is initially bound to a specific site on the carrier protein. This requirement for binding allows carriers to be highly selective for a particular substrate to be transported. Carriers therefore specialize in the transport of specific ions or organic metabolites. Binding causes a conformational change in the protein, which exposes the substance to the solution on the other side of the membrane. Transport is complete when the substance dissociates from the carrier's binding site. Because a conformational change in the protein is required to transport an individual molecule or ion, the rate of transport by a carrier is many orders of magnitude slower than that through a channel. Carrier-mediated transport (unlike transport through channels) can be either passive transport or secondary active transport. Passive transport via a carrier is sometimes called facilitated diffusion, although it resembles diffusion only in that it transports substances down their gradient of electrochemical potential, without an additional input of energy.

Primary active transport, called pumps, requires direct energy source

To carry out active transport, a carrier must couple the energetically uphill transport of a solute with another, energy-releasing event so that the overall free-energy change is negative. Primary active transport is coupled directly to a source of energy, such as ATP hydrolysis, an oxidation-reduction reaction (as in the electron transport chain of mitochondria and chloroplasts), or the absorption of light by the carrier protein (such as bacteriorhodopsin in halobacteria). Membrane proteins that carry out primary active transport are called pumps. Most pumps transport ions, such as H+ or Ca2+. However, pumps belonging to the ATP-binding cassette (ABC) family of transporters can carry large organic molecules. For the plasma membranes of plants, fungi, and bacteria, as well as for plant tonoplasts and other plant and animal endomembranes, H+ is the principal ion that is electrogenically pumped across the membrane. The plasma membrane H+-ATPase generates the gradient of electrochemical potential of H+ across the plasma membrane, while the vacuolar H+-ATPase and the H+-pyrophosphatase (H+-PPase) electrogenically pump protons into the lumen of the vacuole and the Golgi cisternae.

Secondary active transport uses stored energy

In plant plasma membranes, the most prominent pumps are those for H+ and Ca2+, and the direction of pumping is outward from the cytosol to the extracellular space. Another mechanism is needed to drive the active uptake of mineral nutrients such as NO3-, SO42-, and PO43-; the uptake of amino acids, peptides, and sucrose; and the export of Na+, which at high concentrations is toxic to plant cells. The other important way that solutes are actively transported across a membrane against their gradient of electrochemical potential is by coupling the uphill transport of one solute to the downhill transport of another. This type of carrier-mediated cotransport is termed secondary active transport. Secondary active transport is driven indirectly by pumps. The gradient of electrochemical potential for H+ referred to as proton motive force (PMF), represents stored free energy in the form of the H+ gradient. The proton motive force generated by electrogenic H+ transport is used in secondary active transport to drive the transport of many other substances against their gradients of electrochemical potential. There are two types of secondary active transport: symport and antiport. Symport is the transport process when two substances move in the same direction through the membrane. Antiport refers to coupled transport in which the energetically downhill movement of protons drives the active (energetically uphill) transport of a solute in the opposite direction. In both types of secondary transport, the ion or solute being transported simultaneously with the protons is moving against its gradient of electrochemical potential, so its transport is active.

Cations are transported by both cation channels and cation carriers

Transport across biological membrane is energized by one primary active transport system coupled to ATP hydrolysis. The transport of one ionic species – for example, H+ – generates an ion gradient and an electrochemical potential. Many other ions or organic substrates can then be transported by a variety of secondary active transport proteins, which energize the transport of their substrates by simultaneously carrying one or two H+ down their energy gradient. Thus protons circulate across the membrane, outward through the primary active transport proteins and back into the cell through the secondary active transport proteins.

The relative contributions of each type of cation transport mechanism differ depending on the membrane, cell type, and biological phenomenon under investigation. Some of the cation channels are highly selective for specific ionic species, such as potassium ions. Others allow passage of a variety of cations, sometimes including Na+, even though this ion is toxic when overaccumulated.

