Physiological mechanism of leaf N:P ratio as the nutrient limitation diagnostic tool: from the perspective of P fractions

Abstract Leaf nitrogen to phosphorus ratio (N:P) has widely been used to determine plant N and P limitations at community or larger scales. However, the underlying physiological mechanism of this rule of thumb has seldom been explored. Here, we quantify how leaf P fractions vary with leaf N:P and estimate the critical values and dynamics of leaf P fractions along the N:P gradient—from N limitation to P limitation, based on data collected from 143 unfertilized wild species in peer-reviewed literature. As leaf N:P increases, all P-fraction concentrations decline, with lipid P, inorganic P and residual P concentrations showing steeper declines than total P; the allocation-proportion ratios of metabolic P, nucleic acid P and lipid P shift from approximately ≤3:3:9 (corresponding to N:P ≤ 10), to 3:3:5 (N:P = 15) and to ≥3:3:3 (N:P ≥ 20). These findings suggest that most leaf P-fraction concentrations are more sensitive than total P to leaf N:P variations and reveal complicated P-use strategies of coordination (metabolic P and nucleic acid P) and trade-off (lipid P vs. metabolic P and nucleic acid P). This study provides a physiological explanation for using the leaf N:P ratio as a nutritional diagnosis and helps to better understand plant adaptive P-use strategies across diverse N and P availability conditions.