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
Am. Gilmore et O. Bjorkman, 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, Planta, 192(4), 1994, pp. 537-544
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.