ADENINE-NUCLEOTIDES AND THE XANTHOPHYLL CYCLE IN LEAVES .2. COMPARISON OF THE EFFECTS OF CO2- AND TEMPERATURE-LIMITED PHOTOSYNTHESIS ON PHOTOSYSTEM-II FLUORESCENCE QUENCHING, THE ADENYLATE ENERGY CHARGE AND VIOLAXANTHIN DE-EPOXIDATION IN COTTON

The relationships among the leaf adenylate energy charge, the xanthoph yll-cycle components, and photosystem II (PSII) fluorescence quenching were determined in leaves of cotton (Gossypium hirsutum L. cv. Acala) under different leaf temperatures and different intercellular CO2 con centrations (C-i). Attenuating the rate of photosynthesis by lowering the C-i at a given temperature and photon flux density increased the c oncentration of high-energy adenylate phosphate bonds (adenylate energ y charge) in the cell by restricting ATP consumption (A.M. Gilmore, O. Bjarkman 1994, Planta 192, 526-536). In this study we show that decre ases in photosynthesis and increases in the adenylate energy charge at steady state were both correlated with decreases in PSII photochemica l efficiency as determined by chlorophyll fluorescence analysis. Atten uating photosynthesis by decreasing C-i also stimulated violaxanthin-d e-epoxidation-dependent nonradiative dissipation (NRD) of excess energ y in PSII, measured by nonphotochemical fluorescence quenching. Howeve r, high NRD levels, which indicate a large trans-thylakoid proton grad ient, were not dependent on a high adenylate energy charge, especially at low temperatures. Moreover, dithiothreitol at concentrations suffi cient to fully inhibit violaxanthin de-epoxidation and strongly inhibi t NRD, affected neither the increased adenylate energy charge nor the decreased PSII photochemical efficiency that result from inhibiting ph otosynthesis. The build-up of a high adenylate energy charge in the li ght that took place at low C-i and low temperatures was accompanied by a slowing of the relaxation of non-photochemical fluorescence quenchi ng after darkening. This slowly relaxing component of nonphotochemical quenching was also correlated with a sustained high adenylate energy charge in the dark. These results indicate that hydrolysis of ATP that accumulated in the light may acidify the lumen and thus sustain the l evel of NRD for extended periods after darkening the leaf. Hence, sust ained nonphotochemical quenching often observed in leaves subjected to stress, rather than being indicative of photoinhibitory damage, appar ently reflects the continued operation of NRD, a photoprotective proce ss.