High nitrogen-driven photosynthesis limitation in shade-demanding species mediated by stomatal conductance through plasma membrane intrinsic proteins

Photosynthesis is susceptible to nitrogen (N) surplus. However, the mechanism of reduced photosynthesis by N surplus need to be further verified in the shade-demanding and high-N-sensitive species. Photosynthetic capacity, N allocation, stomatal parameters, the correlation between PIP (encoding plasma membrane intrinsic proteins) expression and total stomatal conductance (g_t) were analyzed in Panax notoginseng grown under the levels of low N (LN), moderate N (MN) and high N (HN). N content per unit leaf area (N_area) was increased with increasing N application. HN resulted in an increase in N allocation to the light capture system (P_L) and a decrease in N allocation to the carboxylation system (P_C). Net photosynthetic rate (P_n), stomatal conductance (g_s), the maximum electron transport rate (J_max) and the maximum carboxylation rate (V_cmax) were lower in the LN and HN plants. Under HN condition, the limitation to stomatal conductance (S_L) and limitation to biochemical factors (B_L) were the main factors for reducing photosynthetic efficiency. The stomatal area index (SAI) was more closely correlated with stomatal density (SD) than with stomatal length (SS). g_s was partially correlated with SS or SD. The correlation analysis showed that the relative expression of PIP1.3, PIP1.4 and PIP2.7 is positively correlated with g_t /N_area. The results obtained suggest that high-N-driven limitation to photosynthesis in the shade-demanding and HN-sensitive medicinal crop, such as P. notoginseng, is mainly derived from the decrease in g_s, and partly from the decrease in the carboxylation capacity of RuBP.