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 (g(m)) carbon Isotope discrimination (Delta) 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 (C-1) decreased while the role of chloroplastic concentration (C-c) became increasingly important in explaining the high values of carbon isotope composition (delta C-13) 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 g(m) which is not mediated by leaf N Besides there are functional links between g(m) and photosynthetic capacity (V-cmax) 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 (R-L) is of central importance to the greater role played by NAR in determining variation in RGR. (C) 2010 Elsevier B V All rights reserved