Dynamics of stomatal water relations following leaf excision.

We examined the stomatal response to leaf excision in an evergreen woody shrub, Photinia x 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 degrees C) before being transferred to a glasshouse (temperature range: 20-30 degrees 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: Pe, Pg and pi g, respectively) from stomatal conductance (gs) and bulk leaf water potential (psi l), using a hydromechanical model of gs. psi l was calculated from cumulative post-excision transpirational water loss using empirical relationships between psi l and relative water content obtained on similar leaves. Inferred Pg and Pe both declined immediately after leaf excision. Inferred pi g also declined after a lag period. The kinetics of pi 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 gs and for leaf water status regulation.