© 2000 Heron Publishing—Victoria, Canada
Effects of drought preconditioning on thermotolerance of photosystem II and susceptibility of photosynthesis to heat stress
in cedar seedlings
Mehdi Ladjal (1), Daniel Epron (2, 3) and Michel Ducrey (1)
1. Unité de Recherche Forestière Méditerannéenne, INRA, Avenue A. Vivaldi, F-84000 Avignon, France / 2. Laboratoire de Biologie et Ecophysiologie, Institut des Sciences et des Techniques de l'Environnement, Université de Franche-Comté,
Pôle / 3. Author to whom correspondence should be addressed (Daniel.Epron@pu-pm.univ-fcomte.fr) / Received March 16, 2000
Summary
Changes in photosystem II (PSII) thermotolerance during drought and recovery were studied under controlled conditions in three
Mediterranean cedar species (Cedrus brevifolia Henry, C. libani Loudon and C. atlantica Manetti). The temperature at which the quantum yield of PSII photochemistry was reduced by 15% of its value at 25 °C was
3 to 4 °C higher in drought-treated plants than in well-watered plants. The drought-induced increase in PSII thermotolerance
was already evident 8 days after water had been withheld from the seedlings, when net CO2 assimilation was still at 80% of its initial value, and was visible for up to 12 days after re-watering. When seedlings of
the three species were exposed to temperatures above 45 °C for 5 h, both maximal quantum yield of PSII photochemistry and
net CO2 assimilation rate were significantly reduced in unconditioned seedlings, whereas drought-preconditioned seedlings were almost
unaffected by the heat treatment. Drought-preconditioned seedlings still exhibited a higher tolerance to heat stress than
unconditioned seedlings 60 days after re-watering, although the transient, drought-induced osmotic adjustment had fully disappeared.
Among species, C. atlantica was the most heat sensitive, whereas the heat treatment had no significant effect on the parameters measured in C. brevifolia.
Keywords:
Cedrus, chlorophyll fluorescence, drought, heat acclimation, high temperature stress.
