Effects of root temperature on leaf gas exchange and xylem sap abscisic acid concentrations in six Cucurbitaceae species

Roots of six Cucurbitaceae species were exposed to low (14 degrees C), middle (24 degrees C), and high (34 degrees C) temperatures while aerial parts of plants were maintained at ambient temperatures between 23 and 33 degrees C. The highest dry mass (DM), photon-saturated rate of net photosynthesis (P-Nsat), and stomatal conductance (g(s)) were found at 14 degrees C in figleaf gourd and turban squash plants, at 24 degrees C in cucumber and melon plants, while bitter melon and wax gourd plants had lower DM, P-Nsat, and g(s) at 14 degrees C than at 24 or 34 degrees C. Sub-or supra-optimum root temperatures did not induce photoinhibition but induced slight changes in the quantum efficiency of photosystem 2, PS2 (Phi(PS2)) and photochemical quenching(q(p)). Meanwhile, xylem sap abscisic acid (ABA) concentration followed a contrastingchange pattern to that of g(s). Thus the change in P-Nsat was mainly due to the change in g(s) and roots played an important role in the regulation of stomatal behaviour by deliveringincreased amount of ABA to shoots at sub-or supra-optimum root temperatures.