Differential photosynthetic performance and photoprotection mechanisms of three Mediterranean evergreen oaks under severe drought stress

The ability of three Mediterranean oaks (Quercus coccifera L., Quercus ilex ssp. ballota (Desf.) Samp and Quercus suber L.) to cope with intense drought was investigated. Water stress reduced stomatal conductance and photosynthesis in these species. Drought-mediated changes in photosynthetic-related parameters allowed the characterisation of the specific photo-protective mechanisms. Specifically, Q. suber downregulated photosynthetic electron transport rates (ETR) closing PSII reaction centres (i.e. decreasing photochemical quenching) and through an antheraxanthin (A) + zeaxanthin (Z)-mediated diminished intrinsic PSII efficiency (Phi(exc.).). These changes were lower in Q. coccifera and Q. ilex ssp. ballota, which decreased further ETR photo-inactivating PSII centres (evidenced by their low predawn F(v)/F(m) ratios at high water stress). The predawn F(v)/F(m) ratio decreased in Q. coccifera largely due to F(m) decreases, whereas in Q. ilex ssp. ballota F(v)/F(m) decreases were due to F(0) increases, below -4 MPa. These F(v)/F(m) decreases were well correlated with increases in the A + Z photo-protective pigments. An analysis of dark respiration and photorespiration as alternative electron sinks under intense drought stress also revealed interspecific differences. The largest imbalance between electrons generated and consumed increased potentially oxidative damage in Q. suber. Subsequently, only Q. suber showed loss of chlorophyll, which is one of the main targets of oxidative damage. Data suggest that Q. coccifera and Q. ilex ssp. ballota seem more able than Q. suber to withstand highly xeric conditions. Therefore, our results question the consideration of Mediterranean evergreen oaks as a homogeneous physiological group.