Adaptive photosynthetic strategies of the evergreen Mediterranean species and their response to stress factors

Most temperature based models require a parameter to link temperature with plant functioning; photosynthesis may be an appropriate indicator for plant functional limitations imposed by environmental factors, particularly air temperature, which controls the magnitude of the photosynthetic response. The intrinsic link between net photosynthesis and biomass production suggests that photosynthesis and its response to drought is likely to play a major role in determining the ability of species to persist in the distribution area despite increasing drought stress. Global change effects on Mediterranean region are likely to produce warmer and drier conditions, water deficit, and more frequent and stronger drought periods. Increased drought stress may be a discriminant of species distribution and abundance determining changes in vegetation in the long-time. The main objective of this study was to analyze differences in physiological traits among species cooccurring in the Mediterranean maquis. Plant survival in a given environment depends largely upon whether it can photosynthesize and keep its water loss lower than the supply; rates of these two gas exchange processes may play a major role in determining the outcome of competition among species. The pronounced sensitivity of photosynthesis to heat can be used to detect early damage in plant tissue; we employed photosynthesis as a stress temperature indicator. The overall results of this research underline that the evergreen species of the Mediterranean maquis had different photosynthetic responses to drought stress: R. officinalis, E. arborea and E. multiflora had the lowest PN rates in favourable periods, the highest decrease during drought but the highest recovery capacity at the first rainfall, when the drought stress lasts over a short period. A. unedo and C. incanus had the highest PN rates during favourable periods, a high decrease during drought and a low recovery capacity. Q. ilex, P. latifolia and P. lentiscus had intermedial PN rates in favourable periods, a low reduction during drought and a high recovery capacity. The upper leaf temperature threshold beyond which photosynthetic rates drop below half of its maximum value is the highest in Q. ilex and P. latifolia, i.e. they photosynthesize at sufficient rates at air temperatures highest than 37 °C. These functional traits associated to a large and deep root system result in a higher capacity of these species to adjust photosynthetic rates under severe drought, favouring biomass accumulation. On the contrary, R. officinalis seems to be particularly affected by a severe drought stress period, the low WUE and CE, and the low tolerance to high air temperatures. It loses ca. 50% of its leaves at the end of a severe drought period. Q. ilex and P. latifolia might be at a competitive advantage relative to the other considered species in regard to increasing drought stress which is expected to occur in the Mediterranean Basin with the ongoing global climatic change.