Stomatal conductance of dehydrating leaves
The effect of the chemical treatments on the stomatal conductance (gs) of leaves was assessed in relation to Ψ as measured by balancing pressure. We used 24 randomly selected leaves of P. dulcis (10 FC, 7 ABA, 7 control) and 20 leaves of P. communis (7 FC, 6 ABA, 7 control). For each leaf, gs was measured periodically during dehydration (from Ψ close to zero to turgor loss point) using a portable photosynthesis system (LI-6400, Li-Cor, Lincoln, NE, USA) in darkness, with an air flow of 1,000 μmol s−1 and RH set at 70%. The Ψ was recorded after each gs measurement. Initial trials showed that at room conditions, leaves – especially FC treated leaves – dehydrated rather fast, limiting the number of measurements that could be performed when still turgid. Thus, to reduce the speed of dehydration, leaves were kept in dark, small containers filled with wet paper towels between measurements. The relationship between gs and Ψ was assessed by fitting data to a linear mixed-effect model with treatment as fixed factor and leaf as random factor. Data were compared at the average Ψ value (least-square mean) predicted for all the treatments and both species using analysis of variance (ANOVA) and Tukey tests. The model was also used to estimate gs at Ψ values when maximum flux (Qmax) and maximum hydraulic conductance (Kmax) were reached and evaluate potential correlations. Statistical analyses were performed with R (v.3.5.1, the R Foundation for Statistical Computing, Vienna, Austria).