In-season heat stress compromises postharvest quality and low-temperature sweetening resistance in potato (Solanum tuberosum L.)

High soil temperature during bulking and maturation of potatoes alters postharvest carbohydrate metabolism to attenuate genotypic resistance to cold-induced sweetening and accelerate loss of process quality. The effects of soil temperature during tuber development on physiological processes affecting retention of postharvest quality in low-temperature sweetening (LTS) resistant and susceptible potato cultivars were investigated. 'Premier Russet' (LTS resistant), AO02183-2 (LTS resistant) and 'Ranger Russet' (LTS susceptible) tubers were grown at 16 (ambient), 23 and 29 A degrees C during bulking (111-164 DAP) and maturation (151-180 DAP). Bulking at 29 A degrees C virtually eliminated yield despite vigorous vine growth. Tuber specific gravity decreased as soil temperature increased during bulking, but was not affected by temperature during maturation. Bulking at 23 A degrees C and maturation at 29 A degrees C induced higher reducing sugar levels in the proximal (basal) ends of tubers, resulting in non-uniform fry color at harvest, and abolished the LTS-resistant phenotype of 'Premier Russet' tubers. AO02183-2 tubers were more tolerant of heat for retention of LTS resistance. Higher bulking and maturation temperatures also accelerated LTS and loss of process quality of 'Ranger Russet' tubers, consistent with increased invertase and lower invertase inhibitor activities. During LTS, tuber respiration fell rapidly to a minimum as temperature decreased from 9 to 4 A degrees C, followed by an increase to a maximum as tubers acclimated to 4 A degrees C; respiration then declined over the remaining storage period. The magnitude of this cold-induced acclimation response correlated directly with the extent of buildup in sugars over the 24-day LTS period and thus reflected the effects of in-season heat stress on propensity of tubers to sweeten and lose process quality at 4 A degrees C. While morphologically indistinguishable from control tubers, tubers grown at elevated temperature had different basal metabolic (respiration) rates at harvest and during cold acclimation, reduced dormancy during storage, greater increases in sucrose and reducing sugars and associated loss of process quality during LTS, and reduced ability to improve process quality through reconditioning. Breeding for retention of postharvest quality and LTS resistance should consider strategies for incorporating more robust tolerance to in-season heat stress.