Figure 2.

Mean response of instantaneous stomatal conductance, foliage ABA level and leaf water potential in sunflower cv. Yellow Empress after whole plants (n = 3, ± SD, and n = 6, ± SD for initial ABA level) were exposed to a step change in VPD from 0.75 kPa (white circles) to 3.25 kPa (black circles; change denoted by a vertical dashed line). The horizontal lines indicate the minimum leaf water potential necessary to trigger the accumulation of foliage ABA level in bench-dried branches (Fig. 1). Leaf water potential was measured 5 min after the VPD transition to capture the most negative leaf water potential values before stomatal began to close, no significant change in either stomatal conductance or ABA levels occurred within the first 5 min following the VPD transition (data not shown). Asterisks denote a significant change in leaf water potential among the three conditions or significant change in foliage ABA level compared with the initial one (Student’s t-test; **P < 0.01).