A variety of carriers also move cations into plant cells. There are two families of transporters that specialize in K+ transport across plant membranes: the KUP/HAK/KT family and the HKTs. A third family, the cation-H+ antiporters (CPAs), mediates electroneutral exchange of H+ and other cations, including K+ in some cases.

Anion transporters have been identified

Nitrate (NO3-), chloride (Cl-), sulfate (SO4-), and phosphate (PO43-) are the major inorganic ions in plant cells, and malate2- is the major organic anion. The free-energy gradient for all of these anions is in the direction of passive efflux. Several types of plant anion channels have been characterized by electrophysiological techniques, and most anion channels appear to be permeable to a variety of anions. In contrast to the relative lack of specificity of anion channels, anion carriers that mediate the energetically uphill transport of anions into plant cells exhibit selectivity for particular anions. Plants have transporters for various organic anions, such as malate and citrate.

Phosphate availability in the soil solution often limits plant growth. Phosphate-H+ symporters with lower affinity for phosphate have also been identified in plants and have been localized to membranes of intracellular organelles such as plastids and mitochondria. Another group of phosphate transporters, the phosphate translocators, are located in the inner plastid membrane, where, in exchange for uptake of inorganic phosphate, they function to release phosphorylated carbon compounds derived from photosynthesis to the cytosol.

Aquaporins forms water channels in membranes

Water channels, or aquaporins, are a class of proteins that are relatively abundant in plant membranes. Many aquaporins do not result in ion currents when expressed in oocytes, consistent with a lack of ion transport activity, but when the osmolarity of the external medium is reduced, expression of these proteins results in swelling and bursting of the oocytes. The bursting results from rapid influx of water across the oocyte plasma membrane, which normally has very low permeability to water. These results confirm that aquaporins form water channels in membranes. Some aquaporin proteins also transport uncharged solutes (e.g., NH3), and there is some evidence that aquaporins act as conduits for carbon dioxide uptake into plant cells. Aquaporin activity is regulated by phosphorylation as well as by pH, calcium concentration, heteromerization, and reactive oxygen species.

Plasma membrane H+-ATPases are important for the regulation of cytoplasmic pH and for the control of cell turgor

The outward active transport of protons across the plasma membrane creates gradients of pH and electrical potential that drive the transport of many other substances (ions and uncharged solutes) through the various secondary active transport proteins. H+-ATPase activity is also important for the regulation of cytoplasmic pH and for the control of cell turgor, which drives organ (leaf and flower) movement, stomatal opening, and cell growth. Plant plasma membrane H+-ATPases are encoded by a family of about a dozen genes. In general, H+-ATPase expression is high in cells with key functions in nutrient movement, including root endodermal cells and cells involved in nutrient uptake from the apoplast that surrounds the developing seed. Like other enzymes, the plasma membrane H+-ATPase is regulated by the concentration of substrate (ATP), pH, temperature, and other factors. In addition, H+-ATPase molecules can be reversibly activated or deactivated by specific signals, such as light, hormones, or pathogen attack.

Plant cells increase their size primarily by taking up water into a large central vacuole. Therefore, the osmotic pressure of the vacuole must be kept sufficiently high for water to enter from the cytoplasm. The tonoplast regulates the traffic of ions and metabolites between the cytosol and the vacuole, just as the plasma membrane regulates their uptake into the cell. The vacuolar H+-ATPase (also called V-ATPase) differs both structurally and functionally from the plasma membrane H+-ATPase.

Solute transport

Phloem translocation

Phloem translocation moves the products of photosynthesis from mature leaves to areas of growth and storage. It also transmits chemical signals and redistributes ions and other substances throughout the plant body.

