Response of photosynthetic nitrogen use efficiency in Betula utilis to altitudinal variation along Balang Mountain, Sichuan, China.

To better understand the response and adaptation of plants to altitudinal changes, four sites at the altitude of 2200 m, 2500 m, 3100 m and 3400 m on Balang Mountain were selected to test and calculate the eco-physiological parameters in leaves of Betula utilis, including photosynthetic nitrogen use efficiency (PNUE), CO ₂ diffusion conductance (stomatal conductance g _s and mesophyll conductance g _m ) and nitrogen allocation in each component (fractions of leaf nitrogen allocated to Rubisco P _R , to bioenergetics P _B , to light-harvesting components P _L , and to cell wall P _CW ). Their changes with altitudinal variations and the relationships between leaf PNUE and the other parameters were analyzed. The results showed that PNUE, P _R , and P _B of the leaves were relatively higher at 2500 m and 3100 m. With the increases of altitude, g _s and g _m increased and P _L decreased. The correlations between P _R , P _B and PNUE were significant, indicating that P _R and P _B were the main factors driving the changes in leaf PNUE in response to altitudinal variations. Besides, the fraction of leaf nitrogen allocated to photosynthetic apparatus (P _P ) was relatively higher at 2500 m and 3100 m. With increasing altitude, P _CW decreased and the fraction of leaf nitrogen allocated to the other components (P _other ) increased, which suggested that B. utilis leaves tended to allocate more nitrogen to the other components instead of the photosynthetic apparatus and cell wall with the increasing altitude to well adapt environmental changes.