FIGURE 5.

Effects of COMT1 silencing, exogenous melatonin and heat stress either alone or combined on the maximum photochemical efficiency of photosystem II (F_v/F_m) and CO₂ assimilation rate. (A) F_v/F_m shown in pseudo color images, the false color code depicted at the bottom of the image ranges from 0 (black) to 1 (purple); (B) F_v/F_m values; and (C) the light-saturated rate of CO₂ assimilation (A_sat). Tomato seedlings at five-leaf stage were exposed to 40°C temperature for 9 h before analysis. Plants were pretreated with 10 μmol L⁻¹ melatonin on leaves at 8 h prior to imposition of the heat stress. F_v/F_m and A_sat both were measured on the third fully expanded leaves; however, plants were dark adapted for 30 min before recording the F_v/F_m by using an imaging pulse amplitude modulated (PAM) fluorimeter (IMAG-MAXI; Heinz Walz, Effeltrich, Germany). The results are expressed as the mean values ± SE, n = 6. Different letters indicate significant differences (P < 0.05) according to the Tukey’s test.