Pathways of translocation

An analysis of phloem exudate provides more direct evidence in support of the conclusion that photoassimilates are translocated through the phloem. Unfortunately, phloem tissue does not lend itself to analysis as easily as xylem tissue does. This is because the translocating elements in the phloem are, unlike xylem vessels and tracheids, living cells when functional. The distinguishing feature of phloem tissue is the conducting cell called the sieve element. Also known as a sieve tube, the sieve element is an elongated rank of individual cells, called sieve-tube members, arranged end-to-end. Unlike xylem tracheary elements, phloem sieve elements lack rigid walls and contain living protoplasts when mature and functional. The protoplasts of contiguous sieve elements are interconnected through specialized sieve areas in adjacent walls. Where the pores of the sieve area are relatively large and are found grouped in a specific area, they are known as sieve plates. Sieve plates are typically found in the end walls of sieve-tube members and provide a high degree of protoplasmic continuity between consecutive sieve-tube members. Additional pores are found in sieve areas located in lateral walls. In addition to sieve elements, phloem tissue also contains a variety of parenchyma cells. Some of these cells are intimately associated with sieve-tube members and for this reason are called companion cells. The interdependence of the sieve-tube member and companion cells is reflected in their lifetime – the companion cell remains alive only so long as the sieve-tube member continues to function. Companion cells are believed to provide metabolic support for the sieve-tube member.

Materials translocated in the phloem

Phloem sap can be collected from aphid stylets or, alternatively, from some plants by simply making an incision into the bark. If done carefully, to avoid cutting into the underlying xylem, the incision opens the sieve tubes and a relatively pure exudate can be collected in very small microcapillary tubes for subsequent analysis. As might be expected, the chemical composition of phloem exudate is highly variable. It depends on the species, age, and physiological condition of the tissue sampled. Even for a particular sample under uniform conditions, there may be wide variations in the concentrations of particular components between subsequent samples. For example, an analysis of phloem exudate from stems of actively growing castor bean (Ricinus communis) shows that the exudate contains sugars, protein, amino acids, the organic acid malate, and a variety of inorganic anions and cations. The inorganic anions include phosphate, sulphate, and chloride – nitrate is conspicuously absent – while the predominant cation is potassium. Some plant hormones (auxin, cytokinin, and gibberellin) were also detected, but at very low concentrations. The principal constituent of phloem exudate in most species is sugar. In castor bean it is sucrose, which comprises approximately 80 percent of the dry matter. A survey of over 500 species representing approximately 100 dicotyledonous families confirms that sucrose is almost universal as the dominant sugar in the phloem stream.

It is interesting to speculate on why sucrose is the preferred vehicle for long-distance translocation of photoassimilate. One possibility is that sucrose, a disaccharide, and its related oligosaccharides are nonreducing sugars. On the other hand, all monosaccharides, including glucose and fructose, are reducing sugars. Reducing sugars have a free aldehyde or ketone group that is capable of reducing mild oxidizing agents. Some oligosaccharides, such as sucrose, are nonreducing sugars because the acetal link between the subunits is stable and nonreactive in alkaline solution. The exclusive use of nonreducing sugars in the translocation of photoassimilate may be related to this greater chemical stability.

The pressure-flow model, a passive mechanism for phloem transport

Any comprehensive theory for phloem translocation must take into account a number of factors. These include: (1) the structure of sieve elements, including the presence of active cytoplasm, P-protein (phloem protein), and resistances imposed by sieve plates; (2) observed rapid rates of translocation (50 to 250 cm hr-1) over long distances; (3) translocation in different directions at the same time; (4) the initial transfer of assimilate from leaf mesophyll cells into sieve elements of the leaf minor veins (called phloem loading); and (5) final transfer of assimilate out of the sieve elements into target cells (called phloem unloading).

The most credible and generally accepted model for phloem translocation is one of the earliest. Originally proposed by E. Münch in 1930 but modified by a series of investigators since, the pressure-flow hypothesis remains the simplest model and continues to earn widespread support among plant physiologists. The pressure-flow mechanism is based on the mass transfer of solute from source to sink along a hydrostatic (turgor) pressure gradient. Translocation of solute in the phloem is closely linked to the flow of water in the transpiration stream and a continuous recirculation of water in the plant. The principle of pressure flow can be easily demonstrated in the laboratory by connecting two osmometers (Figure 1.20).

