Long-term acclimation of mesophyll conductance, carbon isotope discrimination and growth in two contrasting Picea asperata populations exposed to drought and enhanced UV-B radiation for three years

Two Picea asperata Mast. populations originating from wet and dry climate regions of China were subjected to enhanced UV-B radiation, drought and their combination in a greenhouse for 3 years. Drought treatment caused a significant decrease in photosynthesis, mesophyll conductance (gm), carbon isotope discrimination (Δ) and growth characteristics when compared with well-watered treatment. In contrast to the great effects of drought stress, enhanced UV-B radiation showed some but not as substantial negative effects on these parameters. Little interaction effect between drought and UV-B radiation was detected, and the drought effect in combination with enhanced UV-B was not more pronounced than with no enhanced UV-B radiation. These results suggest that enhanced UV-B radiation does not aggravate the drought effect on P. asperata seedlings. The results also showed that the proportional role of the intercellular CO2 (Ci) decreased, while the role of chloroplastic concentration (Cc) became increasingly important in explaining the high values of carbon isotope composition (δ13C), when the water stress progressed in time. In addition, multivariate causal models proposed that there is a direct causal relationship between specific leaf area (SLA) and gm, which is not mediated by leaf N. Besides, there are functional links between gm and photosynthetic capacity (Vcmax). On the other hand, the study showed that net assimilation rate (NAR) is the main driving force for changes in relative growth rate (RGR), especially in low-water environments, and the degree of acclimation of respiration in the light (RL) is of central importance to the greater role played by NAR in determining variation in RGR.