PHOTOSYNTHESIS AND SHOOT WATER STATUS OF SEEDLINGS FROM DIFFERENT OAK SPECIES SUBMITTED TO WATERLOGGING

Stress effects induced on shoot photosynthesis and leaf water status by root hypoxia due to waterlogging have been assessed on saplings of Quercus robur, Q petraea, Q rubra and Q palustris in 2 successive experiments. Daily (first experiment) and weekly (second experiment) measurements of leaf gas exchange were made during 2 and 7 wk of waterlogging with a water table at 3 (1st) and 6 cm below the soil surface (2nd experiment). Net CO2 assimilation rate (A), and leaf conductance to CO2 (g) were rapidly and strongly affected by waterlogging in almost every case. CO2 diffusion analysis of gas exchange data revealed that both stomatal and non stomatal limitations apparently induced this decline. Predawn leaf water potential remained high in all cases, indicating that reductions in photosynthesis were not due to altered leaf water status. Possible mechanisms relating root hypoxia and leaf physiology are discussed. Within this general framework, some species-related differences could be detected: reactions of Q robur were in general much more limited than those of Q rubra and Q palustris, being virtually absent when the water table remained at 6 cm below soil surface. This observation could be connected with the ability of Q robur to produce more adventitious roots when waterlogged. No significant long term trend parallelling phases of root decay and subsequent root regeneration could be observed in photosynthesis for this species.