| Citrus, as an evergreen fruit trees, is mainly distributed in over130countries and has become the largest fruit as well as one of the three major trading agricultural products, and also is one of the highest economic and nutritional value fruit in the world. In China, most of the citrus were cultivated in the subtropical monsoon areas, where the rainfall is uneven distributed and citrus orchard located in hilly to avoid the land utilization conflict with grain, oil and cotton crops, so citrus crops have to suffer from the shortage of water source and poor soil preserving capacity of water and nutrient. All of those ultimately results in a serious problems to citrus plantation-seasonal drought. Fruit quality is the crucial factor to citrus’economic and nutritional value. So it has great theoretical and practical significance to study the influence of drought stress to fruit quality and related metabolism regulator. In this paper, Yamasitaka [Citrus unshiu Marc] were used as materials to study the effects of drought stress on the content of chlorophyll and starch in leaf, fruit quality and gene expression related to citrate-metabolism in fruit. The main results are as follows:1. Drought stress significantly decreased the contents of chlorophylls, carotenoids and starch in leaves. It is indicated that drought stress effected photosynthesis and photosynthate accumulation.2. When under condition of40%drought stress, the growth of citrus fruits was inhibited obviously. Drought stress significantly decreased the contents of ascorbic acid content and juice recovery in fruit, causing significant decrease in fruit size and weight. But, the content of fruit soluble solids increased.3. Quality analysis indicated that sucrose and citric acid were the main sugar and organic acid type in fruits. From July15to November15, as water stress extended, concentrations of main sugars and citric acid within per gram sarcocarp as well as per-fruit content of citric acid increased, but per-fruit content of sugars was reduced, whcih resulted in the decline of sugar to acid ratio and fruit quality.4. Furthermore, compared with control, quality of the fruits under drought stress during the early stage, interim stage, late stage, early plus interim stage and whole stage of its fruit development were analyzed. The result showed that all of those drought stress treatments caused obvious decrease in fruit size and weight, but the impact extent was different. It was showed that the longer the intensity of drought stress time was, the stronger the growth of fruits was inhibited. Within the early stage, interim stage and late stage treatments, which had the same intensity of drought stress, the fruits under drought stress of the interim stage were smallest and lightest, followed by the fruits under drought stress of the early stage, and yet the fruits under drought stress of the late stage were only slightly smaller than control. So, it mean that the fruits had a certain extent growth recovering after water recovery under drought stress of the early stage, but had little growth recovering after water recovery under drought stress of the interim stage. The influence of drought stress within the interim stage on fruit size and weight was minimum.5. The effect of drought stress on fruit quality in different phases showed that excepting drought stress in the early stage, all other drought stress treatments increased the content of fruit soluble solids. Meanwhile the longer time of drought stress, the more obvious of the increasing. It showed the same trends in concentrations of main sugars. While, compared with control, the per-fruit content of sugars was reduced of all treatments excepting drought stress in the late stage. It was found that drought stress in the late stage was conducive to sugar accumulation in the whole fruit, but drought stress in the other four stages was disadvantageous. Meanwhile, all the five drought stress treatments lead to a decline of sucrose proportion in fruits, which indicated that citrus fruit was more inclined to accumulate the simple sugars like fructose and glucose under drought stress.6. While to the content of organic acids, the result showed that, in all drought stress treatments, the concentrations and per-fruit content of total organic acid and citric acid in citrus fruit increased, especially the treatment of drought stress in the late stage, which had a highest concentrations and per-fruit content of total organic acid. So, all five drought stress treatments resulted in accumulation of citric acid as well as decline of sugar to acid ratio and fruit quality. Therefore,54days’ continuing drought stress in the late stage of fruit development significantly enhanced the sugar content within whole fruit, but also resulted in a significant increase of organic acids. The sugar to acid ratio of fruit decline. So appropriately reducing the intensity or times of drought stress in the late stage of fruit development may promote the fruit quality in production.7. The relative gene expression of citrate synthase (CS), isocitrate dehydrogenase (IDH) and aconitase (ACO) which were closely involved in citrate-metabolism showed much difference. The expression level of CS in fruit was clearly increased while the expression level of IDH down-regμLated under water stress. The expression level of ACO was increased at the late stage of fruit development. The up-regμLation of CS gene and down regμLation of IDH gene might be one of the reasons of citric acid accumulation.8. The results of gene expression under drought stress in different phases indicated that the relative gene expression levels of CitCS1and CitCS2, the two members of CS gene family, in five drought treatments increase to varying degrees when stress occurs. However, the relative gene expression levels of CitIDH and CitlDH1downregulated in the peak time of citric acid accumulation near the time of81days after treatment, while CitNADPIDH downregulated in the peak time of citric acid degradation near the time of108or135days after treatment, which might be one of the reasons of citric acid accumulation. But the expression level of CitIDH, CitIDH1and CitNADPIDH in fruit was increased as a whole compare with control. Whether that would strengthen the activity of isocitrate dehydrogenase or only as a response to citric acid accumulation, which required further research. |
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