Stomatal exchange

What functions do the stomates serve in gas exchange with the atmosphere ... 

Photosynthesis and gas exchange of leaves are affected by many stresses including drought, flooding, salinity, chilling, high temperature, soil compaction and inadequate nutrition. Many, but not all, of these stresses have symptoms in common. For example, stomatal conductance and the rate of assimilation of CO2 per unit leaf area often decrease when stress occurs. Further, it is possible that several of the stresses may exert their effects, in part, by increasing the levels of the hormone abscisic acid (ABA) in the leaf epidermis. This hormone is known to close stomata when applied to leaves. 

In this chapter we review recent developments, concentrating mostly on the effects of water stress because of its great importance, but trying where possible to identify general problems. As noted above, accumulation of ABA in leaves is common under stress, and we review its effects on gas exchange. If the sole direct effect of ABA were to reduce stomatal aperture, then assimilation rate would also decrease because of a lower intercellular partial pressure of C02,pi. Some experiments have suggested that pi is little affected by ABA and that the capacity for CO2 fixation has actually decreased. The situation has become complicated with the observation (Terashima et al., 1988) that application of ABA can cause non-uniform... 

Bradford, K.J., Sharkey, T.D. Farquhar, G.D. (1983). Gas exchange, stomatal behaviour, and 6 C values of the acca tomato mutant in relation to abscisic acid. Plant Physiology, 72, 245-50. 

Guard cells Specialized epidermal cells, crescent-shaped, contain chloroplasts, form defines stomatal pore Regulate stomatal aperture/pore for gas exchange... 

Transpiration rate and stomatal resistance are the most commonly measured plant gas exchange parameters using IRGA. Their behaviour in A. thaliana plants exposed to heavy metal ion stress is demonstrated in Figure 2 B and C. 

Leaf gas exchange rates are highly dependent on local climatic factors influencing C02 diffusion and evaporation rates, especially temperature lapse rates. The dependency of gas-exchange parameters on local climatic factors and leaf anatomy may account for the wide variability in leaf stomatal responses and stable isotope composition over elevation transects found in different species and different regions. 

The value of E in relation to like the value of gj, indicates the evaporative nature of the surface so E < E q or E/Ee, < 1 reflects surface dryness or stomatal closure as well as the balance of energy exchange between the atmosphere and the underlying surface. By definition, E > Ef, can be caused only by advection. As implied above with respect to the partitioning of temperature, this may also result from the entrainment of dry air from above the convective boundary layer that develops daily over the earth surface. To further illustrate the relation between E and Ef, in terms of surface characteristics, it is helpful to write the Penman-Monteith equation (Monteith and Unsworth, 1990),... 

A minute pore in the epidermis of plant leaves or stems. Stoma, which are bordered by guard cells that regulate the size of the opening, function in gas exchange between the plant and the external environment. The stomatal apparatus or stomate consists of the stoma plus guard cells, stratification... 

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