Figure 1.20 A physical model of the pressure-flow hypothesis for translocation in the phloem (source: Taiz L., Zeiger E., 2010)

Assimilate translocation begins with the loading of sugars into sieve elements at the source. Typically, loading would occur in the minor veins of a leaf, close to a photosynthetic mesophyll or bundle-sheath cell. The increased solute concentration in the sieve element lowers its water potential and, consequently, is accompanied by the osmotic uptake of water from the nearby xylem. This establishes a higher turgor or hydrostatic pressure in the sieve element at the source end. At the same time, sugar is unloaded at the sink end – a root or stem storage cell, for example. The hydrostatic pressure at the sink end is lowered as water leaves the sieve elements and returns to the xylem. So long as assimilates continue to be loaded at the source and unloaded at the sink, this pressure differential will be maintained, water will continue to move in at the source and out at the sink, and assimilate will be carried passively along. According to the pressure-flow hypothesis, solute translocation in the phloem is fundamentally a passive process; that is, translocation requires no direct input of metabolic energy to make it function.

Photosynthate distribution: allocation and partitioning

The photosynthetic rate determines the total amount of fixed carbon available to the leaf. However, the amount of fixed carbon available for translocation depends on subsequent metabolic events. The regulation of the distribution of fixed carbon into various metabolic pathways is termed allocation. The vascular bundles in a plant form a system of "pipes" that can direct the flow of photosynthates to various sinks: young leaves, stems, roots, fruits, or seeds. However, the vascular system is often highly interconnected, forming an open network that allows source leaves to communicate with multiple sinks. Under these conditions, what determines the volume of flow to any given sink? The differential distribution of photosynthates within the plant is termed partitioning.

The carbon fixed in a source cell can be used for storage, metabolism, and transport:

  • Synthesis of storage compounds. Starch is synthesized and stored within chloroplasts and, in most species, is the primary storage form that is mobilized for translocation during the night. Plants that store carbon primarily as starch are called "starch storers".

  • Metabolic utilization. Fixed carbon can be utilized within various compartments of the photosynthesizing cell to meet the energy needs of the cell or to provide carbon skeletons for the synthesis of other compounds required by the cell.

  • Synthesis of transport compounds. Fixed carbon can be incorporated into transport sugars for export to various sink tissues. A portion of the transport sugar can also be stored temporarily in the vacuole.

Transport of signaling molecules

Besides its major function in the long-distance transport of photosynthate, the phloem is also a conduit for the transport of signaling molecules from one part of the organism to another. Such long-distance signals coordinate the activities of sources and sinks and regulate plant growth and development. The signals between sources and sinks might be physical or chemical. Physical signals such as turgor change could be transmitted rapidly via the interconnecting system of sieve elements. Molecules traditionally considered to be chemical signals, such as proteins and plant hormones, are found in the phloem sap, as are mRNAs and small RNAs, which have more recently been added to the list of signal molecules. The translocated carbohydrates themselves may also act as signals.

Shoots produce growth regulators such as auxin, which can be rapidly transported to the roots via the phloem, and roots produce cytokinins, which move to the shoots through the xylem. Gibberellins (GA) and abscisic acid (ABA) are also transported throughout the plant in the vascular system. Plant hormones play a role in regulating source-sink relationships. They affect photosynthate partitioning in part by controlling sink growth, leaf senescence, and other developmental processes. Plant defense responses against herbivores and pathogens can also change allocation and partitioning of photoassimilates, with plant defense hormones such as jasmonic acid mediating the responses.

It has long been known that viruses can move in the phloem, traveling as complexes of proteins and nucleic acids or as intact virus particles. More recently, endogenous RNA molecules and proteins have been found in phloem sap, and at least some of these can function as signal molecules or generate phloem-mobile signals. To be assigned a signaling role in plants, a macromolecule must meet a number of significant criteria:

  • The macromolecule must move from source to sink in the phloem.

