Variation in leaf N allocation and mesophyll conductance to CO2 in four tree species under low soil P stress in subtropical China

Low P (LP) levels in leaves can affect their photosynthetic N-use efficiency (PNUE), internal N allocation, and mesophyll conductance to CO 2 (gm). The changes in leaf internal N allocation and gm in N-fixing trees and the consequent changes in PNUE under low soil P treatments are not well understood. In this study, we exposed seedlings of Dalbergia odorifera, Erythrophleum fordii (N-fixing trees), Castanopsis hystrix, and Betula alnoides (non-N-fixing trees) to three levels of soil P. The effects were not consistent among species, and LP had no specific effect on N-fixing species. Saturated net CO2 assimilation rate (Asat) values in D. odorifera and C. hystrix were remarkably lower under LP than under high P (HP) because Cc in D. odorifera and Vcmax and Jmax in C. hystrix were reduced. Narea values in D. odorifera and C. hystrix were also reduced under LP, and the degree of reduction of Narea was larger than that of Asat, which resulted in decreased PNUE in these species. PR and gm in D. odorifera and PR, PB, and gm in C. hystrix significantly decreased under LP and were internal factors affecting the variation in PNUE in these two trees. P CW was significantly and linearly related to PR only in C. hystrix, indicating that more N was invested in the cell walls to resist the damage caused by low soil P, at the expense of Rubisco N. Our results showed that soil P deficiency affected leaf N utilization, photosynthetic efficiency, and seedling growth.