Post bloom spraying apple trees with prohexadione-calcium and gibberellic acid affects vegetative growth, fruit mineral content and bitter pit incidence

Post bloom spraying of apple orchards with prohexadione-calcium (ProCa, an inhibitor of gibberellins synthesis) might reduce vegetative growth and therefore increase calcium (Ca) transport into the fruit. On the other hand, post bloom spraying with gibberellins (GAs) is used commercially on apple orchards to improve fruit skin finish, but this might promote fruit growth, increase vegetative growth and reduce Ca content in the fruit. Bitter pit (BP) is a physiological disorder caused by Ca deficiency in apples, which is also associated with increased activity of GAs in the plant. This study was carried out to assess the effects of post bloom orchard spraying with ProCa and gibberellic acid (GA(3)) on vegetative growth, fruit mineral content and BP incidence. The experiment was carried out in Southern Brazil. 'Catarina' and 'Fuji' apple trees were sprayed with water (control), ProCa (319 mg a.i. L‑1) or GA(3) (319 mg a.i. L‑1) at the petal fall stage (when shoots were 5-10 cm long), with reapplication 20 days later. During the trial, trees of both cultivars were not sprayed with calcium chloride in all treatments (to encourage the occurrence of BP). In both cultivars, yield and average fruit weight at harvest was not different among treatments, while vegetative growth was reduced by ProCa and increased by GA3 in comparison to the control. In fruit of both cultivars harvested at the commercial maturity and then left in cold storage (0±0.5°C/90-95% RH) for four months, followed by five days of shelf life (20±4°C/60-70% RH), ProCa reduced while GA3 increased the occurrence of BP in comparison to the control. In both cultivars, fruit from trees treated with ProCa had higher content of Ca and lower (K+Mg+N)/Ca ratio in the peel tissue at the distal end than those from trees treated with GA(3). The results show that post bloom spraying with GA(3) increases the risk of BP. On the other hand, the inhibition of active GAs synthesis by post bloom spraying with ProCa reduces vegetative growth, reduces the contents of K, Mg and N while increases the content of Ca in the fruit, and therefore reduces the risk of BP.