A spatially explicit model of patchy stomatal responses to humidity

Abstract

Stomata of leaves can exhibit either temporally stable, spatially homogeneous behaviour or complex spatial and temporal dynamics, depending on environmental and physiological conditions. To test the ability of accepted physiological mechanisms to describe these patterns, we developed a simple, spatially explicit model of stomatal responses to humidity that incorporated hydraulic interactions among stomata. Model results showed qualitative agreement with experimental evidence for a number of phenomena: (1) at high humidities, whole-leaf steady-state conductance is a monotonic function of humidity; (2) the initial stomatal response following a perturbation in humidity is in the direction opposite to the final response, and (3) spatial dynamics include patch formation and selforganization similar to that observed in actual leaves. These comparisons do not eliminate other explanations, but do suggest that novel mechanisms need not be invoked to explain the diversity of spatial and temporal patterns of stomatal behaviour in leaves.

The Ball–Berry–Leuning and Tardieu–Davies stomatal models: synthesis and extension within a spatially aggregated picture of guard cell function

“…The main reason for adopting Eqn 3a in this study, as previously (Dewar 1995), is that it is mathematically consistent with the empirical BBL and TD models (as shown below), a 'top-down' philosophy that is reflected in the opening sentence of the Introduction. In contrast, Appendix 2 shows that spatially explicit models describing the known mechanical advantage of subsidiary cells, as currently formulatedspecifically, the model of Haefner, Buckley & Mott (1997) which incorporates hydraulic interactions across the leafare not yet fully consistent with the BBL and TD models. A spatially explicit 'bottom-up' explanation of the BBL and TD models may require a comprehensive description of mechanical, osmotic and hydraulic interactions across the leaf (see Appendix 2 for further details).…”

“…The outcome of this collective mechanical behaviour -as expressed in terms of the relationship between g, P g and P e -would depend on the transmission of pressure across the epidermis, and thus on the common areas and elastic properties of the walls dividing neighbouring epidermal cells. Further theoretical justification of the BBL ¥ TD model, incorporating the known mechanical advantage of subsidiary cells, may require a scaling-up analysis of mechanical and osmotic relationships across the epidermis, possibly through extension of the spatially explicit model of hydraulic interactions proposed by Haefner et al (1997).…”

“…The ion pumping assumption proposed by Haefner et al (1997) This simplified analysis does not exclude the possibility that the BBL and TD models can be explained using the full two-dimensional version of Haefner et al's model, with an alternative prescription for the pumping parameter m 0 , and/or with a spatially varying (rather than constant) value of p s . Appendix 1 suggests that mechanical interactions across the leaf epidermis may also need to be represented.…”

Biotic and abiotic consequences of differences in leaf structure

“…Although the area-averaged effect of stomatal opening is to confer a conductance that is controlled physiologically, not all stomata are open equally. A typical histogram of apertures is unimodal, but transients in light or humidity can induce broader and even multimodal distributions (Buckley et al, 1997). Aperture responds to at least three major physiological state variables (Tardieu, 1994 ;Tardieu & Simmoneau, 1998) : (1) photosynthetic metabolites, so that conductance keeps pace with need for CO # substrate (in the leaf interior ; Mott, 1988) ; (2) hormones or regulators, particularly abscisic acid or ABA (as a water-stress signal), primarily from the roots (Blackman & Davies, 1985 ;implicating cytokinins ;Zhang & Davies, 1990 ;Tardieu et al, 1993Tardieu et al, , 1996 ; and (3) hydraulic linkages.…”

Interactions among stomata in response to perturbations in humidity

“…These interaetions were caused by changes in epidermal turgor associated with changes in the transpiration rate of individual stomata and by the effect of epidermal turgor on stomata aperture, Haefner et al (1997) presented a eomputer model showing that, in the presenee of such interactions, stomata exhibit stable, homogeneous behaviour at high humidities and oseillatory, heterogeneous behaviour at low humidities. The portions of their model describing the functioning of each individual stoma are similar to previously published models and are based on generally accepted aspects of stomatal physiology.…”