László Lévai, Szilvia Veres (2013)
University of Debrecen
The exchange of oxygen, carbon dioxide and the loss of water in the leaf occur through pores called stomata. Guard cells are cells surrounding each stoma. Guard cells drive stomatal movement through changes in their osmotic pressure. When the guard cells are turgid the stomatal pore is large. This turgidity is caused by the accumulation of potassium ions in the guard cells. As potassium ion levels increase in the guard cells, the water potential of the guard cells drops, and water enters the guard cells. This actively accumulates potassium ions from neighboring epidermal cells due to proton pump-generated membrane potential. Part of this response is due to the activation of blue-light dependent receptors that transiently activate the plasma membrane H+-ATPase.
When the guard cells have lost water, it causes the cells to become flaccid and the stomatal pore to close. This may occur when the plant has lost an excessive amount of water. In addition, it generally occurs daily as light levels drop and the use of CO2 in photosynthesis decreases.
Stomatal opening is tightly controlled to meet the demand for CO2 for photosynthesis and restrict the loss of water during drought. On hot and sunny days, guard cells will often receive conflicting signals, high light promoting stomatal opening and low humidity inducing stomatal closure. This raises the question, how guard cells process these signals in such a way that opposing responses are prevented. Guard cell responses to blue light and ABA are clearly time dependent. After receiving these signals, guard cells show a maximal response after a few minutes, which levels off afterwards. Once the response has passed its maximum guard cells transport can be altered again by other signals. The response to photosynthetic light differs in this respect, since here a negative feedback system couples the influx of CO2 through stomata to the demand for CO2 in the leaf.
There are many signals that induce stomatal closure, among these the best known signal is probably ABA. In the signaling pathway towards stomatal closure, there are several secondary messengers, such as Ca2+, H2O2 and NO that contribute to the stomatal closure. Passive loss of turgor pressure also results in stomatal closure. Hydro passive stomatal closure occurs when the water evaporation from the guard cells is too low to be balanced by water movement into these cells. The water content in the cells is then rapidly reduced to the extent where the osmotic pressure is reduced and the cells lose turgor pressure and shrink. When this happens the guard cells are unable to maintain the shape and the stomatal pore is covered. Passive stomatal closure is important in ferns and Lycopods.