Effect of winter cold stress on photosynthetic activity of Mediterranean evergreen species.

Long-term exposure to a combination of high light levels and suboptimal growth temperatures during winter causes a depression of photosynthesis (1,2). In many chilling-sensitive species, exposure to low air temperatures under high irradiance leads to pronounced functional impairment of photosynthesis (3); moreover, during winter the shorter photoperiod negatively influences plant metabolism in Mediterranean ecosystem (4). The mildness of the Mediterranean winter may allow a relatively high photosynthetic activity in evergreen plants (5); nevertheless, a very cold winter and frost events can be expected every decade in the Mediterranean Basin (2,6). Winter stress may, thus, represents an additional limitation to Mediterranean evergreen species production (7). Plants are able to adapt their photosynthesis within a certain range to the prevailing environmental conditions and sensitivity of photosynthesis to stress varies among plant species (8). The main objective of this research was to compare the photosynthetic response of the Mediterranean evergreen species (Q.ilex, P.latifolia, P.lentiscus, A.unedo, C. incanus, E. arborea, E.multiflora, R. officinalis) under severe winter conditions (low air temperature and high irradiance). The leaf damage after the stress period (presence of brown necrotic areas), according to (9) was analysed. The results on the whole show that the considered species have the same photosynthetic response to winter stress, nevertheless the rate of decrease is different. At low winter air temperatures the photosynthetic rate was decreased on an average 41 – 71 % , paralleled by a lowering of carbossilation efficiency (ratio between net photosynthesis and sub-stomatal CO2 concentration). E.multiflora is the species having the highest photosynthetic decrease (71 %) and Q. ilex the lowest one (41 %). The species sensitivity to winter stress factors results from a higher leaf damage, according to (3); after the stress period necrosis was not observed in P.latifolia, P.lentiscus, E.arborea. Most temperature based models require a parameter to link temperature with plant metabolism, and an appropriate parameter is photosynthesis which provides an indicator for functional limitations imposed by environmental factors (8), particularly air temperature which controls the magnitude of the photosynthetic response (10). The intrinsic link between photosynthesis and biomass production suggests that photosynthesis and its response to heat and cold periods may play a major role in determining the ability of evergreen species to persist in their distribution areas, also in consideration of global change.