Drill seeding and 5-ALA improve photosynthetic efficiency and ion balance of rice seedlings under salt stress

Rice (Oryza sativa L.) seedling raising is a vital step in rice cultivation. Salt stress severely impairs the growth of rice seedlings, whereas 5-aminolevulinic acid (5-ALA) plays a crucial role in plant responses to various abiotic stresses. In this study, we combined agronomic practices with chemical regulation. We selected two sowing methods, namely drill sowing and broadcast sowing. Treatments were applied at the three-leaf stage. These included foliar application of 40 mg·L− 1 5-aminolevulinic acid (5-ALA) and application of 0.4% (w/w) NaCl based on dry soil weight. This resulted in a soil electrical conductivity of 5500 ± 50 µS·cm− 1. Three biological replicates were set for each treatment, and sampling was conducted on the 3rd, 6th, and 9th days after salt stress imposition. The results showed that, under salt stress, excessive accumulation of Na+ and Cl− in rice seedling leaves damaged the photosynthetic apparatus and significantly inhibited seedling growth. Exogenous foliar application of 5-ALA promoted the synthesis of photosynthetic pigments by increasing the content of precursors for chloroplast synthesis. Meanwhile, 5-ALA increased K+ and Ca2+ levels significantly and reduced Na+ content by 26.96%. It also enhanced antioxidant enzyme activity and alleviated oxidative damage. As a result, gas exchange and chloroplast fluorescence parameters in seedlings improved, the photosynthetic rate increased by 49.39%, and carbohydrate accumulation was enhanced. Compared with broadcast sowing, drill sowing combined with 5-ALA application resulted in higher levels of chloroplast synthesis precursors and greater carbohydrate accumulation. These reserves provided additional energy for chlorophyll synthesis and for the repair of the photosynthetic apparatus under salt stress. This enhanced peroxide-scavenging capacity, maintained photosynthetic apparatus stability, improved ion homeostasis, and gave drill sowing stronger salt tolerance than broadcast sowing. Taken together, drill sowing and foliar application of 5-ALA significantly maintain the stability of the photosynthetic system and ion homeostasis in rice seedlings under salt stress. This enhances salt tolerance. These findings lay a theoretical foundation for future field trials and the use of saline-alkali soils.