Stomatal conductance correlates with photosynthetic activity in Quercus ilex L. during the year

Abstract

Net photosynthesis (Anet) and stomatal conductance (gs) of Quercus ilex L. shrubs growing in the Mediterranean maquis, developing along the coast on sand dunes near Rome, were monitored throughout several clear days between February and November 1996. The favourable leaf temperature determining 90-100% of the highest Anet was 17-29°C. Anet decreased over 50% when Tleaf was respectively under 11°C and over 36°C, indicating the more pronounced decline in mesophyll activity. Anet and gs both increased from the end of winter to the beginning of spring and they decreased during the dry period. The highest rates of transpiration were observed in summer, despite the lowest stomatal conductance. The combined effects of decreased assimilation and increased water loss per transpiration resulted in a reduction of water use efficiency in photosynthesis during summer. Nevertheless stomatal limitation of CO2 uptake was not prolonged after hot periods, and a rapid increase of gs and Anet was possible when environmental conditions became more favourable. Therefore Quercus ilex is able to assume carbon at the highest rates during periods of low evaporative demand and it minimises its water loss in summer, decreasing stomatal conductance.