The influence of relative sun exposure on sunscald susceptibility, peel metabolism, and storage outcome of 'Granny Smith' apples

Manoella Cabral Mendoza

Although sunlight is crucial for optimal apple production, excessive sun exposure can negatively impact appearance and alter fruit quality and metabolism. ‘Granny Smith’ is notably prone to sunscald, a sun stress-related postharvest disorder that manifests as the superficial darkening of the peel, rendering the fruit unmarketable and causing economic losses for growers and packers. A lower chlorophyll-to-carotenoid ratio in ‘Granny Smith’ peel is associated with increased sunlight exposure and a higher likelihood of sunscald development. In this research, the chlorophyll-to-carotenoid ratio was used to classify apples according to their relative sun exposure, as determined by sunscald susceptibility categories: high, moderate, low, and very low.The spatial distribution of apples within the tree canopy was recorded over two growing seasons to elucidate the relationship between relative sun exposure and sunscald risk. Nondestructive assessments of the chlorophyll to carotenoid ratio exhibited a strong correlation with peel pigment content, particularly stronger during the year when the exposed fruit was subjected to elevated photooxidative stress. As anticipated, apples with higher relative sun exposure demonstrated greater propensity for sunscald development. Notably, minor fluctuations in relative sun exposure led to divergent sunscald risk, even within neighboring apples, highlighting the critical role of specific canopy position in determining sunscald susceptibility. To investigate the influence of relative sun exposure on fruit quality and ripening, ‘Granny Smith’ apples were harvested from three orchards over three consecutive years and subsequently stored in air for up to six months before quality analyses. During the final year, additional assessments were conducted at 4 and 5 months of storage to monitor ripening progression and sunscald incidence. Apples with higher sun exposure exhibited elevated soluble solids, reduced acidity, and greater ripening variability than those with lower exposure. Ripening was more uniform among fruits in the low and very low sunscald risk categories. The incidence of sunscald increased overtime, resulting in a total loss of 11% of the fruit after six months of storage, potentially leading to a 21% decrease in revenue. Metabolomic profiling of ‘Granny Smith’ apple peel by relative sun exposure revealed significant shifts in peel metabolism. Apples with higher relative sun exposure exhibited lower levels of chlorophyll and accumulation of photoprotective compounds, such as flavonol glycosides and xanthophylls. Biosynthesis of membrane structural components (stigmasterol, β-sitosteryl linoleate, and β-sitosteryl linolenate), cuticle metabolites (p-coumaryl eicosonoate and docosonoate), and stress signaling compounds (Ceramide, DAG) were also elevated with increased sun exposure. These metabolic adaptations reflect coordinated defense mechanisms against photooxidative stress. Although this approach has certain limitations, non-destructive evaluation of the chlorophyll-to-carotenoid ratio at harvest offers significant benefits for the apple industry. The strong correlation between this ratio and relative sun exposure provides a data-driven means of assessing sunscald risk prior to symptom expression, while also serving as an indicator of the fruit’s metabolic state and potential postharvest performance. Sorting strategies based on relative sun exposure enable tailored cold chain management, aligning storage conditions with fruit susceptibility to sunscald and expected ripening behavior. This targeted approach can improve quality consistency and reduce economic losses for growers and packers.

The influence of relative sun exposure on sunscald susceptibility, peel metabolism, and storage outcome of 'Granny Smith' apples

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