Summary• The capability to withstand and to recover from severe summer droughts is becoming an important issue for tree species in central Europe, as dry periods are predicted to occur more frequently over the coming decades.• Changes in leaf gas exchange, chlorophyll a fluorescence and leaf compounds related to photoprotection were analysed in young Quercus pubescens trees under field conditions during two summers (2004 and 2005) of progressive drought and subsequent rewatering.• Photochemistry was reversibly down-regulated and dissipation of excess energy was enhanced during the stress phase, while contents of leaf pigments and antioxidants were almost unaltered. Plant water status was restored immediately after rewatering. Net photosynthesis ( P n ) measured at ambient CO 2 recovered from inhibition by drought within 4 wk. P n measured at elevated CO 2 -to overcome stomatal limitations -was restored after a few days.• A network of photoprotective mechanisms acted in preserving the potential functionality of the photosynthetic apparatus during severe drought, leading to a rapid recovery of photosynthetic activity after rewatering. Thus, Q. pubescens seems to be capable of withstanding and surviving extreme drought events.

Inhibition of the net photosynthetic CO 2 assimilation rate ( P n ) by high temperature was examined in oak ( Quercus pubescens L.) leaves grown under natural conditions. Combined measurements of gas exchange and chlorophyll (Chl) a fluorescence were employed to differentiate between inhibition originating from heat effects on components of the thylakoid membranes and that resulting from effects on photosynthetic carbon metabolism. Regardless of whether temperature was increased rapidly or gradually, P n decreased with increasing leaf temperature and was more than 90% reduced at 45 ∞ ∞ ∞ ∞ C as compared to 25 ∞ ∞ ∞ ∞ C. Inhibition of P n by heat stress did not result from reduced stomatal conductance ( g s ), as heat-induced reduction of g s was accompanied by an increase of the intercellular CO 2 concentration ( C i ). Chl a fluorescence measurements revealed that between 25 and 45 ∞ ∞ ∞ ∞ C heat-dependent alterations of thylakoid-associated processes contributed only marginally, if at all, to the inhibition of P n by heat stress, with photosystem II being remarkably well protected against thermal inactivation. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) decreased from about 90% at 25 ∞ ∞ ∞ ∞ C to less than 30% at 45 ∞ ∞ ∞ ∞ C. Heat stress did not affect Rubisco per se , since full activity could be restored by incubation with CO 2 and Mg 2 + . Western-blot analysis of leaf extracts disclosed the presence of two Rubisco activase polypeptides, but heat stress did not alter the profile of the activase bands. Inhibition of P n at high leaf temperature could be markedly reduced by artificially increasing C i . A high C i also stimulated photosynthetic electron transport and resulted in reduced non-photochemical fluorescence quenching. Recovery experiments showed that heat-dependent inhibition of P n was largely, if not fully, reversible. The present results demonstrate that in Q. pubescens leaves the thylakoid membranes in general and photosynthetic electron transport in particular were well protected against heat-induced perturbations and that inhibition of P n by high temperature closely correlated with a reversible heat-dependent reduction of the Rubisco activation state.

To study the role of the plastidial a-glucan phosphorylase in starch metabolism in the leaves of Arabidopsis, two independent mutant lines containing T-DNA insertions within the phosphorylase gene were identified. Both insertions eliminate the activity of the plastidial a-glucan phosphorylase. Measurement of other enzymes of starch metabolism reveals only minor changes compared with the wild type. The loss of plastidial a-glucan phosphorylase does not cause a significant change in the total accumulation of starch during the day or its remobilization at night. Starch structure and composition are unaltered. However, mutant plants display lesions on their leaves that are not seen on wild-type plants, and mesophyll cells bordering the lesions accumulate high levels of starch. Lesion formation is abolished by growing plants under 100% humidity in still air, but subsequent transfer to circulating air with lower humidity causes extensive wilting in the mutant leaves. Wilted sectors die, causing large lesions that are bordered by starch-accumulating cells. Similar lesions are caused by the application of acute salt stress to mature plants. We conclude that plastidial phosphorylase is not required for the degradation of starch, but that it plays a role in the capacity of the leaf lamina to endure a transient water deficit.