Energy dissipation in drought-avoiding and drought-tolerant tree species at midday during the Mediterranean summer

Photosynthetic performance was monitored during two consecutive summers in four co-occurring evergreen Mediterranean tree species growing on a south-f acing rocky slope. In response to midday water stress, the drought-avoiding species Pinus halepensis Mill. exhibited marked stomatal closure (g(s)) bu t no changes in stem water potential (Psi(s)), whereas the drought-tolerant species Quercus coccifera L., Q. ilex ssp. ballota (Desf.) Samp. and Junip erus phoenicea L. displayed declines in midday g(s) and Psi(s). The higher resistance to CO2 influx in needles of P. halepensis compared with the othe r species did not result in either a proportional increase in non-radiative dissipation of excess energy or photo-inactivation of photosystem II (PSII ). No significant differences were found among species either in the de-epo xidation state of the xanthophyll cycle (DPS) or in the pool of its compone nts on a total chlorophyll basis (VAZ). Despite contrasting midday assimila tion rates, the three drought-tolerant species all exhibited a pronounced d rop in photochemical efficiency at midday that was characterized by a decre ase in the excitation capture efficiency of the open PSII centers. Although photoinhibition was not fully reversed before dawn, it apparently did not result in cumulative photo-damage. Thus, the drought-avoiding and drought-t olerant species employed different mechanisms for coping with excess light during the midday depression in photosynthesis that involved contrasting mi dday photochemical efficiencies of PSII and different degrees of dynamic ph otoinhibition as a photo-protective mechanism. These behaviors may be relat ed to the different mechanisms employed by drought-avoiding and drought-tol erant species to withstand water deficit.