Allocation of absorbed light energy into photochemistry and dissipation in a semi-deciduous and an evergreen Mediterranean woody species during winter

The importance of photodissipation in the protection from excessive solar energy absorption during winter was evaluated in shrubs of Cistus albidus L. (semi-deciduous) and Quercus ilex L. (evergreen). Diurnal and seasonal changes in chlorophyll a (Chl a) fluorescence-related parameters of leaves with different microclimates (south- and north-facing leaves) were assessed from early winter to late spring in a Mediterranean plant community. Maximal photochemical efficiency of photosystem II (PSII) (F-v/F-m) was lowest on sunny and cold winter days, but low values on overcast (and cold) days indicated that the whole winter season was potentially stressful. Photoinhibition was greater in south than in north leaves during winter. There was a decreased proportion of absorbed light utilised in photochemistry, and an increase in that proportion diverted into dissipation processes on cold sunny days. Under low light conditions (e.g. of north-facing leaves throughout the winter, or all the canopy on overcast days), a greater proportion of light energy was allocated to photochemistry than in bright conditions, although still smaller than the dissipated fraction. Allocation to photochemistry prevailed over dissipation only in spring. The phenological and structural characteristics of C. albidus protect it from excessive light loads, and reduce the need for investment in physiological mechanisms to avoid their harmful effects. Quercus ilex lacks such protection, and relies essentially on increased and sustained photodissipative activity at the physiological level.