Effect Of Short-term Low Nitrogen Supply On The Biosynthesis Of AsA And AsA-GSH Recycling In Cucumber During Fruiting

Fresh fruits and vegetables are rich in AsA,and also are the main source for human to obtain AsA(Cu Cumis sativus.L).Cucumber is one of the most important vegetable crops,and it is also one of the most widely planted vegetables.The nitrogen,phosphorus,potassium fertilizer are demanded much,but facilities of nitrogen use efficiency is low in the current production.The excessive application of nitrogen can not significantly increase fruit yield,but fruit quality decreased.Therefore,we should ensure the normal growth of crops and its yield,and improve the nitrogen use efficiency.AsA content is an important index of the quality of cucumber fruits,the study of AsA bio-synthesis and regulation mechanism under the low nitrogen cultivation is an important subject.This experiment was carried out with greenhouse cucumber(Cucumis sativus L.9930)in nutrient solution.The plants were treated with three different levels of nitrogen supplies for 7 days when the female flowers opened.The three levels of experimental treatments were: control,12 mM;T1,0 m M,T2,3 mM.In the study,the antioxidant enzyme system activity of leaves and fruits,the AsA and soluble sugar contents and the expression of AsA bio-synthetic and recycling genes involve in pathway.The results are as follows:1.Short-term low nitrogen supply could activate the antioxidant system,antioxidant enzyme activity and antioxidant content of cucumber plants significantly.Compared with control,the SOD activity of T1 leaves was increased by 41.8% on the 5th day,the POD activity increased by 55% on the 3rd day,and the CAT activity of T1 and T2 leaves increased by 82.06% and 62.86% respectively.The contents of T-AsA and AsA increased significantly.The content of T-As A in fruits and leaves of T1 were higher than control about 69.54% and 22.4%,respectively.The content of AsA in fruits was significantly increased,the highest increasing was 30.2%.Moreover the content of AsA in leaves increased by 26.3%.At the same time the total soluble sugar content of fruits increased significantly and that of T1 increase by about 1 times on the 3rd day.The total sugar content in leaves increased by31.8% and 9.1% respectively,and it is significant compared with control.2.The expression level of genes involved in AsA biosynthesis pathways of cucumber fruits and leaves was regulated by short-term low nitrogen supply.The expression of GMPs,GPPs,GGP and PMI in the L-galactose pathway was up-regulated,and the expression levels of these genes in leaves were increased by 2-45 times.At the same time,the GalURs of D-galacturonic acid pathway and MIOXs of inositol pathway also had high expression level.Compared with control,the expression level of GalURs increased up to 3 times,and the expression level of MIOXs increased by 5 times.Additionally the expression levels of leaves were higher than control by 2-45 times.3.The expression level of genes involved in the AsA-GSH recycling of cucumber fruits and leaves was regulated by short-term low nitrogen supply.Compared with control,the expression level of AsA oxidative genes(AOs and APXs)was significantly up-regulated,and the expression of AOs infruits of T1 increased by 1-9 times.The expression level of AO-4 in fruit of T2 increased by 4 times.Furthermore the expression level of AOs inleaves increased significantly,and the expression of AO-4was up-regulated by 25 times in the first day.The expression levels of DHAR-1 and DHAR-2 in fruits were up-regulated by 2.5-fold and 3-fold,respectively.And the expression level of MDHARs increased by 0.5-3 times.The expression levels of DHARs in leaves of T1 and T2 increased significantly,and they were up-regulated by 15-fold and 6-fold,respectively.Meanwhile the level of MDHAR-3 in leaves increased by 8 times and it was high evidently compared with control.4.The enzyme activities of AsA-GSH cycling of cucumber fruits and leaves were regulated under short-term low nitrogen supply.Under the condition of low nitrogen supply,the APX activity of T1 fruit was higher than that of control,and increased by 51.9%.The activity of AO and APX in T1 leaves increased significantly,which increased by 34.6% and 50% compared with control,respectively.MDHAR and DHAR activity was significantly increased on the third day during treatment.And they increased by 57.7% and 50.2%,respectively.The GR activity in the fruit was significantly increased by 55% on the first day.But it increased by 44.37% in the leaves of treatment on 7th day.Overall,in our study we found that the antioxidant system was activated,the content of AsA and other antioxidant substances increased,the content of soluble sugar increased.Moreover the expression levels of genes involved in AsA biosynthesis pathways and AsA-GSH cycling were up-regulated,and enzyme activities of AsA-GSH cycling were significantly increased under short-term low nitrogen supply.Therefore,the genes and enzymes related to the AsA biosynthesis pathways and AsA-GSH cycling can respond to short-term low nitrogen supply so that they can regulate the regeneration and accumulation of AsA.