Mechanisms regulating fruit calcium content and susceptibility to bitter pit in cultivars of apple

Abstract

This work was carried out to investigate the mechanisms regulating fruit calcium (Ca) content and susceptibility to bitter pit (BP) in 'Fuji' and 'Catarina' apples (cultivars with low and high susceptibility to BP, respectively). Fruit of both cultivars were harvested at commercial maturity in an orchard located in southern Brazil, and assessed for xylem functionality and lenticels pore area (mm2 mm-2 of skin) at the distal end. Fruits were also cold stored for four months (0.0±0.5°C/95% RH) and then separated in lots without and with BP. Fruits of both cultivars without and with BP were assessed at the distal (blossom end), middle (equatorial region) and proximal (peduncle end) portions for pectin methylesterase (PME) activity in the skin and flesh tissues, and at distal portion for peroxidase (POD) and ascorbate peroxidase (APX) activities and Ca content in the flesh tissue. At harvest, 'Catarina' apples had lower xylem functionality and lower lenticels pore area at the distal end than 'Fuji' apples. 'Catarina' had higher incidence of BP after cold storage (12.7%) than 'Fuji' (2.3%). In both cultivars, fruit with BP had lower Ca content in the flesh at the distal end. Fruits with BP showed at the distal region higher POD activity in the flesh in 'Fuji' and higher PME activity in the skin in 'Catarina' than fruit without BP. Only in 'Fuji', fruits with BP had higher PME activity in the flesh tissue at the middle and distal portion than fruits without BP. The results show that impairment of xylem functionality, and reduced lenticels pore area and high PME activity in the skin tissue at the distal portion of the fruit might reduce fruit Ca content and increase the incidence of BP, especially in 'Catarina', a cultivar highly susceptible to the disorder.