Nitrendipine-Treatment Increases Cork Spot Disorder Incidence in Pear ‘Akituki’ (Pyrus pyrifolia Nakai.) by Altering Calcium Distribution Inside the Fruit

‘Akituki’ (Pyrus pyrifolia Nakai.) is a very popular and profitable pear cultivar in China. However, its high susceptibility to cork spot disorder has limited its expansion of cultivated area. The mechanisms of cork spot disorder have been discussed extensively, focusing on Ca(2+) deficiency, yet no consensus has been made. In this study, we applied nitrendipine (NI) as a Ca(2+) uptake inhibitor to explore the role of calcium in cork spot disorder occurrence. Results showed that NI treatment on the fruit remarkably increased the incidence of cork spot disorder; alteration of mineral contents happened at the early developmental stage of the fruit, especially on the outer flesh and the peel of the fruit; and this gap was filled gradually along with the expansion of the fruit. Significant differences in the expression levels of Ca(2+) transport-related genes were found in the inner flesh, outer flesh and peel during the fruit growth period. The observation of free Ca(2+) localization indicated the intracellular imbalance of Ca(2+) in the NI-treated fruit. In conclusion, NI treatment reduced the calcium content in the fruit at an early developmental stage, altered the related expression of genes and influenced the cellular Ca(2+) balance in the fruit, which prompted the occurrence of cork spot disorder. Measures for the prevention and control of cork spot disorder should be taken at the early stage of the fruit development in the field.