Loss of the precise control of photosynthesis and increased yield of non-radiative dissipation of excitation energy after mild heat treatment of barley leaves

The aftereffects of a short exposure of intact barley leaves to moderately elevated temperature (40 degrees C, 5 min) on the induction transients and the irradiance dependencies of photosynthesis and chlorophyll fluorescence are presented. This mild heat treatment strongly reduced the oscillations i n the rate of photosynthesis and in the yield of chlorophyll fluorescence. However, only a 25% irreversible inhibition of maximum photosynthetic capac ity of photosystem II (PSII) measured by oxygen evolution was produced and the intrinsic quantum yield of PSII measured by the chlorophyll fluorescenc e ratio (F-m - F-o)/F-m decreased by only 15%. In contrast, the above treat ment increased radiationless dissipation processes in PSII by a factor of t wo. In heat-treated leaves, photosynthesis was not saturated even by strong light. Both Delta pH-dependent quenching of excitons in PSII (including fo rmation of zeaxanthin) and state 1/state 2 transition were found to be stim ulated. Heat exposure enhanced the control of PSII activity by PSI, as evid enced by a significant increase in the quenching effect of far-red light on the maximum yield of chlorophyll fluorescence. It was deduced that after m ild heat treatment, the photosynthetic apparatus in leaves lacks the precis e coordinating control of electron transport and carbon metabolism owing to the inability of PSII to support electron transport at a level adequate fo r carbon metabolism. This effect was not related to the small irreversible thermal damage to PSII, but was rather due to a significant increase in non -photochemical quenching of excitation energy.