Ördög Vince, Molnár Zoltán (2011)
Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem
Several environmental factors influence the photosynthesis, which shows direct responses to the environmental factors like light, ambient CO2 concentrations, and temperature, as well as indirect responses (mediated through the effects of stomatal control) like humidity and soil moisture. However, under any particular conditions, the rate of photosynthesis is limited by the slowest step in the process, the so-called limiting factor. Therefore, at any given time, photosynthesis can be limited either by light or by CO2 concentration, but not by both factors at the same time.
Photosynthesis is the primary function of leaves
Leaves are exposed to different spectra and quantities of light that result in photosynthesis. The light reaching the plant is a flux and that flux can be measured in either energy or photon units. Irradiance (energy) is expressed in watts per square meter (W m-2; 1 W = 1 joule s-1). Photon irradiance is the number of incident quanta, expressed in moles per square meter per second (mol m-2 s-1; 1 mol of light = 6.02 x 1023 photons). The photosynthetically active radiation (PAR, 400-700nm) may also be expressed in terms of energy (W m-2) but is more commonly expressed as quanta (mol m-2 s-1). Under direct sunlight, PAR irradiance is about 2000 µmol m-2 s-1 (900 W m-2) at the top of a dense forest canopy, but may be only 10 µmol m-2 s-1 (4.5 W m-2) at the bottom of the canopy. While roughly 1.3 kW m-2 of radiant energy from the sun reaches Earth, less than 5% of this energy is ultimately converted into carbohydrates by a photosynthesizing leaf. Significant fraction of the absorbed light is lost as heat and a smaller amount is lost as fluorescence.
Leaf anatomy maximizes light absorption
The anatomy of the leaf is highly specialized for light absorption. The epidermis is typically transparent to visible light. Below the epidermis, the top layers of photosynthetic cells are called palisade cells. Some leaves have several layers of columnar palisade cells. To increase the efficiency of photosynthetic structures within palisade cells, chloroplasts have high surface-to-volume ratios. Below the palisade layers is the spongy mesophyll, where the cells are very irregular in shape and are surrounded by large air spaces. The large air spaces generate many interfaces between air and water that reflect and refract the light, thereby randomizing its direction of travel. This phenomenon is called interface light scattering. Some environments, such as deserts, have so much light that it is potentially harmful to leaves. In these environments leaves often have special anatomical features, such as hairs, salt glands, and epicuticular wax, that increase the reflection of light from the leaf surface, thereby reducing light absorption by as much as 40%.
Leaf angle and leaf movement can control light absorption
Under natural conditions, leaves exposed to full sunlight at the top of canopy tend to have steep leaf angles, which allow more sunlight to penetrate into the canopy. It is common to see the angle of leaves within a canopy decrease (become more horizontal) with increasing depth in the canopy. Some plants control light absorption by solar tracking, that is, their leaves continuously adjust the orientation of their laminae such that they remain perpendicular to the sun’s rays. Many species, including alfalfa, cotton, soybean, bean, and lupine, have leaves capable of solar tracking (Figure 2.16). Solar tracking is a blue-light response.
Figure 2.16 Leaf movement in sun-tracking plants: (A) initial orientation, and (B) orientation 4 hours after exposure to light (source: Taiz L., Zeiger E., 2010)
The term heliotropism used to describe sun-induced leaf movements, we call leaves that maximize light interception by solar tracking diaheliotropic. Some solar tracking plants can also move their leaves so that they avoid full exposure to sunlight, thus minimizing heating and water loss. These sun avoiding leaves are called paraheliotropic. Some plant species have leaves that can display diaheliotropic movements when they are well watered and paraheliptropic movements when they experience water stress. Diaheliotropic solar tracking appears to be a feature common to wild plants that are short-lived and must complete their life cycle before the onset of drought. Paraheliotropic leaves are able to regulate the amount of sunlight incident on the leaf to a nearly constant value. Often only one-half to two-thirds of full sunlight may be advantageous under conditions of water stress or excessive solar radiation.
Plants acclimate and adapt to sun and shade environments
Acclimation is a growth process in which each newly produced leaf has a set of biochemical and morphological characteristics suited to the particular environment in which it unfolds. In some plant species the mature leaf will abscise and a new leaf will develop that is better suited for the new environment. However, some species of plants are not able to acclimate when transferred from a sunny to a shady environment. These plants are adapted to either a sunny or a shady environment. When plants adapted to deep shade conditions are transferred into full sunlight, the leaves experience chronic photoinhibition and leaf bleaching, and the plants eventually die. Shade leaves have more total chlorophyll per reaction centre, have a higher ratio of chlorophyll b to chlorophyll a, and are usually thinner that sun leaves. Sun leaves have more rubisco, are thicker, and have longer palisade cells than leaves grown in the shade. The adaptive response of some shade plants is to produce a 3:1 ratio of photosystem II to photosystem I reaction centers, compared with the 2:1 ratio found in sun plants. Other shade plants add more antenna chlorophyll to PSII to increase absorption by this photosystem and better balance to flow of energy through PSII and PSI. These changes appear to enhance light absorption and energy transfer in shady environment.