  • The macromolecule must be able to leave the sieve element-companion cell complex in sink tissues. Alternatively, the macromolecule might trigger the formation of a second signal that transmits information to the sink tissues surrounding the phloem; that is, it might initiate a signal cascade.

  • Perhaps most important, the macromolecule must be able to modify the functions of specific cells in the sink.

Plasmodesmata have been implicated in nearly every aspect of phloem translocation, from loading to long-distance transport (pores in sieve areas and sieve plates are modified plasmodesmata) to allocation and partitioning. The mechanism of plasmodesmatal transport (called trafficking) can be either passive (non targeted) or selective and regulated.

Nutritional deficiencies

Requirements for mineral elements change during the growth and development of a plant. In crop plants, nutrient levels at certain stages of growth influence the yield of the economically important tissues (tuber, grain, and so on). To optimize yields, farmers use analyses of nutrient levels in soil and in plant tissue to determine fertilizer schedules.

Analysis of plant tissues reveals mineral deficiencies

Soil analysis is the chemical determination of the nutrient content in a soil sample from the root zone. Both the chemistry and the biology of soils are complex, and the results of soil analyses vary with sampling methods, storage conditions for the samples, and nutrient extraction techniques. Perhaps more important is that a particular soil analysis reflects the levels of nutrients potentially available to the plant roots from the soil, but soil analysis does not tell us how much of a particular mineral nutrient the plant actually needs or is able to absorb. This additional information is best determined by plant tissue analysis.

Proper use of plant tissue analysis requires an understanding of the relationship between plant growth (or yield) and the concentration of a nutrient in plant tissue samples (Figure 1.21). Three zones (deficiency, adequate, and toxic) are identified in the response of growth to increasing tissue concentrations of a nutrient. When the nutrient concentration in a tissue sample is low, growth is reduced. In this deficiency zone of the curve, an increase in nutrient availability is directly related to an increase in growth or yield. As nutrient availability continues to increase, a point is reached at which further addition of the nutrient is no longer related to increases in growth or yield, but is reflected in increased tissue concentrations. This region of the curve is called the adequate zone. The point of transition between the deficiency and adequate zones of the curve reveals the critical concentration of the nutrient (Figure 1.21), which may be defined as the minimum tissue content of the nutrient that is correlated with maximal growth or yield. As the nutrient concentration of the tissue increases beyond the adequate zone, growth or yield declines because of toxicity (this region of the curve is the toxic zone).

Figure 1.21 Relationship between yield (or growth) and the nutrient content of the plant tissue (source: Taiz L., Zeiger E., 2010)

Because agricultural soils are often limited in the elements nitrogen, phosphorus, and potassium, many farmers routinely take into account, at a minimum, growth or yield responses for these elements. If a nutrient deficiency is suspected, steps are taken to correct the deficiency before it reduces growth or yield. Plant analysis has proved useful in establishing fertilizer schedules that sustain yields and ensure the food quality of many crops.

Some essential elements can be recycled from older to younger leaves, others are relatively immobile

An important clue in relating an acute deficiency symptom to a particular essential element is the extent to which an element can be recycled from older to younger leaves. Some elements, such as nitrogen, phosphorus, and potassium, can readily move from leaf to leaf; others, such as boron, iron, and calcium, are relatively immobile in most plant species. If an essential element is mobile, deficiency symptoms tend to appear first in older leaves. Deficiency of an immobile essential element becomes evident first in younger leaves. Although the precise mechanisms of nutrient mobilization are not well understood, plant hormones such as cytokinins appear to be involved.

Inadequate supply of an essential element is manifested by characteristic deficiency symptoms

Mineral deficiencies disrupt plant metabolism and function. In hydroponic culture, withholding of an essential element can be readily correlated with a given set of symptoms.

Diagnosis of soil-grown plants can be more complex for the following reasons:

  • deficiencies of several elements may occur simultaneously in different plant tissues,

  • deficiencies or excessive amounts of one element may induce deficiencies or excessive accumulations of another,

  • some virus-induced plant diseases may produce symptoms similar to those of nutrient deficiencies.

