Rachis browning in grapes: temperature influence, ethylene and respiration rates, and evaluation techniques

Rachis browning is a key postharvest challenge affecting the visual quality and marketability of table grapes (Vitis vinifera L.), yet its physiological basis remains poorly understood, particularly across different cultivars. This study investigated the relationship between rachis browning, endogenous ethylene production, and respiration in two table grape varieties—‘Autumn Crisp’ and ‘Sweet Globe’—under cold (−0.5 °C) and ambient (20 °C) storage. Grapes were evaluated over 12 days for quality parameters, weight loss, gas exchange (CO₂ and ethylene), and rachis browning, assessed visually and through digital image analysis. Overall, storage at 0.5 °C effectively reduced browning and ethylene production compared to 20 °C. However, varietal differences were pronounced: ‘Sweet Globe’ exhibited higher rachis browning than ‘Autumn Crisp’ at 0.5 °C despite similar ethylene and respiration levels, indicating variety-specific susceptibility. At 20 °C, increased ethylene and respiration coincided with more severe browning, particularly in ‘Autumn Crisp’. A strong correlation was found between visual and image-based assessments, though discrepancies emerged under high-stress conditions. The results suggest that while ethylene and respiration are associated with rachis browning, they do not fully explain the varietal differences observed, emphasizing the need to consider cultivar-specific physiological responses. Furthermore, image-based evaluation offers a more consistent method for quantifying browning severity. This study advances our understanding of rachis browning physiology in grapes and highlights the importance of tailored postharvest strategies and objective assessment tools to extend shelf life and maintain visual quality.