Photosynthetic responses to light by the intact leaf
In the dark CO2 is given off by the plant because of mitochondrial respiration. With increasing irradiance photosynthetic CO2 assimilation eventually reaches a point at which photosynthetic CO2 uptake exactly balances the respiratory CO2 release. This is called the light compensation point. Light compensation points of sun plants range from 10 to 20 µmol m-2 s-1, whereas corresponding values for shade plants are 1 to 5 µmol m-2 s-1. The linear relationship between photon flux and photosynthetic rate persists at light levels above the light compensation point. The slope of this linear portion of the curve reveals the maximum quantum yield of photosynthesis for the leaf. Leaves of sun and shade plants show very similar quantum yields. This is because the basic biochemical processes that determine quantum yield are the same for these two types of plants. The quantum yield of photochemistry is about 0.95. However, the photosynthetic quantum yield is lower (0.125 for C3 plants). The quantum yields for CO2 of C3 and C4 leaves vary between 0.04 and 0.06 mole of CO2 per mole of photons. If C3 leaves are exposed to low O2 concentrations, photorespiration is minimized and the quantum yield increases to about 0.09 mole of CO2 per mole of photons. At higher photon fluxes, the photosynthetic response to light starts to level off and eventually reaches saturation. The light-response curve of most leaves saturates between 500 and 1000 µmol m-2 s-1, well below full sunlight. However, because the photosynthetic response of the intact plant is the sum of the photosynthetic activity of all the leaves, only rarely is photosynthesis light-saturated at the level of the whole plant.
Leaves must dissipate excess light energy
When exposed to excess light, leaves must dissipate the surplus absorbed light energy so that it does not harm the photosynthetic apparatus. Heat production and the xanthophylls cycle appears to be important avenues for dissipation of excess light energy. The xanthophylls cycle comprises the three carotenoids violaxanthin, antheraxanthin, and zeaxanthin. Experiments have shown that zeaxanthin is the most effective of the three xanthophylls in heat dissipation. The zeaxanthin content increases at high irradiances and decreases at low irradiances. In leaves growing under full sunlight, zeaxanthin and antheraxanthin can make up 60% of the total xanthophyll cycle pool at maximal irradiance levels attained at midday (Figure 2.17). Contrary to the diurnal cycling of this pool observed in summer, zeaxanthin levels remain high all day during the winter. Presumably this mechanism maximizes dissipation of light energy, thereby protecting the leaves against photooxidation during winter.
Figure 2.17 Diurnal changes in xanthophyll content as a function of irradiance in sunflower (source: Taiz L., Zeiger E., 2010)
An alternative means of reducing excess light energy is to move the chloroplasts so that they are no longer exposed to high light. Under high light, the chloroplasts move to the cell surfaces that are parallel to the incident light, thus avoiding excess absorption of light. Such chloroplast rearrangement can decrease the amount of light by the leaf about 15%. Chloroplast movement in leaves is a typical blue-light response.
Photosynthetic response to temperature
Stomatal opening influences both leaf temperature and the extent of transpiration water loss. A leaf with an effective thickness of 300 µm of primarily water would warm up to a very high temperature if all available solar energy were absorbed and no heat were lost. This heat load is dissipated by emission of long-wave radiation (at about 10,000 nm), by sensible heat loss, and by evaporative (or latent) heat loss (Figure 2.18):
Radiative heat loss: all objects emit radiation in proportion to their temperature. However, the maximum wavelength is inversely proportional to its temperature, and leaf temperatures are low enough that the wavelength emitted are not visible to the human eye.
Sensible heat loss: if the temperature of the leaf is higher than that of the air circulating around the leaf, the heat is convected (transferred) from the leaf to the air.
Latent heat loss: because the evaporation of water requires energy, when water evaporates from a leaf (transpiration), it withdraws large amounts of heat from the leaf and cools it.
Sensible heat loss and evaporative heat loss are the most important processes in the regulation of leaf temperature, and the ratio of the two fluxes is called the Bowen ratio. In water-stressed crop, partial stomatal closure reduces evaporative cooling and the Bowen ratio is increased. The amount of evaporative heat loss is influenced by the degree to which stomata remain open. Plants with very high Bowen ratios conserve water, but also ensure very high leaf temperatures.