Nutrient deficiency symptoms in a plant are the expression of metabolic disorders resulting from the insufficient supply of an essential element. These disorders are related to the roles played by essential elements in normal plant metabolism and function.

Although each essential element participates in many different metabolic reactions, some general statements about the functions of essential elements in plant metabolism are possible. In general, the essential elements function in plant structure, metabolism, and cellular osmoregulation. More specific roles may be related to the ability of divalent cations such as calcium or magnesium to modify the permeability of plant membranes. In addition, research continues to reveal specific roles for these elements in plant metabolism.

In the discussion that follows, we will describe the specific deficiency symptoms and functional roles of the mineral essential elements. According to the four basic group of essential elements, deficiency symptoms of plant minerals can be classified as:

  • Group 1: deficiencies in mineral nutrients that are part of carbon compounds (N, S),

  • Group 2: deficiencies in mineral nutrients that are important in energy storage or structural integrity (P, Si, B),

  • Group 3: deficiencies in mineral nutrients that remain in ionic form (K, Ca, Mg, Cl, Mn, Na),

  • Group 4: deficiencies in mineral nutrients that are involved in redox reactions (Fe, Zn, Cu, Ni, Mo).

Deficiencies in mineral nutrients that are part of carbon compounds (N, S)

This first group consists of nitrogen and sulfur. Nitrogen availability in soils limits plant productivity in most natural and agricultural ecosystems. By contrast, soils generally contain sulfur in excess. Some of the most energy-intensive reactions in life convert the highly oxidized, inorganic forms, such as nitrate and sulfate, that roots absorb from the soil into the highly reduced forms found in organic compounds such as amino acids within plants.

Nitrogen is the mineral element that plants require in the greatest amounts. It serves as a constituent of many plant cell components, including amino acids, proteins, and nucleic acids. Therefore nitrogen deficiency rapidly inhibits plant growth. If such a deficiency persists, most species show chlorosis (yellowing of the leaves), especially in the older leaves near the base of the plant. Under severe nitrogen deficiency, these leaves become completely yellow (or tan) and fall off the plant. Younger leaves may not show these symptoms initially because nitrogen can be mobilized from older leaves. Carbohydrates not used in nitrogen metabolism may also be used in anthocyanin synthesis, leading to accumulation of that pigment. This condition is revealed as a purple coloration in leaves, petioles, and stems of nitrogen-deficient plants of some species, such as tomato and certain varieties of maize (Zea mays).

Sulfur is found in amino acids (cystine, cysteine, and methionine) and is a constituent of several coenzymes and vitamins, such as coenzyme A, S-adenosylmethionine, biotin, vitamin B1, and pantothenic acid, which are essential for metabolism. Many of the symptoms of sulfur deficiency are similar to those of nitrogen deficiency, including chlorosis, stunting of growth, and anthocyanin accumulation. The chlorosis caused by sulfur deficiency, however, generally arises initially in mature and young leaves, rather than in old leaves as in nitrogen deficiency, because sulfur, unlike nitrogen, is not easily remobilized to the younger leaves in most species.

Deficiencies in mineral nutrients that are important in energy storage or structural integrity (P, Si, B)

This group consists of phosphorus, silicon, and boron. Phosphorus and silicon are found at concentrations within plant tissue that warrant their classification as macronutrients, whereas boron is much less abundant and is considered a micronutrient.

Phosphorus (as phosphate, PO43-) is an integral component of important compounds of plant cells, including the sugar-phosphate intermediates of respiration and photosynthesis as well as the phospholipids that make up plant membranes. It is also a component of nucleotides used in plant energy metabolism (such as ATP) and in DNA and RNA. Characteristic symptoms of phosphorus deficiency include stunted growth in young plants and a dark green coloration of the leaves. As in nitrogen deficiency, some species may produce excess anthocyanins, giving the leaves a slight purple coloration.

