The Impact Of Plastic Film Mulch On The Accumulation Of Sugar And Acid And Their Related Enzymes In Citrus Fruit

Mulching cultivation is one of the most important practices for improving citrus quality. The experiment was performed on6-year-old Egan1#Ponkan (Citrus reticulata cv. Egan1#) trees grafted on Poncirus trifoliata. In the present study, we investigated the activity responses of sucrose and citric acid metabolizing related enzymes in fruits under film mulch, as well as sugar and acid accumulation. The objective of this study was to identify factors involved in increasing soluble carbohydrate and acid accumulation in citrus fruit in response to soil plastic film mulch during fruit development and ripening stage. The main results were as follows:1. Changes of soil temperature and water potential under plastic mulchThe soil temperature is significantly higher than the control due to the plastic mulch. Soil water potential under plastic-mulching conditions was significantly lower than the control after12days of mulch treatment, implying that osmotic adjustment would occur.2. The effect of plastic mulch on the contents of starch, soluble sugar, proline and net photosynthetic rate in citrus leavesDate from total soluble sugar in the leaves suggest that only at the24days after plastic mulch, total soluble sugar concentration was slightly higher in the mulching trees than that in the control. Net photosynthetic rate and content of starch had no significant differences between treatment and control.Proline concentration was higher in leaves at48days of plastic mulch and maintained this different until the late of maturity. Since proline is a kind of osmolytes, and its significant increase is an osmoregulation index under the drought stress. So we believed that osmotic adjustment did occur in the trees under plastic mulch.3. Changes of sugar accumulation and activities of sucrose-metabolizing enzymesIt was found that plastic film mulch did not change the total soluble sugar(TSS) accumulation trend in the segment membrane, which was increased as fruit developed, reached to the maximal at the early maturation and then decreased to the end. However, the total soluble sugar concentration in the segment membrane from mulched tree was significantly higher after the middle September than that from the control tree. In the juice sacs, the total soluble solution (TSS) and soluble sugars (sucrose, glucose and fructose) were increased by plastic film mulch and were obviously higher at about12days after mulch treatment than that of the control. Sucrose-metabolizing enzymes were analyzed during the mulch period in both segment membrane and juice sacs. In the segment membrane, activities of Acid invertase(AI) and Sucrose synthase(SS) of cleavage direction were increased significantly by mulch treatment whereas activities of other enzymes including Neutral invertase(NI), SS of synthetic direction and Sucrose phosphate synthase(SPS) did not change much by the mulch treatment as compared with those from the control. In the juice sacs, only the SS activity was affected obviously by the mulch treatment. Of them, the SS activity of cleavage direction from mulched trees was significantly lower than that from the control trees while the SS activity of synthetic direction from mulched trees was significantly higher than that from the control trees.Based on the present data, we could conclude that the increase of sugar accumulation in the juice sacs under mulch treatment attributes to the increase of sink strength through increasing SS activity of cleavage direction, AI activity in segment membrane and SS activity of synthetic direction (SS-SD) in the juice sacs, and decreasing SS activity of cleavage direction (SS-CD)in the juice sacs.4. Changes of acid accumulation and activities of citric acid metabolizing-related enzymesFruit acidity was significantly increased in the end of treatment by mulch treatment. The change trend of citric acid was not changed by mulched treatment. However, citric acid content was significantly increased in the late of mulch treatment by mulch treatment.Activities of enzymes directly related with citric acid synthesis and degradation were investigated either in this study. Phosphoenolpyruvate carboxylase (PEPC) and Citrate synthase(CS) were directly related with citric acid synthesis, and did not change obviously under plastic mulch treatment. Aconitase (Aco) and Isocitrate dehydrogenase (IDH) were directly related with citric acid degradation. It was interestingly found that plastic film mulch did not change obviously the mit-Aco activity but decreased the activities of cyt-Aco and cyt-IDH which were significantly lower than those in the control fruits after the end of September. Hence, the significant decrease of activities of cyt-Aco and cyt-IDH reduced the ability of citric acid utilization, which was assumed as the probable reason for the significant increase of fruit acidity under plastic film mulch.