Comparative photochemical and phenomorphological responses to winter stress of an evergreen (Quercus ilex L.) and a semi-deciduous (Cistus albidus L.) Mediterranean woody species

The impact of winter stress on plants from a Mediterranean area was evaluat ed through comparison of photosystem II (PS II) efficiencies and phenomorph ological characteristics of two Mediterranean woody species - Quercus ilex (evergreen) and Cistus albidus (semi-deciduous). The studies were carried o ut in NE Spain, at two sites with different mean winter temperatures. The r esults showed that reductions of the efficiency of PS II may occur in Medit erranean plant communities during winter, and are especially remarkable on colder days. The extent and duration of the decrease in PS II efficiency (p hotoinhibition) depended not only on the climatic conditions, but also on t he site, on the species considered and on the position of leaves in the pla nt canopy. Increased photoinhibition at the colder site was only clear for C. albidus. Nevertheless, the efficiencies of PS II were always higher in C . albidus than in Q. ilex. Phenological patterns, morphology and leaf incli nation may protect C. albidus leaves from potentially photoinhibitory condi tions in winter. Morphological and structural photoprotection is apparently not so well developed in Q. ilex, which probably relies more on increased protection at the photochemical level of its long-lived leaves. As has been reported in relation to summer-drought stress, Q. ilex has possibly develo ped a strategy of tolerance to photoinhibition, whereas C. albidus relies p referentially on avoidance features. (C) 2000 Editions scientifiques et med icales Elsevier SAS.