Exogenous GA₃ alleviates low nitrogen stress by enhancing leaf nitrogen assimilation and limiting the transport of photosynthates to roots in apple plants

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Gibberellins (GAs) are crucial for improving plant adaptation to abiotic stresses; however, their physiological functions in apple plants under low-nitrogen (N) stress remain unclear. In this work, a hydroponic approach was used to explore the effects of exogenous GA₃ on carbon (C)-N metabolism and allocation in apple seedlings under low-N stress. The results indicated that the application of exogenous GA₃ (50 mg·L⁻¹) enhanced the concentration of endogenous GA₃ in the leaves, alleviated the inhibition of photosynthesis and leaf growth caused by low-N stress, and significantly increased plant biomass (21%). In terms of N metabolism, exogenous GA₃ enhanced the transcription levels of nitrate transporters and increased leaf N-assimilating enzyme activities, leading to an increase in nitrate and soluble protein content. Additionally, exogenous GA₃ significantly increased the distribution ratio of ¹⁵N in the leaves and the expression of MdNRT1.5 in the roots, indicating that more N was utilized in the leaves. Regarding C metabolism, exogenous GA₃ improved the utilization of photosynthetic products by leaves under low N stress. Specifically, GA₃ application enhanced the activities of sorbitol dehydrogenase and sucrose synthase in the leaves, while reducing the expression of sugar unloading-related genes in the roots, leading to significant increases in the ¹³C distribution ratio and energy charge (EC) in the leaves. In conclusion, our study demonstrated that exogenous GA₃ promotes the recovery of leaf growth and function under low-N stress by enhancing the allocation and utilization of C and N in the leaves.