Effects of high temperature exposure on chlorophyll fluorescence of Phalaenopsis leaves

The effect of high temperature on photosystem II efficiency was studied in leaves of 16 month-old Phalaenopsis 'Sweetheart' plants grown under controlled cabinets. The various temperatures applied to leaves were 28, 35, and 47°C, for 30, 45, and 60 minutes, respectively. The distal half of individual leaves was divided into a shaded zone and illuminated zone and monitored the photosynthetic yield (PSII efficiency) by Walz MINI-PAM system. A PSI Fluorcam 700MF chlorophyll fluorescence imaging system was used to measure the dark-adapted F_v/F_m of the different leaf zones before and at the end of high temperature treatment, and then after 24 hours of recovery at 28°C. The Phalaenopsis leaves maintained a high photosynthetic yield (within 83% to 100% of maximum values) over a wide range of leaf temperatures, from 21°C to 45°C. The critical temperature, at which photosynthesis yield began to decrease sharply, was approximately 45°C. Leaf zones exposed to high temperature (47°C leaf temperature) showed an approximately 22% recovery in mean F_v/F_m after being transferred to 28°C growth cabinet for 24 h. After the 47°C leaf treatment, CF imaging system revealed the variation in the F_v/F_m distribution increased significantly and after 24 h of recovery reduced this variation substantially. The main source of variation in the F_v/F_m results was variation between leaves rather than within leaves. 'High temperature stress test' based on CF technology could be developed to sort Phalaenopsis plants by temperature tolerance levels to guarantee the certain quality for commercial application.