IN-SITU ESTIMATION OF NET CO2 ASSIMILATION, PHOTOSYNTHETIC ELECTRON FLOW AND PHOTORESPIRATION IN TURKEY OAK (Q-CERRIS L) LEAVES - DIURNAL CYCLES UNDER DIFFERENT LEVELS OF WATER-SUPPLY

Diurnal time courses of net CO2 assimilation rates, stomatal conductan ce and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L,) under natur al summer conditions in central Italy, Combined measurements of gas ex change and chlorophyll a fluorescence under low O-2 concentrations all owed the demonstration of a linear relationship between the photochemi cal efficiency of PSII (fluorescence measurements) and the apparent qu antum yield of gross photosynthesis (gas exchange), This relationship was used under normal O-2 to compute total light-driven electron fluxe s, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation, This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-drive n electron flow, net CO2 assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon d eviations as soon as a severe drought stress occurred, whereas photoch emical efficiency and maximal fluorescence underwent large but reversi ble diurnal decreases, The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to pho torespiration. The rate of carbon loss by photorespiration (R(1)) reac hed mean levels of 56% of net assimilation rates, The potential applic ation of this technique to analysis of the relative contributions of t hermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.