Influence of fruit maturity on fruit quality and skin structural composition of ‘Picnic’ apples at harvest and during cold storage

Abstract

Fruit maturity highly influences the quality and storability of apples. Hence, this study investigated the influence of fruit maturity on the fruit quality and skin structural deformation of ‘Picnic’ apples at harvest and during cold storage (0 °C) for 6 months. Apples were harvested based on the 1‒8 scales of Cornell’s starch pattern index (SPI) method at low (SPI-6), medium (SPI-7), and high (SPI-8) maturity stages, respectively. The SPI-8 fruit was the heaviest at harvest, with higher soluble solids content, a deeper red skin coloration, and lower firmness and titratable acidity (TA) compared to other fruit. However, during postharvest storage, the SPI-8 fruit ripened faster than the SPI-6 and -7 fruit, resulting in reduced firmness, TA, weight, and red coloration. The micro-cracks that appeared at harvest were increasingly enlarged during storage, particularly in the skins of SPI-8 fruit. However, the lenticels and opened lenticel density were not noticeably different among SPI treatments. The cuticle thickness and morphological properties of epidermal cells steadily increased during storage in all the fruit maturity stages. The cuticular wax-associated genes such as MdCER1, MdCER6, and MdLACS1 were up-regulated, while MdCER4, MdWSD1, and MdMAH1 were down-regulated, particularly in the SPI-8 fruit compared to that of SPI-6 and -7 fruit. Overall, these results suggest that the ‘Picnic’ apples harvested at higher maturity stages improve fruit size, pigment coloration, and sugar levels, but reduce firmness and acidity, leading to greater loss in fruit quality and skin structural integrity during postharvest storage.