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 CO₂ (g_m). The changes in leaf internal N allocation and g_m 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 (A_sat) values in D. odorifera and C. hystrix were remarkably lower under LP than under high P (HP) because C_c in D. odorifera and V_cmax and J_max in C. hystrix were reduced. N_area values in D. odorifera and C. hystrix were also reduced under LP, and the degree of reduction of N_area was larger than that of A_sat, which resulted in decreased PNUE in these species. P_R and g_m in D. odorifera and P_R, P_B, and g_m 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 P_R 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.