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Climatic constraints on diurnal variations in photosynthetic traits were investigated in oaks ( Quercus pubescens  Willd.) growing in the Swiss Alps. The measurement period included the summer of 2003, when central Europe experienced a record-breaking heat wave. During the summer, a combination of moderate heat and drought caused a reduction in photosynthetic CO 2  assimilation rate ( P  n ) by mid-morning, which increased by the afternoon. More extreme drought and heat caused a sharp day-long reduction in  P  n . These effects were closely related to changes in stomatal conductance ( g  s ), but low  g  s  was unaccompanied by low intercellular CO 2  concentrations ( C  i ). Around midday, a combination of heat and drought increased  C  i , indicating metabolic limitation of photosynthesis. Chlorophyll a (Chl a) fluorescence measurements revealed reversible down-regulation of photosystem (PS) II activity during the day, which was accentuated by heat and drought and correlated with diurnal variation in zeaxanthin accumulation. A combination of heat and drought reduced leaf Chl a + b concentrations and increased ratios of total carotenoids, xanthophyll-cycle carotenoids and lutein to Chl a + b. The combination of summertime heat and drought altered the 77 K Chl fluorescence emission spectra of leaves, indicating changes in the organization of thylakoid membranes, but it had no effect on the amounts of the major light-harvesting Chl-a/b-binding protein of PSII (LHCII), Rubisco, Rubisco activase, Rubisco-binding protein (cpn-60), phosphoribulokinase and chloroplast ATP synthase. The results demonstrate that  Q. pubescens  can maintain photosynthetic capacity under adverse summer conditions.

Impact of an exceptionally hot dry summer on photosynthetic traits in oak ( Quercus pubescens ) leaves