Effects of exogenous γ-aminobutyric acid (GABA) on nitrogen metabolism of oriental melon seedlings under elevated rhizosphere CO2

Rhizosphere CO2 concentration is affected by multiple factors in agricultural production. When it exceeds a defined threshold, the growth of oriental melon seedlings will be suppressed, giving rise to metabolic dysfunctions that undermine the yield and quality of oriental melons. Hence, it is indispensable to identify efficacious mitigation approaches to cultivating high-quality oriental melons in controlled settings. Recent investigations have manifested that exogenous gamma-aminobutyric acid (GABA) plays a crucial role in mediating responses to abiotic stress; however, the mechanisms which through GABA regulates nitrogen metabolism remain elusive under elevated rhizosphere CO2 concentration. This study adopted an automatic rhizosphere CO2 treatment system, with normal atmospheric CO2 concentration of 0.037% as the control, rhizosphere CO2 concentration of 1% as the stress treatment (T), and exogenous spraying of 50 mmol/L GABA as the mitigation treatment (TG) under CO2 stress conditions. The results showed that the elevated rhizosphere CO2 concentration significantly inhibited the nitrogen metabolism of oriental melon seedlings. The N-15 tracer technology verified that foliar spray of 50 mmol/L GABA greatly improved the nitrogen uptake and distribution capacity of oriental melon seedlings under the condition of elevated rhizosphere CO2 concentration. In addition, 50 mmol/L exogenous GABA significantly increased the nitrogen assimilation utilization and nitrogen metabolism-related enzyme activities of oriental melon seedlings under elevated rhizosphere CO2 concentration, and significantly up-regulated the expression levels of key enzyme genes and nitrogen transporter genes of nitrogen metabolism. In conclusion, when oriental melon seedlings were subjected to high CO2 stress in rhizosphere, the nitrogen metabolism inhibited by elevated rhizosphere CO2 concentration could be alleviated by exogenous application of 50 mmol/L GABA.