Effects of γ-aminobutyric acid on carbon and nitrogen metabolism in leaves of Cabernet Gernischt grape

【Objective】Carbon metabolism and nitrogen metabolism are the most basic and important physiological processes in plant. The ability of carbon and nitrogen metabolism can directly affect the quality and yield of crops. It has been shown that regulating carbon and nitrogen metabolism is an important measure to improve plant yield and quality. The aim of this study was to investigate the effect of γ-aminobutyric acid (GABA) on carbon and nitrogen metabolism in grape and to screen the optimal regulation concentration.【Methods】Using 10-year-old wine grape Cabernet Gernischt as test material, the effects of different concentrations of γ-aminobutyric acid (GABA) on the carbon and nitrogen metabolism and key enzyme activities in the leaves of grape were studied. A total of 4 GABA concentration treatments were set up: 5 mmol·L-1 (T1), 10 mmol·L-1 (T2), 15 mmol·L-1 (T3), 20 mmol·L-1 (T4), and distilled water treatment was used as the control (CK). The leaves of grape plants with the same growth and without diseases and pests were sprayed at the flowering stage, fruit setting stage, fruit expansion stage and fruit color conversion stage. Each treatment had 3 replicates and each replicate had 5 plants. The amount of spray was controlled by the appearance of initial drip from the leaves. The leaf sampling time was 8:00 am on the third day after treatment, and the leaves were sampled again at mature stage. After freezing in liquid nitrogen, the samples were placed in an ultra-low temperature refrigeratorat -80 ℃ for later use.【Results】Compared with CK, the exogenous GABA treatments increased the activities of sucrose synthase (SuSy), sucrose phosphate synthase (SPS), acid invertase (AI) and neutral invertase (NI) related to the carbon metabolism in the grape leaves, and increased the contents of starch, soluble sugar, fructose, glucose and sucrose. The GABA treatments also enhanced the activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT), and also increased the content of nitrate nitrogen (NO3 - ·N) and the endogenous GABA content. The exogenous GABA treatments significantly increased the contents of starch, soluble sugar, NO3 - ·N and the activities of SuSy- s, NR, GS, GDH, GOT and GPT in the leaves of Cabernet Gernischt during the growth period, and significantly increased the contents of fructose, glucose, sucrose, NH4 + ·N and the activities of SuSy-c, SPS, AI, NI and GOGAT, and also increased the content of endogenous GABA. Specifically, the endogenous GABA content in the grape leaves increased significantly at flowering stage, fruit setting stage, veraison stage and maturity stage after the exogenous 10 mmol·L-1 GABA treatment. The starch content in the leaves increased significantly in the growth period, the fructose content increased significantly at the fruit setting stage, the expanding stage and the mature stage, the glucose content increased significantly at the expanding stage and the turning stage, the sucrose content increased significantly at the expanding stage and the mature stage, and the total soluble sugar increased significantly from the flowering stage to the mature stage. The activity of carbon metabolism-related enzyme AI increased significantly at the expansion stage, the turning stage and the mature stage, and the activity of NI increased significantly at the fruit setting stage and the mature stage. At the same time, the GABA treatments significantly increased the SPS activity at the fruit setting stage, the expansion stage and the mature stage, respectively. The activity of SuSy- s increased significantly at the fruit setting stage, the turning stage and the mature stage, and the activity of SuSy-c increased significantly at the expansion stage, the turning stage and the mature stage. Similarly, after the exogenous GABA treatments, the content of NO3 - ·N in grape leaves increased significantly from flowering stage to maturity stage, and maintained a higher level of NO3 - · N at the late stage of grape growth compared with the control, while the content of NH4 + ·N had no significant difference with those of the CK except for that at maturity stage. The activity of GOGAT, GS and NR increased significantly from fruit setting stage to maturity stage, the activity of GDH increased significantly from flowering stage to maturity stage, and the activity of GOT and GPT increased significantly except for maturity stage.【Conclusion】During the growth period, the GABA treatments increased the carbon metabolism and nitrogen metabolism related substances and enzyme activities in the grape leaves. It was speculated that the exogenous GABA treatments increased the carbon and nitrogen metabolism activity of the grape leaves, and affected the carbon metabolism of the leaves as a nitrogen source, which strengthened the relationship between nitrogen metabolism and carbon metabolism to some extent. The exogenous GABA increased the endogenous GABA in the grape leaves, thereby enhanced the activities of enzymes related to carbon metabolism and nitrogen metabolism, increased the accumulation of starch and soluble sugar, and promoted the absorption of nitrate nitrogen (NO3 - · N) and the transformation of ammonium nitrogen (NH4 + · N). 10 mmol·L- 1 exogenous GABA had the best effect.