Plants deficient in silicon are more susceptible to lodging (falling over) and fungal infection. Silicon is deposited primarily in the endoplasmic reticulum, cell walls, and intercellular spaces as hydrated, amorphous silica (SiO2•nH2O). It also forms complexes with polyphenols and thus serves as an alternative to lignin in the reinforcement of cell walls.

Boron-deficient plants may exhibit a wide variety of symptoms, depending on the species and the age of the plant. A characteristic symptom is black necrosis of young leaves and terminal buds. The necrosis of the young leaves occurs primarily at the base of the leaf blade. Apical dominance may also be lost, causing the plant to become highly branched. Structures such as the fruits, fleshy roots, and tubers may exhibit necrosis or abnormalities related to the breakdown of internal tissues.

Deficiencies in mineral nutrients that remain in ionic form (K, Ca, Mg, Cl, Mn, Na)

This group includes some of the most familiar mineral elements: the macronutrients potassium, calcium, and magnesium and the micronutrients chlorine, manganese, and sodium. These elements may be found as ions in solution in the cytosol or vacuoles, or they may be bound electrostatically or as ligands to larger, carbon-containing compounds.

Potassium, present within plants as the cation K+, plays an important role in regulation of the osmotic potential of plant cells. It also activates many enzymes involved in respiration and photosynthesis. The first observable symptom of potassium deficiency is mottled or marginal chlorosis, which then develops into necrosis primarily at the leaf tips, at the margins, and between veins. In many monocots, these necrotic lesions may initially form at the leaf tips and margins and then extend toward the leaf base. Because potassium can be mobilized to the younger leaves, these symptoms appear initially on the more mature leaves toward the base of the plant.

Calcium ions (Ca2+) are used in the synthesis of new cell walls. It is required for the normal functioning of plant membranes and has been implicated as a second messenger for various plant responses to both environmental and hormonal signals. Characteristic symptoms of calcium deficiency include necrosis of young meristematic regions such as the tips of roots or young leaves, where cell division and cell wall formation are most rapid. Necrosis in slowly growing plants may be preceded by a general chlorosis and downward hooking of young leaves. The root system of a calcium-deficient plant may appear brownish, short, and highly branched.

In plant cells, magnesium ions (Mg2+) have a specific role in the activation of enzymes involved in respiration, photosynthesis, and the synthesis of DNA and RNA. Magnesium is also a part of the ring structure of the chlorophyll molecule. A characteristic symptom of magnesium deficiency is chlorosis between the leaf veins, occurring first in older leaves because of the mobility of this cation.

The element chlorine is found in plants as the chloride ion (Cl-). It is required for the water-splitting reaction of photosynthesis through which oxygen is produced. Plants deficient in chlorine develop wilting of the leaf tips followed by general leaf chlorosis and necrosis. The leaves may also exhibit reduced growth. Roots of chlorine-deficient plants may appear stunted and thickened near the root tips. Chloride ions are highly soluble and are generally available in soils. Therefore chlorine deficiency is only rarely observed in plants grown in native or agricultural habitats.

Manganese ions (Mn2+) activate several enzymes in plant cells. In particular, decarboxylases and dehydrogenases involved in the citric acid (Krebs) cycle are specifically activated by manganese. The best-defined function of manganese is in the photosynthetic reaction through which oxygen (O2) is produced from water. The major symptom of manganese deficiency is intervenous chlorosis associated with the development of small necrotic spots.

Most species utilizing the C4 and crassulacean acid metabolism (CAM) pathways of carbon fixation require sodium ions (Na+). In these plants, sodium appears vital for regenerating phosphoenolpyruvate, the substrate for the first carboxylation in the C4 and CAM pathways. Under sodium deficiency, these plants exhibit chlorosis and necrosis, or even fail to form flowers. Many C3 species also benefit from exposure to low levels of sodium ions.

Deficiencies in mineral nutrients that are involved in redox reactions (Fe, Zn, Cu, Ni, Mo)

This group of five micronutrients consists of the metals iron, zinc, copper, nickel, and molybdenum. All of these can undergo reversible oxidations and reductions (e.g., Fe2+ ↔ Fe3+) and have important roles in electron transfer and energy transformation. They are usually found in association with larger molecules such as cytochromes, chlorophyll, and proteins (usually enzymes).

