Leaf functional traits of four evergreen species growing in Mediterranean environmental conditions

In this study, physiological and biochemical responses of native and introduced evergreens to the Mediterranean region, co-occurring under ambient conditions, are presented. Carbon and nitrogen compounds have been investigated in fully expanded leaves of the Mediterranean evergreens Laurus nobilis and Nerium oleander and introduced to the Mediterranean region Asian evergreens Ligustrum japonicum and Pittosporum tobira in May (end of the growing season), July (middle of the drought season), October (beginning of the wet season), and January (middle of the cold season). The above-mentioned species are subjected to seasonal fluctuations of climatic stimuli, in the eastern Mediterranean region. During the most vigorous vegetative growth in spring, the species maintained elevated chlorophyll, soluble sugar, and starch content, at rising water potential and CO2 assimilation. Leaf proline content increased during unfavourable drought conditions, concomitantly with reduced leaf osmotic potential; in addition, declining water availability, during the dry season, had a significant impact on leaf water potential and turgor. Total lipid and nitrogen content increased during the wet and cold seasons. In general, lipid content was higher in mature leaves of the Mediterranean evergreens (L. nobilis and N. oleander) throughout the year, in comparison with that of the introduced evergreens (L. japonicum and P. tobira), while fatty acid composition seems to be species specific. Although differences in leaf attributes have been investigated between native and introduced species in the Mediterranean region, fully expanded leaves of the studied species did not appear to respond differently to the seasonality of the Mediterranean ecosystem.