Demonstration of thermal dissipation of absorbed quanta during energy-dependent quenching of chlorophyll fluorescence in photosynthetic membranes

When plant leaves or chloroplasts are exposed to illumination that exceeds their photosynthetic capacity, photoprotective mechanisms such as described by the energy-dependent (non-photochemical) quenching of chlorophyll fluor escence are involved. The protective action is attributed to an increased r ate constant for thermal dissipation of absorbed quanta, We applied photoac oustic spectroscopy to monitor thermal dissipation in spinach thylakoid mem branes together with simultaneous measurement of chlorophyll fluorescence i n the presence of inhibitors of opposite action on the formation of Delta p H across the thylakoid membrane (tentoxin and nigericin/valinomycin). A lin ear relationship between the appearance of fluorescence quenching during fo rmation of the Delta pH and the reciprocal variation of thermal dissipation was demonstrated. Dicyclohexylcarbodiimide, which is known to prevent prot onation of the minor light-harvesting complexes of photosystem II, signific antly reduced the formation of fluorescence quenching and the concurrent in crease in thermal dissipation, However, the addition of exogenous ascorbate to activate the xanthophyll de-epoxidase increased non-photochemical fluor escence quenching without affecting the measured thermal dissipation, It is concluded that a portion of energy-dependent fluorescence quenching that i s independent of de-epoxidase activity can be readily measured by photoacou stic spectroscopy as an increase in thermal deactivation processes. (C) 199 8 Federation of European Biochemical Societies.