Impact of an exceptionally hot dry summer on photosynthetic traits in oak (Quercus pubescens) leaves
P. Haldimann (1, 2, 3), A. Gallé (1) and U. Feller (1)
1. Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland / 2. Present address: Department of Plant Molecular Biology, Biophore – Biology building, University of Lausanne, CH-1015 Lausanne,
Switzerland / 3. Corresponding author () / Received June 3, 2007; accepted August 16, 2007; published online March 3, 2008
Summary
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
CO2 assimilation rate (Pn) by mid-morning, which increased by the afternoon. More extreme drought and heat caused a sharp day-long reduction in Pn. These effects were closely related to changes in stomatal conductance (gs), but low gs was unaccompanied by low intercellular CO2 concentrations (Ci). Around midday, a combination of heat and drought increased Ci, 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.
