Objectives To clarify the physiological pathways by which γ-aminobutyric acid (GABA) irrigation promotes the growth of soybean seedlings under low nitrogen stress.

Methods A sand culture experiment was carried out using non-nodulating soybean (referred to as L, sourced from the Chinese Academy of Agricultural Sciences with the code WDD01795) and its nodulating mutant (L-M, obtained through natural mutation) as experimental materials. The experimental treatments included a normal nitrogen control (CK), low nitrogen stress (LN), and low nitrogen stress with GABA application (LN+GABA). At the V1 stage of soybean seedlings (designated as day 1), GABA was irrigated for three consecutive days. The key physiological and morphological indicators of soybean seedlings were determined at 5, 10, 15, and 20 days of treatment.

Results Compared to CK, LN significantly inhibited the growth-related physiological and morphological indicators of both the tested materials. Compared with LN, LN+GABA treatment significantly improved plant heights, stem diameters, leaf areas, root lengths, root volumes, leaf dry weights, stem dry weights, root dry weights, and growth rates of the two tested soybean materials on 20 and 30 days of treatment, the growth rate of nodulating and non-nodulating soybean materials increased by 36.53% and 52.44% on average, respectively. The contents of NO₃⁻, NH₄⁺, soluble protein, and free amino acids in the roots, stems, and leaves, as well as the activities of key nitrogen assimilation and transfer enzymes NR, GS, GOGAT, GDH, GOT, and GPT in the roots and leaves of the two materials were also increased significantly. On the 20th and 30th day, the nitrogen accumulation in the roots, stems, and leaves of nodulating soybean materials increased by an average of 42.35%, 22.05%, and 41.70%, respectively, while the increases in those of non-nodulating soybean were 23.56%, 34.23%, and 30.97%. Additionally, GABA significantly increased the contents of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids, and enhanced the gas exchange parameters of photosynthesis including net photosynthetic rate, stomatal conductance, and transpiration rate, but reduced intercellular CO₂ concentration.

Conclusions Irrigating soybean seedlings with GABA boosts the activity of pivotal enzymes involved in nitrogen assimilation, fostering nitrogen accumulation and elevating the contents of major nitrogen-containing compounds within plants. Consequently, this enhances the photosynthetic pigment content and improves photosynthetic parameters in the shoots, thereby promoting the growth of soybean seedlings under nitrogen stress. Moreover, compared to nodulating soybeans, exogenous GABA exhibits a more pronounced promotional effect on the absorption, assimilation, and utilization of nitrogen in non-nodulating soybeans under low-nitrogen stress.