Mechanism analysis: Nitrogen and potassium synergy regulate nitrogen distribution in photosynthetic system to enhance Panax notoginseng resistance to light stress

Panax notoginseng often faces varying levels of light stress during its growth, which can hinder efforts to improve its yield and quality. Nitrogen (N) and potassium (K) are known play important role in photosynthesis, while the synergistic effect and mechanism in enhancing the resistance to light stress of P. notoginseng have yet to be well understood. In this study, field experiments and pot trials were conducted to investigate the regulatory effects of the combined application of N and K on N allocation in P. notoginseng leaves under light stress, as well as on metabolic products of photosynthesis and the accumulation of saponin. Results showed that compared with medium light (ML: optimal light intensity, 450 μmol/m2/s), both low light (LL: 60 μmol/m2/s) and high light (HL: 800 μmol/m2/s) stress significantly reduced the net photosynthetic rate (Pn) (32.45% and 12.23%, respectively) and photosynthetic nitrogen use efficiency (PNUE) (10.01% and 19.21%, respectively) of P. notoginseng leaves. Compared with N0K0 (N: K=0.0045:0.0045 kg/m2), N1K2 (N: K=0.0225:0.045 kg/m2) treatment significantly increased Pn and PNUE under LL or HL stress in P. notoginseng, with an average increase of 52.80% and 31.61%, respectively. The allocation of carboxylation nitrogen (Ncb) and electron transport nitrogen (Net) were identified as the main limiting factors for PNUE under light stress. However, compared with N0K0, N1K2 treatment mitigated these limitations by increasing Rubisco content at an average of 0.29 ng/g FW, and adjusting the Ncb ratio by 7.31% to 21.11%, primarily through conversion of non-protein nitrogen (Nnp) to photosynthetic nitrogen (Npsn). In addition, compared with N0K0, N1K2 regulated the activity of carbohydrate metabolism enzymes, resulting in an average increase of 25.72% in starch content and a mean decrease of 6.48% in sucrose content under LL or HL stress, ultimately promoting the accumulation of total saponins in P. notoginseng (11.95% to 29.67%). In summary, under light stress, N1K2 synergy improves LL or HL stress resistance of P. notoginseng by expanding and supplementing the Npsn pool in its leaves, thereby promoting the accumulation of carbohydrate and saponins.