A mathematical model of stomatal conductance is presented. It is based on whole-plant and epidermal hydromechanics, and on two hypotheses: (1) the osmotic gradient across guard cell membranes is proportional to the concentration of ATP in the guard cells; and (2) the osmotic gradient that can be sustained per unit of ATP is proportional to the turgor pressure of adjacent epidermal cells. In the present study, guard cell [ATP] is calculated using a previously published model that is based on a widely used biochemical model of C 3 mesophyll photosynthesis. The conductance model for Vicia faba L. is parameterized and tested As with most other stomatal models, the present model correctly predicts the stomatal responses to variations in transpiration rate, irradiance and intercellular CO 2 . Unlike most other models, however, this model can predict the transient stomatal opening often observed before conductance declines in response to decreases in humidity, soil water potential, or xylem conductance. The model also explicitly accommodates the mechanical advantage of the epidermis and correctly predicts that stomata are relatively insensitive to the ambient partial pressure of oxygen, as a result of the assumed dependence on ATP concentration.

Abstract. Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO2 and water vapour added), which we termed ‘helox’. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water‐vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO2 concentration at the evaporating surfaces (ci), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant ci, suggesting heterogeneous (patchy) stomatal closure.

Stomatal conductance (g s ) typically declines in response to increasing intercellular CO 2 concentration (c i ). However, the mechanisms underlying this response are not fully understood. Recent work suggests that stomatal responses to c i and red light (RL) are linked to photosynthetic electron transport. We investigated the role of photosynthetic electron transport in the stomatal response to c i in intact leaves of cocklebur (Xanthium strumarium) plants by examining the responses of g s and net CO 2 assimilation rate to c i in light and darkness, in the presence and absence of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and at 2% and 21% ambient oxygen. Our results indicate that (1) g s and assimilation rate decline concurrently and with similar spatial patterns in response to DCMU; (2) the response of g s to c i changes slope in concert with the transition from Rubisco-to electron transport-limited photosynthesis at various irradiances and oxygen concentrations; (3) the response of g s to c i is similar in darkness and in DCMU-treated leaves, whereas the response in light in non-DCMU-treated leaves is much larger and has a different shape; (4) the response of g s to c i is insensitive to oxygen in DCMU-treated leaves or in darkness; and (5) stomata respond normally to RL when c i is held constant, indicating the RL response does not require a reduction in c i by mesophyll photosynthesis. Together, these results suggest that part of the stomatal response to c i involves the balance between photosynthetic electron transport and carbon reduction either in the mesophyll or in guard cell chloroplasts.

A clear correlation between the presence of stomata on both surfaces and factors such as habitat, growth form, and physiology has yet to emerge in the literature. However, certain loose trends with these factors are evident, and these are reviewed along with evidence for hypostomaty as the primitive form. It is proposed that the effect of developing stomata on the upper surface as well as the lower is to increase maximum leaf conductance to CO 2 . Plants with a high photosynthetic capacity, living in full-sun environments, and experiencing rapidly fluctuating or continuously available soil water (as opposed to seasonal or long-term soil water depletion), are identified as deriving an adaptive advantage from a high maximum leaf conductance. The correlation between groups of plants fitting the above conditions and those noted to be amphistomatic is remarkable. Plants not fitting the conditions are found to be largely hypostomatic.