The changes of intracellular calcium concentration and distribution in the hard end pear (Pyrus pyrifolia cv. ‘Whangkeumbae’) fruit

Hard end is a physiological disorder of pear fruit that is frequently observed in the 'Whangkeumbae' (Pyrus pyrifolia) variety, however, the mechanisms that are involved in its development are poorly understood. In this study, we explored the causes of hard end disorder in pear fruit in relation to calcium deficiency. During fruit development, the ratio of Ca/N, Ca/K, Ca/Mg and the content of B were significantly lower in the hard end fruit as compared to normal fruit. However, no calcium deficiency was detected in the soil and leaves of the orchard where the hard end fruit were located. Additionally, the Ca2+ influx in the calyx of hard end fruit was lower than that of normal fruit at 90 d after anthesis. The free Ca2+ and storage Ca2+ in the flesh cells of hard end fruit were less than that of normal fruit during fruit development, while an opposite tendency was observed at 120 d after anthesis (harvest day). In hard end fruit, the Ca2+ transport-related gene, PpCNGC1 (Cyclic nucleotide-gated ion channel 1), was up-regulated; whereas the Ca2+ sensor-related genes of PpCIPKs, PpCDPK28 and PpCML41 were all down-regulated. Spraying with a 2% calcium chloride (CaCl2) solution inhibited the incidence rate of hard end disorder and decreased fruit firmness and lignin content during storage. Additionally, the ratio of Ca/N, Ca/K, Ca/Mg and the content of B all increased on harvest day. Our study suggests that low Ca2+ influx leads to less Ca2+ into the pear fruit, which results in an intracellular imbalance of Ca2+ and consequently triggers the development of hard end disorder.