Research paperExogenous GABA application improves the NO3−-N absorption and assimilation in Ca(NO3)2-treated muskmelon seedlings

- We examined plant growth, NO3−-N assimilation enzyme and transaminase activities, and endogenous GABA concentration in muskmelon seedlings.
- Foliar spraying with 50 mM of GABA regulates the NO3−-N absorption and assimilation and alleviates Ca(NO3)2-induced growth inhibition.
- Exogenous GABA application improves salt tolerance of muskmelon.

The potential of exogenous γ-aminobutyric acid (GABA) application to improve the NO3−-N absorption and assimilation in Ca(NO3)2-treated muskmelon seedlings was investigated in 'Yipintianxia 208', a salt-sensitive melon cultivar grown in a deep-flow hydroponic system. Plants were treated under control or 80 mM Ca(NO3)2 stress conditions with or without foliar spraying 50 mM GABA. We found that under Ca(NO3)2 stress, the activities of nitrate reductase (NR), glutamate synthetase (GS), and glutamate amino-transferase (GOGAT) in muskmelon seedlings were significantly reduced, while the activities of glutamate dehydrogenase (GDH), glutamate oxaloacetate transaminase (GOT), and glutamate pyruvate aminotransferase (GPT) were enhanced. These changes lead to a decrease in the content of NO3−-N and an increase in that of NH4+-N in muskmelon roots and leaves, which further severely inhibited the plant growth. Exogenous GABA application effectively improved the absorption of NO3−-N in muskmelon roots and leaves under Ca(NO3)2 stress and the NH4+-N assimilation by enhancing the activities of nitrogen assimilation enzymes (NR, GS, and GOGAT) in muskmelon seedlings. Exogenous GABA also reduced the NH4+ release by limiting the GDH deamination, thus further alleviating the ammonia toxication induced by Ca(NO3)2 stress. Our results suggest that exogenous GABA applications can relieve the nitrogen metabolic disorders caused by Ca(NO3)2 stress and eventually promote plant growth. In addition, Exogenous GABA regulated the transaminase (GOT and GPT) activities, maintaining the balance of the amino acid metabolism. Furthermore, the accumulation of the functioning as a nitrogen source endogenous GABA was promoted, eventually improving the resistance of muskmelon seedlings to Ca(NO3)2 stress.