METHODS OF FRUIT TISSUE SAMPLING TO QUANTIFY CALCIUM AND MAGNESIUM CONTENTS TO SEGREGATE APPLES FOR BITTER PIT INCIDENCE

Abstract

Susceptibility to bitter pit (BP) has been associated with low Ca content and high Mg/Ca ratio in the fruit, which is highly affected by fruit genotype. This study was carried out to identify the best fruit tissue sampling method to assess Ca and Mg contents to segregate apples for BP occurrence. ‘Fuji’ and ‘Catarina’ apples (with low and high susceptibilities to BP, respectively) were segregated in lots without and with BP symptoms, after four months of cold storage (0±0.5°C/95% RH), and then analyzed for Ca and Mg contents by three fruit tissue sampling methods: peel+flesh, peel, and flesh. For peel+flesh, a wedge-shaped segment was cut longitudinally from the fruit, discarding the core tissue. The fruit calyx-end was used for peel (thickness of 2 mm) and flesh (thickness of 2-8 mm) sampling. For peel+flesh and flesh, Ca content was lower and Mg/Ca ratio was higher in fruit with BP in both cultivars. For peel, only in ‘Fuji’, Ca content was lower and Mg/Ca ratio was higher in fruit with BP. In ‘Catarina’, Mg/Ca ratio in the flesh tissue at the calyx-end increased from 1.83 in fruit without BP to 4.56 in fruit with BP (~2.5-fold increase), while in ‘Fuji’ this ratio increased from 0.96 in fruit without BP to 1.29 in fruit with BP (~1.3-fold increase). Therefore, the high susceptibility to BP in ‘Catarina’ might be explained by its high Mg/Ca ratio in the flesh tissue under the peel, at the calyx-end of the fruit, were the symptoms appeared. Nonetheless, the results show that the Mg/Ca ratio in the flesh at the calyx-end is more suitable to segregate apple fruit for BP susceptibility in both cultivars.