Comparative ecophysiological effects of drought on seedlings of the Mediterranean water-saver Pinus halepensis and water-spenders Quercus coccifera and Quercus ilex

Ecophysiological and structural traits of seedlings of the water-saver Pinus halepensis and the water-spenders Quercus coccifera and Q. ilex were studied in response to water stress under greenhouse conditions. Water deficit reduced stomatal conductance (g (s)) and, as a consequence, both net CO2 assimilation (A) and transpiration rate (E) were also reduced. Water stress also emphasized midday down-regulation of the photochemical efficiency (dynamic photoinhibition) reducing quantum yield of noncyclic electron transport (Phi(PSII)), photochemical quenching (qP) and photochemical efficiency of the open reaction centres of PSII (F '(v)/F '(m)) and involved an increase of thermal dissipation of excess energy. However, water stress not only induced dynamic photoinhibition but also brought a reduction in F-v/F-m (chronic photoinhibition). Despite the water-saving strategy of P. halepensis that limited net CO2 assimilation, this species showed a higher photochemical efficiency and lower photoinhibition than Quercus species. This was not the result of a different photochemical quenching but was linked to a higher value of F '(v)/F '(m), indicating a less severe photo-inactivation of PSII. Water stress resulted in a loss of pigment content and in an increase of the carotenoids/chlorophyll ratio, antioxidant capacity and the biomass rate allocated to roots as opposed to that assigned to leaves. P. halepensis showed a lower photoinhibition and antioxidant activity than Quercus species due to its lower pigment content and higher proportion of carotenoids allowing P. halepensis to use, in a more effective way, the lesser excess energy absorbed.