Stomatal heterogeneity in responses to humidity and temperature: Testing a mechanistic model.

The role of stomatal heterogeneity in the response of stomatal conductance (gs ) to the mole fraction difference in water vapour between the inside of the leaf and the ambient air (Δw) was determined using thermography and gas exchange for 3 species. The value of Δw for the leaf was varied in 2 different ways: first by varying air humidity while holding leaf temperature constant and second by varying leaf temperature while holding air humidity constant. Stomatal heterogeneity was explored by examining the response of gs in small areas of the leaf (as determined by thermography) and comparing them to each other and to the average value of gs (as determined by gas exchange). These analyses show that despite substantial heterogeneity in gs values, the response of gs to Δw was qualitatively similar in all areas of the leaf, and all responses of gs to Δw were well predicted by a recently proposed, vapour-phase mechanism for stomatal responses to temperature and humidity. Remarkably, the 2 model parameters, Θ and Z, that depend on leaf anatomy were constant for a given species, and only the maximum conductance varied in different regions of the leaf.