Changes in fruit skin δ Absorbance and heat stress response compounds in relation to sunburn browning severities of 'Buckeye Gala' apple

High temperature deteriorates apple tree performance and fruit growth. In the summer of 2021, heat waves affected the apple production zones in the Pacific Northwest of North America. In a 'Buckeye Gala' experimental trial in Summerland, British Columbia, Canada, heat caused fruit skin sunburn browning (SB), and affected fruit mass, dry matter content, soluble solid content, titratable acidity, firmness and flesh water potential. We studied fruit skin δAbsorbance at 670–720 nm (δA670–720), cuticle characteristics, and key stress response compounds in peel and flesh, to elucidate the SB mechanism. SB was associated with thickened cuticle, chloroplast degradation, decreased anthocyanin content and increased firmness on the affected sunlit side of the fruit. As sunburn damage intensified, anthocyanin content in peel decreased, while the concentration of tartaric ester and total phenolics in flesh increased. The δA670–720, measured using Delta Absorbance (DA) meter, was positively correlated with the total amount of phenolics and tartaric esters in peel and flesh. This suggested that the accumulation of the phenolics and tartaric ester triggered by heat stress contributed to the high δA670–720 despite the degradation/absence of chloroplasts. The study improved our understanding of the impacts of sunburn severities on fruit quality and of the heat stress responses in 'Buckeye Gala' apple, and demonstrated the potential use of DA meter as a rapid, non-destructive tool in facilitating heat stress ecophysiology study and predicting SB severity in red apple varieties.