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

The role of stomatal heterogeneity in the response of stomatal conductance (g_s ) 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 g_s in small areas of the leaf (as determined by thermography) and comparing them to each other and to the average value of g_s (as determined by gas exchange). These analyses show that despite substantial heterogeneity in g_s values, the response of g_s to Δw was qualitatively similar in all areas of the leaf, and all responses of g_s 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.