Yunlong Wang, Tonghua Pan, Qian Wang, Kai Cao, Yu Wang, Zhonghua Bian
Far-red light (FR, 700-800 nm) is a crucial environmental signal which regulates plant photosynthesis and salt stress tolerance, but its regulation mechanism is not well-addressed. In the present study, tomato seedlings under 0 mM NaCl or 150 mM NaCl salinity were treated with supplemental FR (R: FR = 0.8) or without supplemental FR (R: FR = 7.4). The growth components, leaf anatomical structure, photosynthetic capacity, and activities of key enzymes involved in carbon fixation were determined, along with the expression of related genes. Results showed that the increased dry mass with supplemental FR under normal conditions (non-salt stress, i.e., 0 mM NaCl) was due to the increase in net assimilation rate (NAR), while that under salt stress was attributed to the improvement in both NAR and leaf area ratio (LAR). FR supplementation elevated the palisade/spongy tissue ratio (P/S) by 10.92%, improving leaf anatomical traits and water retention capacity. In addition, FR addition improved the activities of PSII and PSI via increasing the actual quantum yields of PSII (ΦPSII) and PSI (ΦPSI) by 9.86% and 5.01%, respectively, and concomitantly decreasing the quantum yields of non-regulated non-photochemical quenching (ΦNO) and acceptor-side limitation in PSI (ΦNA) by 19.77% and 11.36%, respectively. Furthermore, FR supplementation not only increased the activities of Rubisco by 36.36% and Rubisco activase (RCA) by 40.65%, but also upregulated the expression of related genes, thereby resulting in higher CO2 fixation capacity. Synergistic improvements in morphological, anatomical, and photosynthetic properties induced by supplemental FR enhanced photosynthetic performance and further promoted carbon assimilation in salt-stressed tomato seedlings. These findings reveal the FR-mediated salt tolerance mechanism in tomato seedlings, thus providing guidance for alleviating secondary soil salinization in controlled agricultural systems.