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ABSTRACT Diurnal time courses of net CO 2 assimilation rates, stomatal conductance and light‐driven electron fluxes were measured in situ on attached leaves of 30‐year‐old Turkey oak trees ( Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O 2 concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O 2 to compute total light‐driven electron fluxes, 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‐driven electron flow, net CO 2 assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible 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 photorespiration. The rate of carbon loss by photorespiration ( R 1 ) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de‐excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.
Valentini et al. (Thu,) studied this question.