Figure 2.18 The absorption and dissipation of energy from sunlight by the leaf (source: Taiz L., Zeiger E., 2010)
There is an optimal temperature for photosynthesis
The highest photosynthetic rates seen in response to increasing temperature represent the optimal temperature response. Optimal temperature is the point at which the capacities of the various steps of photosynthesis are optimally balanced, with some of the steps becoming limiting as the temperature decreases or increases. Membrane-bound electron transport processes become unstable at high temperatures, cutting off the supply of reducing power and leading to a sharp overall decrease in photosynthesis. Optimal temperatures have strong genetic (adaptation) and environmental (acclimation) components. Plants of different species growing in habitats with different temperatures have different optimal temperatures for photosynthesis. Plants growing at low temperatures maintain higher photosynthetic rates at low temperatures than plants grown at high temperatures.
Photosynthetic responses to carbon dioxide
In the presence of adequate amounts of light, higher CO2 concentrations support higher photosynthetic rates. The reverse is also true: low CO2 concentrations can limit the amount of photosynthesis in C3 plants. Carbon dioxide is a trace gas in the atmosphere, presently accounting for about 0.039%, or 390 parts per million (ppm), of air. Currently the CO2 concentration of the atmosphere is increasing by about 1 to 3 ppm each year. By 2100 the atmospheric CO2 concentration could reach 600 to 750 ppm unless fossil fuel emission are controlled. Carbon dioxide and methane, play a role similar to that of the glass roof in a greenhouse. The increased CO2 concentration and temperature associated with the greenhouse effect can influence photosynthesis. At current atmospheric CO2 concentrations, photosynthesis in C3 plants is CO2 limited, but this situation could change as atmospheric CO2 concentrations continue to rise. Under laboratory conditions, most C3 plants grow 30 to 60% faster when CO2 concentration is doubled (to 600-750 ppm), and the growth rate becomes limited by the nutrient available to the plant.
Carbon dioxide diffuses through the pore into the substomatal cavity and into the intercellular spaces between mesophyll cells. This portion of the diffusion path of CO2 into the chloroplast is a gaseous phase. The remainder of the diffusion path to the chloroplast is a liquid phase, which begins at the water layer that wets the walls of the mesophyll cells and continue through the plasma membrane, the cytosol, and the chloroplast (Figure 2.19). In air of high relative humidity, the diffusion gradient that drives water loss is about 50 times larger than the gradient that drives CO2 uptake. In drier air, this difference can be even larger. Therefore, a decrease in stomatal resistance through the opening of stomata facilitates higher CO2 uptake but is unavoidably accompanied by substantial water loss.
Figure 2.19 Points of resistance to the diffusion of CO2 from outside the leaf to the chloroplasts (source: Taiz L., Zeiger E., 2010)
For most leaves, once CO2 has diffused through the stomata, internal CO2 diffusion is rapid, so limitations on photosynthetic performance within the leaf are imposed by factors other than internal CO2 supply. The capacity of leaf tissue for photosynthetic CO2 assimilation depends to a large extent on its rubisco content.
CO2 imposes limitations on photosynthesis
Increasing intracellular CO2 to the concentration at which photosynthesis and respiration balance each other defines the CO2 compensation point, at which the net efflux of CO2 from the leaf is zero. This concept is analogous to that of the light compensation point. The CO2 compensation point reflects the balance between photosynthesis and respiration as a function of CO2 concentration, whereas the light compensation point reflects that balance as a function of photon flux under constant O2 concentration.
In C3 plants, increasing atmospheric CO2 above the compensation point stimulates photosynthesis over a wide concentration range. At low to intermediate CO2 concentrations, photosynthesis is limited by the carboxylation capacity of rubisco. At high CO2 concentrations, photosynthesis becomes limited by the capacity of the Calvin-Benson cycle to regenerate the acceptor molecule ribulose 1,5-bisphosphate, which depends on electron transport rates. However, photosynthesis continues to increase with increasing CO2 because carboxylation replaces oxygenation on rubisco.
C4 plants can use water and nitrogen more efficiently than C3 plants can. On the other hand, the additional energy cost of the concentrating mechanism makes C4 plants less efficient in their utilization of light. This is probable one of the reasons that most shade-adapted plants in temperate regions are C3 plants.
The ratio of water loss to CO2 uptake is much lower in CAM plants than it is in either C3 or C4 plants. This is because stomata are primarily open only at night, when lower temperatures and higher humidity contribute to a lower transpiration rate. The main photosynthetic constraints on CAM metabolism is that the capacity to store malic acid is limited, and this limitation restricts the total amount of CO2 uptake.