Difference in calcium accumulation in the fruit of two apple varieties and its relationship with vascular bundle development in the pedicel.

Calcium (Ca) is an essential mineral element for plant growth and development that plays a key role in fruit growth and quality formation. To study the absorption and transport of Ca in 'Tonami' (susceptible to bitter pit (BP)) and 'Fuji' (resistant to BP), fruiting branches of 'Tonami' and 'Fuji' were injected with 0.05% 44Ca at the fruitlet stage (37 days after full bloom (DAFB)) and fruit expansion stage (72 DAFB). At the fruitlet and fruit expansion stages, the 44Ca content of fruiting branches increased from high to low in leaves, shoots, and fruit. In fruit, the 44Ca content was highest in the peel and lowest in the flesh. In both 'Tonami' and 'Fuji', Ca uptake was more efficient at the fruitlet stage than at the fruit expansion stage. 'Tonami' had a shorter growth and development time and earlier loss of fruit pedicel xylem structure and functionality than 'Fuji', resulting in less Ca accumulation in the fruit. The low Ca uptake efficiency of 'Tonami' fruit, the short Ca accumulation time, and the high Ca dilution resulted in low 44Ca content in the fruit, which may explain the susceptibility of 'Tonami' fruit to BP disease.