Stomatal and mesophyll limitations to photosynthesis in one evergreen and one deciduous Mediterranean oak species

In the evergreen Quercus rotundifolia and the co-existing deciduous Q. faginea we studied the diurnal variations in photosynthetic capacity (P-max), measured as the rate Of O-2 evolution at photon and CO2 saturation, and in the rate of net CO2 assimilation (P-N) in the field during the period of maximum photosynthetic activity. Our aim was to check the contribution of stomatal and non-stomatal limitations to the diurnal variation in photosynthesis, and to study the differences between both species. Q. faginea leaves displayed lower mass per unit area and higher nitrogen content than Q. rotundifolia leaves. The maximum stomatal conductance and PN in the field were higher in Q. faginea than in Q. rotundifolia. Also Pmax of Q. faginea was higher than that of Q. rotundifolia. Both species attained in the field a high percentage of the Pmax (around 82 % for Q. faginea and 73 % for Q. rotundifolia). This indicates reduced stomatal limitation of photosynthesis under favourable conditions, especially in Q. faginea. P-N underwent a sharp decrease towards mid-day in association with increase in the atmospheric vapour pressure deficit and decrease in the leaf water potential. P-max was also reduced during mid-day. This demonstrated the contribution of mesophyll limitations to the P-N in the two species under stress. The mesophyll limitation of photosynthesis seemed to be similar for both species, independently from the differences in leaf traits between them.