Exogenous GABA application improves the NO 3 − -N absorption and assimilation in Ca(NO 3 ) 2 -treated muskmelon seedlings

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Abstract The potential of exogenous γ-aminobutyric acid (GABA) application to improve the NO 3 − -N absorption and assimilation in Ca(NO 3 ) 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(NO 3 ) 2 stress conditions with or without foliar spraying 50 mM GABA. We found that under Ca(NO 3 ) 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 NO 3 − -N and an increase in that of NH 4 + -N in muskmelon roots and leaves, which further severely inhibited the plant growth. Exogenous GABA application effectively improved the absorption of NO 3 − -N in muskmelon roots and leaves under Ca(NO 3 ) 2 stress and the NH 4 + -N assimilation by enhancing the activities of nitrogen assimilation enzymes (NR, GS, and GOGAT) in muskmelon seedlings. Exogenous GABA also reduced the NH 4 + release by limiting the GDH deamination, thus further alleviating the ammonia toxication induced by Ca(NO 3 ) 2 stress. Our results suggest that exogenous GABA applications can relieve the nitrogen metabolic disorders caused by Ca(NO 3 ) 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(NO 3 ) 2 stress.