Plasticity in maximum stomatal conductance constrained by negative correlation between stomatal size and density: an analysis using Eucalyptus globulus

Maximum stomatal conductance to water vapour and CO₂ ( g _wmax, g _cmax, respectively), which are set at the time of leaf maturity, are determined predominantly by stomatal size ( S ) and density ( D ). In theory, many combinations of S and D yield the same g _wmax and g _cmax, so there is no inherent correlation between S and D , or between S , D and maximum stomatal conductance. However, using basic equations for gas diffusion through stomata of different sizes, we show that a negative correlation between S and D offers several advantages, including plasticity in g _wmax and g _cmax with minimal change in epidermal area allocation to stomata. Examination of the relationship between S and D in Eucalyptus globulus seedlings and coppice shoots growing in the field under high and low rainfall revealed a strong negative relationship between S and D , whereby S decreased with increasing D according to a negative power function. The results provide evidence that plasticity in maximum stomatal conductance may be constrained by a negative S versus D relationship, with higher maximum stomatal conductance characterized by smaller S and higher D , and a tendency to minimize change in epidermal space allocation to stomata as S and D vary.