Effect Of Exogenous Aminobutyric Acid On Nitrate Metabolism In Leaf Vegetables

Vegetables are rich in nutrients and play an important role in people’s life. In recent years, as the areaof protected vegetable cultivation is expanding, it promotes the agricultural development and enrichesmarket supplyment. However, there still exist many problems in vegetable production. Among them, theunscientific utilize of fertilizers, causes serious contamination of vegetables and affects the quality ofvegetables. These contaminated vegetables can threaten human health by the food chain. Leafyvegetables are one of the largest consumption of vegetable varieties, but they are very easy toaccumulate nitrates. Part of nitrates in the leaves of vegetable can be converted into nitrite, which is theimportant reason to many cancers. Lettuce (Lactuca sativa L.), rapeseed (Brassica campestris L.) andleek (Allium tuberosum R.) are the widely cultivated vegetables in the protected facility. Therefore, thestudy about how to reduce the nitrate content in their leaves and the method to improve their qualityhave important significance.GABA, as a four-carbon non-protein amino acids, plays an important role in essing blood pressure,resisting anxiety and strengthening the immune system. Since2009, GABA was approved by the Stateas a new source of food, which promote the development of GABA-rich beverages, tea and food owingto its unique functionality and broad scope of application. Studies have shown that human intaking of30-50mg GABA per day can play a very good role in health care. But few researches on whether leafyvegetables can be enriched GABA as well as GABA have an impact on reducing the nitrate content.Therefore, different experiments were conducted to investigate the effect of exogenous GABA onnitrogen metabolism using lettuce, rapeseed and leek as the materials. Based on experimental results,we propose the optimum applying time, concentration as well as method of exogenous GABA to reducethe nitrate contents in leafy vegetable. Meanwhile, the investigation can clarify the mechanism ofGABA on reducing the nitrogen metabolism and provide a theoretical basis for GABA apply in fieldproduction and producing leafy vegetables of lower nitrate content and higher GABA accumulation.The main results are as follows:1. The first purpose of this paper is to study the effects of applying different concentrations of GABAon the nutritional quality of lettuce and rapeseed by accurate control of the nutrient solution nitrogenconcentration (32mmol/L), The results showd that applying GABA improved the quality of the lettuceand rapeseed by decreasing the nitrate contents And decreasing nitrate reductase (NR) activity. Forlettuce, the effect of applying5mmol/L GABA concentration is the best, which the contents ofnitrate(NO-3-N) and nitrite (NO-2-N) reached lowest value but the ammonium (NH+4-N) content and activities of nitrate reductase (NR), nitrite reductase (NiR), glutamic acid decarboxylase (GAD) reachedthe maximum value on6d after treatments. For rapeseed, the effect of10mmol/L GABA concentrationis the best, the lowest value of NO--3-N and NO2-N concentration and highest value of NH+4-N contentand activities of NR, NiR GAD activities were found on12d after treatments.2. The second purpose of this paper is to investigate the effect of irrigating different concentrationsGABA to nitrate content in leaves of leek using substrate culture which containing nitrogen fertilizer11g/m2. The results showed that GABA irrigation could improve the nutrional quality of leek byincreasing NR activity and decreasing NO-3-N content. The effect of10mmol/L GABA concentration isbetter than other concentration, which manifested that the lowest value of NO--3-N and NO2-N contentand highest value of NH+4-N content and activities of NR, NiR and GAD activities were found on6dafter treatments.3. The third purpose of this paper is to investigate the effect of spraying different concentrationsGABA to nitrate content in leaves of lettuce, rapeseed and leek. The results showed that GABAspraying could improve the nutrional quality of lettuce, rapeseed and leek by increasing NR activity anddecreasing NO-3-N content. The effect of75mmol/L GABA concentration is better than otherconcentration, which manifested that the lowest value of NO-3-N and NO-2-N content and highest valueof NH+4-N content and activities of NR, NiR and GAD activities were found on6d after treatments.4. The fourth purpose of this paper is to investigate the effect of soaking seed by differentconcentrations GABA to nitrate content in leaves of lettuce. The results showed that soaking seed bydifferent concentrations GABA could improve the nutrional quality of lettuce. The effect of10mmol/LGABA concentration is better than other concentration, which manifested that the lowest value ofNO-3-N and NO-2-N content and highest value of NH+4-N content and activities of NR, NiR and GADactivities were found on6d after treatments.5. The results demonstrated that there existed significant difference for decreasing NO-3-N contentstreated with GABA method and for the different vegetable. But the spraying method was better thanadding in nutrient and substrate as well as soaking seed method. For lettuce, the effect of foliar sprayingwas better than soaking seeds method, while soaking seeds was batter than adding method. For rapeseedand leek, the effect of foliar spraying is superior to the effect of adding GABA in nutrient solution.Based on the above results, the method of foliar spray is the best method to improve the quality ofleafy vegetable by decreasing the NO-3-N contents.75mmol/L GABA. Furthermore, the appropriateconcentration of spraying GABA is75mmol/L and the appropriate spraying time is six days beforeharvest.