INTERACTION BETWEEN PHOTON FLUX-DENSITY AND ELEVATED-TEMPERATURES ON PHOTOINHIBITION IN ALOCASIA-MACRORRHIZA

The effects of light and elevated temperatures on the efficiency of en ergy conversion in PSII [O-PSII = (Fm'-Fs)/Fm'], pigment composition a nd heat tolerance of shade-acclimated Alocasia macrorrhiza were invest igated. Leaf discs were exposed for 3 h to high light (HL; 1600 mu mol photons.m(-2).s(-1)) or low light (LL; 20 mu mol photons.m(-2).s(-1)) and a series of constant temperatures ranging from 30 to 49 degrees C . All HL treatments led to rapid and severs decreases in O-PSII. Durin g the 2-h recovery period (LL, 25 degrees C) following the HL treatmen ts. fast and slow recovery phases could be distinguished. Leaf discs t hat had experienced HL and 30 degrees C recovered completely while no recovery of O-PSII was seen after a 3-h exposure to HL and 45 degrees C. A 3-h exposure to 45 degrees C at LL led to a less severe decrease in O-PSII and complete recovery was accomplished after less than 1 h. Under LL conditions a temperature of 49 degrees C was necessary to cau se an irreversible decrease in O-PSII, followed by necrosis the next d ay. Streptomycin had no effect on the degree of reduction and recovery in O-PSII discs exposed to HL and 35-45 degrees C, but partially inhi bited recovery in discs exposed to HL and 30 degrees C. Streptomycin l ed to a more severe decrease in O-PSII at LL and 49 degrees C and comp letely inhibited recovery. Streptomycin had no effect on the conversio n of the xanthophyll-cycle pigments during the treatment or the recove ry. The epoxidation state was roughly the same in all leaf discs after a 3-h HL treatment (0.270-0.346) irrespective of the exposure tempera ture. The back-conversion of zeaxanthin into violaxanthin after a 2-h recovery period was only seen in leaf discs that had been exposed to H L and 30 degrees C. The thermotolerance of shade A. macrorrhiza leaves of 49.0 +/- 0.7 degrees C (determined by fluorescence) coincided with the temperature at which damage occurred in leaf discs exposed to LL. However, under HL the critical temperature under which necrosis occur red was much lower (42 degrees C). The thermotolerance of A. macrorrhi za shade leaves could be increased by a short exposure (< 20 min) to s lightly elevated temperatures.