Stomatal and non-stomatal limitations on leaf carbon assimilation in beech (Fagus sylvatica L.) seedlings under natural conditions

Limitations to diffusion and biochemical factors affecting leaf carbon uptake were analyzed in young beech seedlings (Fagus sylvtica L.) growing in natural gaps of a beech-wood at the southern limit of the species. Half of the seedlings received periodic watering in addition to natural rainfall to reduce the severity of the summer drought. Plant water status was evaluated by measuring predawn water potential. Basic biochemical parameters were inferred from chlorophyll fluorescence and photosynthesis-CO2 curves (A-C-c) under saturating light. The curves were established on three dates during the summer months. The main variables studied included: stomatal and mesophyll conductance to CO2 (g(s) and g(m) respectively), maximum velocity of carboxylation (V-cmax) and maximum electron transport capacity (J(max)) The g(m) was estimated by two methodologies: the curve-fitting and J constant methosds. Seedlings withstood moderate water stress, as the leaf predawn water potential (Psi(pd)) measured during the study was within the range -0.2 to -0.5 MPa. Mild drought caused g(s) and g(m) to decrease only slightly in response to Psi(pd). However both diffusional parameters explained most of the limitations to CO2 uptake. In addition, it should be highlighted that biochemical limitations, prompted by V-cmax and J(max) were related mainly to ontogenic factors, without any clear relationship with drought under the moderate water stress experienced by beech seedlings through the study. The results may help to further understanding of the functional mechanisms influencing the carbon fixation capacity of beech seedlings under natural conditions.