Effects of drought on photosynthesis and on the thermotolerance of photosystem II in seedlings of cedar (Cedrus atlanticaandC. libani)

The rate of net CO2 assimilation (A) in cedar needles declined during drought progression, while the quantum yield of electron transport measured under moderate light and high CO2 (ΔF/Fm) remained almost constant. A slight but significant decrease in ΔF/Fm was observed in severely droughted plants. This suggested that the decline in A during drought progression was mostly due to stomatal closure, and that the photosynthetic apparatus of cedar was only impaired by severe water deficits. A Turkish provenance of C. libani displayed a significantly higher thermotolerance than a French provenance of C. atlantica at whatever levels of drought. Nevertheless, there was a strong interaction between drought and heat stress in both cases. The temperature at which Fv/Fm was lowered to 15% of its original value was increased by more than 3–4 °C in droughted plants. This drought-induced shift in the PSII thermotolerance was still evident when heat treatments were done under high light rather than in darkness.