Changes in cytokinins and auxins levels in the rind of ‘Nules Clementine’ mandarin as related to the fruit position on the tree and the susceptibility to non-chilling rind breakdown disorder

The role of phytohormones in mitigating or enhancing the incidence of non-chilling rind physiological disorder of citrus fruit is largely unknown. Therefore, this study attempted to elucidate cytokinins (CK) and auxin changes in ‘Nules Clementine’ mandarin fruit rind from different canopy positions during postharvest non-chilling cold storage regarding rind breakdown disorder (RBD). Fruit from the inside canopy (IC) and outside canopy (OC) positions of citrus trees were harvested at commercial maturity from Eastern Cape (EC) and Western Cape (WC) provinces of South Africa, respectively. Endogenous levels of CK, auxins, their isomers, and conjugates were analysed using ultra-performance liquid chromatography coupled with a triple quadrupole mass spectrometer (MS/MS) equipped with an electrospray interface. Cis-zeatin-type cytokinins were significantly higher in the OC fruit than in the IC fruit from both provinces during cold storage. Whereas dihydrozeatin concentration (the most abundant CK) was significantly higher in fruit from IC (326.10 and 29.38 nmol kg⁻¹) than in OC (65.75 and 17.72 nmol kg⁻¹) fruit before cold storage, respectively. Fruit without rind disorder had lower CK - conjugates than fruit with the disorder. The IAA concentration was higher in the IC fruit (344.15 nmol kg⁻¹) than in the OC fruit (194.20 nmol kg⁻¹) from EC at week 0 while the IAA concentration of OC fruit from WC was below the limit of detection and IC fruit had 53.20 nmol kg⁻¹ at week 0. Fruit without the disorder had more IAA concentration than fruit with RBD while indole-3-acetylaspartic acid (IAAsp) was higher in fruit with RBD than fruit without the disorder, indicating the preventive role of IAA in the incidence of the disorder. This study is the first to record the negative effect of IAAsp in RBD and revealed the crucial role and synergistic role of CK, IAA and their isomers in RBD of ‘Nules Clementine’ fruit.