The apparent feed-forward response to vapour pressure deficit of stomata in droughted, field-grown Eucalyptus globulus Labill

This study tested a multiplicative model of stomatal response to environment for drought-affected trees of Eucalyptus globulus Labill. growing in southern Australia. The model incorporates a feed-forward response to vapour pressure deficit of ambient air (deltae(a)) and performed well if evaluated using reduced major axis regression and log-transformed data. There was strong evidence from gas-exchange data, leaf water potentials and sapflow measurements of the feed-forward response by stomata to leaf-to-air vapour pressure deficit (deltae(l)). The response of stomata to deltae(l) was irreversible. Stomatal conductance and the rate of net photosynthesis were highly correlated and declined, together with the rate of transpiration, throughout the afternoon as deltae(a) increased despite increasing leaf water potentials. The concentration of CO2 inside leaves (c(i)) increased as stomatal conductance declined indicating increasing non-stomatal limitations to photosynthesis. The stomatal response to deltae(l) of E. globulus in the field is best described as an 'apparent feed-forward response' that probably results from both slowly reversible depression of net photosynthesis and abscisic acid accumulation in guard cells. We suggest that the stomatal response to c(i) may strengthen the link between photosynthetic capacity and stomatal conductance during leaf drying as a result of either drought or large delta e(l).