The Effect Of Calcium On Fruit Creasing Of ‘Hongjiang’ Orange

Creasing in ‘Hongjiang’ orange is a serious preharvest physiological disorder which is manifested as separations of cells at the middle lamella of the albedo tissue,resulting in some creases on the surface of the fruit or even cracking, and causes serious economic loss in citrus fruit industry. ‘Hongjiang’ orange is prevail in south China, but its creasing at the stages of growth reduces the yield and fruit quality. Hence, Studying the mechanism of occurrence of citrus fruits creasing and developing agricultural managements that reduce incidence of creasing are very important.The 8-year-old ‘Hongjiang’ orange in 2014 and the 3-year-old ‘Hongjiang’ orange in2015 were used as experimental material with five different spraying treatments: 0.5%Ca(NO3)2, 2.5 mmol/L La Cl3, 5 mmol/L EGTA, 0.5 mmol/L TFP and water(CK), during the fruits expanding period. This study was to research the effect of calcium on the incidence of citrus fruit creasing, mechanical property of peel, the content of mineral element in peel and the activity of cell wall metabolic enzyme, and explore the molecular mechanism of the occurrence of fruit creasing. The main results are as follows:(1) The effect of different treatments on citrus fruit creasing incidence were analyzed.The results showed that the incidence of creasing fruits under the Ca(NO3)2 application was the lowest(7.01% and 4.89%) and significantly lower than the control in 2015. The highest creasing fruit rate was obtained with La Cl3 treatment and the followed was EGTA treatment.No significant difference of creasing rate was found between TFP treatment and control in2005.(2) The thickness was affected by the position of fruit on the tree and by different applications. The peel thickness of exposed fruits was higher than that of shaded fruits significantly in both 2014 and 2015, while the peel hardness was not different between shaded fruits and exposed fruits. Compared with the control, Ca(NO3)2 treatment increasedthe peel thickness significantly, but it didn’t enhance the peel hardness.(3) The content of Ca and Mg in normal fruit peel was higher than that in creasing fruit.To the contrary, K content was higher in creasing fruit peel. Ca and Mg content were significantly higher in the exposed fruit peel than in the shaded fruit on August and October2014. K content in the exposed fruit was lower than in the shaded fruit significantly.Ca(NO3)2 application increased the Ca content and reduced the K content in the peel especially in later development stage, compared with the control. But it didn’t affect Mg content.(4) Spraying ‘Hongjiang’ orange with Ca(NO3)2 effectively reduced the activity of cell wall metabolism enzyme(PG, PE and Cx). But the inhibitor of calcium(EGTA and La Cl3)increased the the activity of cell wall metabolism enzyme(PG, PE and Cx), resulting in the hydrolysis of cell wall component.(5) The expression of PG, Cx and PE in the normal fruit peel was significantly lower than that in the creasing fruit peel. Meanwhile PG, Cx and PE expression were higher in shaded fruit peel than that in exposed fruits, and significant difference was found respectively on August, October and November 2014. Ca(NO3)2 treatment reduced the expression of PG, Cx and PE, but EGTA treatment was the contrary.(6) Ca M and EXP expressions had no significant difference between normal fruit and creasing fruit, exposed fruit and shaded fruit.In conclusion, the exposed ‘Hongjiang’ orange fruit was less susceptible to creasing occurrence because of the higher calcium content, thicker peel, lower activity of cell wall metabolism enzyme and lower expression of PG, Cx and PE, compared with shaded fruits.Foliar spraying Ca(NO3)2 can effectively increase the calcium content, enhance the peel thickness, inhibit the activity of cell wall metabolism enzyme and decrease the expressions of PG, Cx and PE to reduce the incidence of fruit creasing in citrus.