Interactive effects of waterlogging and irradiance on the photosynthetic performance of seedlings from three oak species displaying different sensitivities (Quercus robur, Q petraea and Q rubra)

Abstract

Potted seedlings from three oak species (Quercus robur, Q petraea and Q rubra), known to present different sensitivities to temporary soil hypoxia, were submitted to two contrasted irradiance regimes in a greenhouse (100% and approximately 35% of full greenhouse irradiance) and shortly after to waterlogging for 2-4 weeks. The experiment was repeated for 2 years. Leaf gas exchange, chlorophyll content and fluorescence, leaf water potential and photosynthetic capacity were recorded several times during the stress. Biomass increment was estimated at the end of the stress, or after 3 weeks of drainage. Despite its late application, 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 contents. Q rubra seedling behaved differently; growth decreased and chlorophyll 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 irradiance stress, and was not alleviated by improved fertilization during the second year. Waterlogging induced severe disorders in photosynthesis.: net CO2 assimilation, stomatal conductance, photosynthetic capacity, chlorophyll content and growth declined. In addition, predawn values 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 photosynthesis is an accurate estimate of the differential sensitivity to waterlogging among genotypes. Shading of the seedlings interacted with waterlogging and limited the extent of induced damage. In particular, stomatal closure and net CO2 assimilation rates were proportionally less affected under shade. Similarly, predawn leaf water potential presented smaller decreases. The decline of predawn values of photochemical efficiency occurred to a much lesser extent under shade. Finally, the behavior of seedling during post-waterlogging stress differentiated significantly 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 assimilation rates. Q petraea recovered with more difficulties: predawn leaf water potential decreased in several plants after the end of waterlogging, and photosynthesis recovered very slowly. Q rubra did not recover at all and several individuals died after the end of waterlogging stress.