Effects of leaf nitrogen allocation on photosynthetic nitrogen-use efficiency of seedlings of three tropical species in Indonesia

Background/Question/Methods

During last a few decades, the economics of nitrogen (N) use in plant have been the key issues in eco-physiology, because N is one of the most essential and limiting components of natural ecosystem. Thus the efficient use of this resources greatly affect growth, survival, competition and distribution of plant species. The photosynthetic N-use efficiency (PNUE; μmol CO₂ [mol^-1 N] s^-1), which is defined as the amount of CO₂ assimilation per unit N content, differs with various leaf traits such as leaf N content (N_L), its allocation among different functional & structural components, intercellular and mesophyll CO₂ concentration and enzyme kinetics. In this study, we investigated the allocation of N_L to a photosynthetic apparatus (N_P) and its effects on the photosynthetic characteristics and PNUE of two-year-old pot-growing seedlings of three tropical tree species (Magnolia montana, Swietenia macrophylla and Acacia mangium) in western Java, Indonesia. Especially, A. mangiumis the only N-fixing species among three species, which widely planted for reforestation and afforestation in Indonesia. We expected this species has distinctive characteristics on N use in photosynthetic processes with other two non N-fixing species.

Results/Conclusions

A. mangium, i.e., N-fixing species among three studied species, had greater N_L than other two species, although the N fractions allocated to N_P, namely, Rubisco (N_R) and bioenergetics (N_B), were greater in S. macrophylla. Across the three species, the maximum assimilation (A_max) and dark respiration rate (R_d) were increased with increasing N_L, N_R and N_B. However, the variation of A_max was more explained by N_R or N_B than N_L, while the variation of R_d was more explained by N_L than N_R or N_B. PNUE exhibited a positive relationship with N_P in all species. However, there were significant differences between two different groups, that is, PNUE of A. mangium, i.e., N-fixing species, was lower than those of M. montana and S. macrophylla, i.e., non N-fixing species, at a given N_P. In addition, the intercellular CO₂ concentration exhibited no correlation with PNUE across the three species, while stomatal conductance was positively correlated with PNUE. However, the slopes of the correlation were steeper in non N-fixing species than N-fixing species. These results imply that there may be different strategies in N use in N-fixing and non-N-fixing species.