Dynamics of Stomatal Water Relations Following Leaf Excision

We examined the stomatal response to leaf excision in an evergreen woody shrub, Photinia × fraseri, using a novel combination of gas exchange, traditional water relations and modelling. Plants were kept outdoors in mild winter conditions (average daily temperature range: -1 to 12°C) before being transferred to a glasshouse (temperature range: 20-30°C) and allowed to acclimate for different periods before experiments. 'Glasshouse plants' were acclimated for at least 9 d, and 'outdoor plants' were acclimated for fewer than 3 d before laboratory gas exchange experiments. The transient stomatal opening response to leaf excision was roughly twice as long in outdoor plants as in glasshouse plants. To elucidate the reason for this difference, we inferred variables of stomatal water relations (epidermal and guard cell turgor pressures and guard cell osmotic pressure: P_e, P_g and ψ_g, respectively) from stomatal conductance (g_s) and bulk leaf water potential (ψ_l), using a hydromechanical model of g_s. ψ_l was calculated from cumulative post-excision transpirational water loss using empirical relationships between ψ_l and relative water content obtained on similar leaves. Inferred P_g and P_e both declined immediately after leaf excision. Inferred ψ_g also declined after a lag period. The kinetics of ψ_g adjustment after the lag were similar in outdoors and glasshouse plants, but the lag period was much longer in outdoor plants. This suggests that the longer transient opening response in outdoor plants resulted from slower induction, not slower execution, of guard cell osmoregulation. We discuss the implications of our results for the mechanism of short-term stomatal responses to hydraulic perturbations, for dynamic modelling of g_s and for leaf water status regulation.