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

Seasonal variation and the differential effect of drought on photosynthetic parameters involved in carbon assimilation in forest species have been poorly studied under natural conditions. Limitations to diffusion and biochemical factors affecting leaf carbon uptake were analyzed in young beech seedlings (Fagus sylvatica L.) growing in natural gaps of a beech-wood at the southern limit of the species. Half of the seedling received periodic watering in addition to natural rainfall to reduce the severity of the summer drought which is typical in the area. Plant water status was evaluated by measuring predawn water potential. Basic biochemical parameters were inferred from photosynthesis-CO2 curves (A-Cc) under saturating light and chlorophyll fluorescence. The curves were established on three dates during the summer months Main variables studied included: stomatal and mesophyll conductance to CO2 (gs and gm respectively), maximum velocity of carboxylation (Vcmax) and maximum electron transport capacity (Jmax). The gm was estimated by two methodologies: the curve-fitting and J constant methods. Seedlings withstood a moderate water stress as the leaf predawn water potential (Ψpd) measured during the study was within the range –0.2 to –0.5 MPa. A mild drought caused gs and gm to decrease only slightly in response to Ψpd. However both diffusional parameters explained most of the limitations to CO2 uptake. Furthermore, non-stomatal limitations were also important limiting net photosynthesis as water stress increased. In addition, it should be highlighted that the biochemical limitations, prompted by Vcmax and Jmax, 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 understand of the functional mechanisms influencing carbon fixation capacity of beech seedlings. This capacity may influence the total carbon balance of beech seedlings, and feed forward to their growth and survival under drought-prone environments such as those in the south of Europe.