Effect of hot water treatments on chilling injury and heat damage in 'satsuma' mandarins: Antioxidant enzymes and vacuolar ATPase, and pyrophosphatase

Susceptibility of 'satsuma' mandarin fruit (Citrus unshiu Marc) to chilling injury limits the marketability of stored fruit. In this study, the effect of hot water dip (HWD) treatments on chilling injury tolerance, fruit quality, activity of antioxidant enzymes, and gene expression of proton transporters in the vacuole were investigated in New Zealand. Some treatments suppressed chilling injury development, but none of them could prevent it completely. The major reduction in chilling injury incidence (proportion of unacceptable fruit) and severity (level of damage) was observed in HWD at 50°C, for 2min. Temperatures higher than 50°C increased fruit peel damage. Chilling injury was paralleled by higher ethylene production and respiration rate in affected fruit, and an increase in the amount of ethanol and acetaldehyde emitted in juice headspace. Afterwards, values decreased gradually with increasing chilling damage. Treatments that reduced chilling injury suppressed anaerobic products, ethylene evolution and respiration. During storage at 2°C for 8 weeks, a decline in catalase (CAT) activity was observed, while peroxidase (POX) activity increased. This rapid increase in POX activity was associated with increased peel damage due to both chilling injury and heat damage. Decreasing chilling injury in HWD was correlated with decreased POX activity and maintenance of CAT activity during storage. Amounts of vacuolar pyrophosphatase (V-PPase) and vacuolar ATPase (V-ATPase) gene transcript in fruit peel were reduced by heat treatment. Storage periods reduced V-PPase and induced V-ATPase gene transcript level in fruit treated at 50°C, but this was not associated with changes in the acid content in the juice. A significant induction (5-6-fold) of ATPase and V-PPase occurred in juice sacs and this may be associated with compensation for a loss of tonoplast integrity in fruit treated at 55°C compared to those at 50°C. We also found that V-ATPase and V-PPase transcript levels in juice sacs responded rapidly to heat treatments but were largely recovered after 8 weeks cold storage for successful (non-damaging) heat treatments. The optimal HWD treatments were 47.5-50°C for 2min.