| Chongqing acts as citrus favorable area, becoming an important center of citrus production and orange juice processing in China, and its industry scale and structure as well as marketability are undergoing upgrade and development. However, Chongqing is well known as "stove" in China, which is one of the high temperature and drought stresses areas, therefore, frequently take place in summer and autumn, which results in severe negative effects on the growth and physiology metabolism of citrus. Newhall navel orange is planted as one of the main varieties in Chongqing, which is characterized by optimal fruit shape, bright color, good quality etc., while, the high temperature stress in summer often leads to abnormal fruit dropping, fruit cracking, smaller fruit, fruit sunburn and tree malnutrition. Ca+as an important intracellular second messenger can regulate various physiological and biochemical processes through a variety of ways. A previous study has been demonstrated that a supply of exgenous calcium in summer by foliage spraying could help to eliminate injury of high temperature stress and to promote the development of fruit quality. In this study, the effects of calcium nitrate, EGTA (calcium chelator) and TFP (calmodulin antagonist) treatments on photosynthesis, photosynthetic pigment content, reactive oxygen species, antioxidant enzyme activities and calcium signal transduction correlated molecules contents etc. have been investigated, these results has contributed to preliminary elucidate the physiological and regulatory role of Ca2+in protecting citrus tree against summer high temperature, which provided effectively theoretical support on invalid photosynthesis and hyperthermal injury under summer high temperature stress in Chongqing and similar ecological conditions area.The main results were presented as following:1. During the period of high temperature, the photosynthesis was enhanced after Ca(NO3)2treatment due to the significant suppression of chlorophyll degradation and the obvious increase in stomatal conductance, intercellular CO2concentration and transpiration rate. After Ca(NO3)2treatment, the hydrogen peroxide and malondialdehyde accumulation were reduced, the superoxide anion generation rate was also delayed. compared to the control, POD and CAT activity in Newhall navel orange leaves treated with Ca(NO3)2were increased8.01%and17.18%, respectively, which might cooperate to prevent cell from damage by oxygen species, confering citrus trees with the increased resistence to summer high temperature stress. As expected, EGTA and TFP treatments had opposite physiological effects compared to Ca(NO3)2.2. According to our data, the contents of [Ca2+]i, CaM, IP3and cAMP of Yujincheng orange (C. sinensis Osbeck ’Yujincheng orange’) seedlings grafted on three rootstocks Citrus junos(C junos Sieb. ex Tanaka), Trifoliate (P. trifoliate (L.) Raf.) and Carrizo (P. trifoliate×C. sinensis Osbeck’Carrizo citrage’), respectively, were all increased to some extent after Ca(NO3)2treatment. The [Ca2+]i contents of Yujincheng orange/Citrus junos leaves were increased31.72%and30.98%compared to that on Trifoliate and on Carrizo, respectively. EGTA and TFP had opposite action as compared to Ca(NO3)2, leading to a decrease in the contents of [Ca2+]i. Our results indicated that citrus rootstock strikingly influenced calcium absorption and transportion, Yujincheng orange seedlings grafted on Citrus junos has good effect of calcium regulation. The regulatory ability of different citrus rootstrocks to resist summer high temperature stress was different, a reasonable choice for rootstock-scion combination to improve the role of resistant to high temperature stress, and these results provided a theoretical basis for regulating stress resistance of rootstock-scion interaction.3. This study found that, during the period of high temperature, IP3and cAMP contents of citrus leaves were significantly increased after treated with Ca(NO3)2, by which the release of intracellular and extracellular calcium stores was promoted. This subsequently increasd [Ca2+]i content, which induce CaM accumulation, finally forming calcium signal transduction pathways in response to high temperature through regulated corresponding physiological processes. Collectively, it showed that, the increased contents of calcium signaling molecules, such as [Ca2+]i, CaM, IP3and cAMP after Ca(NO3)2treatment improved the resistance agaist summer high temperature stress in citrus.4. Sprayed exogenous calcium, TSS and Vc contents of fruit were enchanced during summer high temperature period. At the mature stage, the ratio of TSS to titratable acids and Vc content of mature fruit treated by calcium were significantly higher than that of the CK treatment, and significantly reduced the rotten rate of the storage fruit, the citrus rotten rate was only4.9%after storage90days, but there was significantly inhibited effects of fruit quality after inhibitors EGTA and TFP treatments, and significantly increased the rotten rate of the storage fruit, the citrus rotten rate were up to62%and75%respectively after storage90days. indicating that sprayed exogenous calcium will help to improve the fruit quality during fruit enlargement period, and to improve the fruit storability. |
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