Fig. 2.

Daily trend of leaf water potential (Ψ_L) in Populus nigra plants subject to well-watered (WW, open circles), moderate water stress (WS1, grey circles) and severe water stress (WS2, black circles) conditions. In the box the integrated daily values of Ψ_L represent the cumulative difference in water potential from 0 MPa integrated over a 24-h period. Data (means ± s.d., n = 4) were subjected to one-way ANOVA; different letters indicate significant differences between groups at a significance level of P < 0.05 using Tukey’s test.