Photosynthesis as a temperature indicator in Quercus ilex L.

Net photosynthesis (Pn), stomatal conductance (gs), leaf temperature (LT), transpiration rate (E) and in vivo chlorophyll fluorescence were monitored February 1996-February 1997 in Quercus ilex plants growing in the climax ar ea (Rome) and in the Garda lake region. Photosynthetic activity is an appro priate temperature-dependent functional trait linked to plant metabolism an d performance. We employed photosynthesis as a stress temperature indicator . Regression analysis showed that in such regional climatic conditions, net photosynthetic rates were primarily correlated with temperature. The intro duction of rainfall in the function did not significantly improve the theor etical prevision, in the range of temperature and rainfall analysed. The fa vourable leaf temperatures allowing 90-100% of the highest photosynthetic r ates were 14-28 degrees C, decreasing over 50% when leaf temperature were r espectively below 6 degrees C and over 37 degrees C: the low potential phot ochemical efficiency of 0.71 PSII (Fv/Fm) at Castelporziano during summer, confirmed the state of stress. Transpiration rates remained high with the i ncrease of leaf temperature in summer, in spite of the 46% decrease of stom atal conductance. Nevertheless, this decrease allowed the maintenance of ac ceptable Pn rates in stressful conditions. The potential productivity of Q. ilex lied on high peaks of activity during periods of lower evaporative de mand and a rapid stomatal response to an increase in air temperature and so il water deficit. Although it is difficult to forecast Q. ilex productivity and plant structure, we may hypothesize its future presence in the Mediter ranean Basin by the capability of vegetative activity in a wide range of te mperatures, the high stomatal control in stressful conditions, the high pla sticity index and water use efficiency (WUE). (C) 2000 Elsevier Science B.V . All rights reserved.