Role of Ca2+ and calmodulin in on-tree oleocellosis tolerance of Newhall navel orange

Previous studies have shown that spraying calcium on the canopy can significantly reduce the incidence of on-tree oleocellosis (OTO). However, the OTO regulation with calcium spraying has not been reported clearly. Therefore, the physiologic mechanism of spraying Ca(NO3)2 and calmodulin (CaM) inhibitors (trifluoperazine-TFP) on the canopy of 8-year-old Newhall navel orange on Lichi16-6 trifoliata (P. trifoliate) including the ratio (RO) and degree of OTO (DO) have been studied under high summer temperature. The results showed that exogenous Ca(NO3)2 treatment significantly decreased DO value by increase in CaM content of leaves, peroxidase (POD) activity in leaves, and fruit peels. However, TFP treatment significantly decreased CaM content in leaves, SOD and CAT activity in fruit peels, while the POD activity in fruit peels significantly increased, and the formation of Ca(NO3)2-induced DO tolerance in citrus fruits was weakened by TFP treatment. Exogenous Ca(NO3)2 treatment increased stomatal conductance (Gs) and transpiration rate (Tr), and decreased the daily range of rind oil release pressure (△RORP) significantly. However, TFP treatment had no significant influence on transpiration rate (Tr) and △RORP. The results were consistent with the RO of different treatments. These results confirm that Ca2+ and CaM regulate DO value, and the RO of OTO was mainly related to the regulation of △RORP through water metabolism.