INTERACTIVE EFFECTS OF WATERLOGGING AND IRRADIANCE ON THE PHOTOSYNTHETIC PERFORMANCE OF SEEDLINGS FROM 3 OAK SPECIES DISPLAYING DIFFERENT SENSITIVITIES (QUERCUS-ROBUR, QUERCUS-PETRAEA AND QUERCUS-RUBRA)

Potted seedlings from three oak species (Quercus robur, Q petraea and Q rubra), known to present different sensitivities to temporary soil h ypoxia, were submitted to two contrasted irradiance regimes in a green house (100% and approximately 35% of full greenhouse irradiance) and s hortly after to waterlogging for 2-4 weeks. The experiment was repeate d for 2 years. Leaf gas exchange, chlorophyll content and fluorescence , leaf water potential and photosynthetic capacity were recorded sever al times during the stress. Biomass increment was estimated at the end of the stress, or after 3 weeks of drainage. Despite its late applica tion, after completion of elongation and expansion of leaves, shading significantly reduced growth and modified photosynthetic activity. It generally reduced net assimilation rates and increased chlorophyll con tents. Q rubra seedlings behaved differently: growth decreased and chl orophyll was enhanced as in the two other species but net assimilation rates and photosynthetic capacity were higher in the shade than under full irradiance. This surprising effect probably resulted from high i rradiance stress, and was not alleviated by improved fertilization dur ing the second year. Waterlogging induced severe disorders in photosyn thesis: net CO2 assimilation, stomatal conductance, photosynthetic cap acity: chlorophyll content and growth declined. In addition, predawn v alues of photochemical efficiency of photosystem Il, which are usually close to the optimum value of 0.83, were significantly decreased. The largest dysfunctions occurred in Q rubra seedlings, and the smallest in Q robur, which confirms that the number of disorders in photosynthe sis is an accurate estimate of the differential sensitivity to waterlo gging among genotypes. Shading of the seedlings interacted with waterl ogging and limited the extent of induced damage. In particular, stomat al closure and net CO2 assimilation rates were proportionally less aff ected under shade. Similarly, predawn leaf water potential presented s maller decreases. The decline of predawn values of photochemical effic iency occured to a much lesser extent under shade. Finally, the behavi our of seedlings during post-waterlogging stress differentiated signif icantly the species. None of them recovered completely after 3 weeks. Nevertheless, Q robur behaved best, recovering high levels of predawn water potential, slowly reopening stomata and reincreasing net assimil ation rates. Q petraea recovered with more difficulties: predawn leaf water potential decreased in several plants after the end of waterlogg ing, and photosynthesis recovered very slowly. Q rubra did not recover at all and several individuals died after the end of waterlogging str ess.