Acclimation to humidity modifies the link between leaf size and the density of veins and stomata

The coordination of veins and stomata during leaf acclimation to sun and shade can be facilitated by differential epidermal cell expansion so large leaves with low vein and stomatal densities grow in shade, effectively balancing liquid‐ and vapour‐phase conductances. As the difference in vapour pressure between leaf and atmosphere (VPD) determines transpiration at any given stomatal density, we predict that plants grown under high VPD will modify the balance between veins and stomata to accommodate greater maximum transpiration. Thus, we examined the developmental responses of these traits to contrasting VPD in a woody angiosperm (Toona ciliata M. Roem.) and tested whether the relationship between them was altered. High VPD leaves were one‐third the size of low VPD leaves with only marginally greater vein and stomatal density. Transpirational homeostasis was thus maintained by reducing stomatal conductance. VPD acclimation changed leaf size by modifying cell number. Hence, plasticity in vein and stomatal density appears to be generated by plasticity in cell size rather than cell number. Thus, VPD affects cell number and leaf size without changing the relationship between liquid‐ and vapour‐phase conductances. This results in inefficient acclimation to VPD as stomata remain partially closed under high VPD. In a previous paper we demonstrated how the density of veins and stomata remained tightly coordinated during plastic changes to leaf size in response to irradiance. Here we examine whether a similar process maintains the coordination of veins and stomata in plants grown under contrasting VPD treatments. We find that despite large changes to leaf size there is no significant acclimation of vein and stomatal density. This fascinating result points to contrasting anatomical mechanisms of response to irradiance and VPD, with cell size and cell number being specifically responsive to these different environmental stimuli. We suggest a general scheme for how leaf size, stomatal density and vein density respond to the environment.