Significance of mesophyll conductance for photosynthetic capacity and water-use efficiency in response to alkaline stress in Populus cathayana seedlings

Cuttings of Populus cathayana were exposed to three different alkaline regimes (0, 75, and 150 mM Na2CO3) in a semicontrolled environment. The net photosynthesis rate (P (N)), mesophyll conductance (g (m)), the relative limitations posed by stomatal conductance (L (s)) and by mesophyll conductance (L (m)), photosynthetic nitrogen-use efficiency (PNUE), carbon isotope composition (delta C-13), as well as specific leaf area (SLA) were measured. P (N) decreased due to alkaline stress by an average of 25% and g (m) decreased by an average of 57%. Alkaline stress caused an increase of L (m) but not L (s), with average L (s) of 26%, and L (m) average of 38% under stress conditions. Our results suggested reduced assimilation rate under alkaline stress through decreased mesophyll conductance in P. cathayana. Moreover, alkaline stress increased significantly delta C-13 and it drew down CO2 concentration from the substomatal cavities to the sites of carboxylation (C (i)-C (c)), but decreased PNUE. Furthermore, a relationship was found between PNUE and C (i)-C (c). Meanwhile, no correlation was found between delta C-13 and C (i)/C (a), but a strong correlation was proved between delta C-13 and C (c)/C (a), indicating that mesophyll conductance was also influencing the C-13/C-12 ratio of leaf under alkaline stress.