Iron has an important role as a component of enzymes involved in the transfer of electrons (redox reactions), such as cytochromes. As in magnesium deficiency, a characteristic symptom of iron deficiency is intervenous chlorosis. These symptoms, however, appear initially on younger leaves because iron, unlike magnesium, cannot be readily mobilized from older leaves. Under conditions of extreme or prolonged deficiency, the veins may also become chlorotic, causing the whole leaf to turn white.

Many enzymes require zinc ions (Zn2+) for their activity, and zinc may be required for chlorophyll biosynthesis in some plants. Zinc deficiency is characterized by a reduction in internodal growth, and as a result plants display a rosette habit of growth in which the leaves form a circular cluster radiating at or close to the ground. The leaves may also be small and distorted, with leaf margins having a puckered appearance. These symptoms may result from loss of the capacity to produce sufficient amounts of the auxin indole-3-acetic acid (IAA).

Like iron, copper is associated with enzymes involved in redox reactions, through which it is reversibly oxidized from Cu+ to Cu2+ An example of such an enzyme is plastocyanin, which is involved in electron transfer during the light reactions of photosynthesis. The initial symptom of copper deficiency is the production of dark green leaves, which may contain necrotic spots. The necrotic spots appear first at the tips of young leaves and then extend toward the leaf base along the margins.

Urease is the only known nickel-containing (Ni2+) enzyme in higher plants, although nitrogen-fixing microorganisms require nickel (Ni+ through Ni4+) for the enzyme that reprocesses some of the hydrogen gas generated during fixation (hydrogen uptake hydrogenase). Nickel-deficient plants accumulate urea in their leaves and consequently show leaf tip necrosis.

Molybdenum ions (Mo4+ through Mo6+) are components of several enzymes, including nitrate reductase and nitrogenase. The first indication of a molybdenum deficiency is general chlorosis between veins and necrosis of older leaves. In some plants, such as cauliflower or broccoli, the leaves may not become necrotic, but instead may appear twisted and subsequently die. Flower formation may be prevented, or the flowers may abscise prematurely.

Treating nutritional deficiencies

Many traditional and subsistence farming practices promote the recycling of mineral elements. The main losses of nutrients from such agricultural systems ensue from leaching that carries dissolved ions, especially nitrate, away with drainage water. In the high-production agricultural systems of industrialized countries, a large proportion of crop biomass leaves the area of cultivation, and returning crop residues to the land where the crop was produced becomes difficult at best. This unidirectional removal of nutrients from agricultural soils make it important to restore the lost nutrients to these soil through the addition of fertilizers. Most chemical fertilizers contain inorganic salts of the macronutrients nitrogen, phosphorus, and potassium. Fertilizers that contain only one of these three nutrients are termed straight fertilizers, like superphosphate, ammonium nitrate. Fertilizers that contain two or more mineral nutrients are called compound fertilizers or mixed fertilizers, and the numbers on the package label, such as “10-14-10”, refer to the percentages of N, P as P2O5 and K as K2O, respectively, in the fertilizer. With long-term agricultural production, consumption of micronutrients can reach a point at which they, too, must be added to the soil as fertilizers.

Organic fertilizers, in contrast to chemical fertilizers, originate from the residues of plant or animal life or from natural rock deposits. Before crop plants can acquire the nutrient elements from these residues, the organic compounds must be broken down, usually by the action of soil microorganisms through a process called mineralization. Mineralization depends on many factors, including temperature, water and oxygen availability, and the type and number of microorganisms present in the soil. As a consequence, rates of mineralization are highly variable, and nutrients from organic residues become available to plants over periods that range from days to months to years. This slow rate of mineralization hinders efficient fertilizer use, so farms that rely solely on organic fertilizers may require the addition of substantially more nitrogen or phosphorus.