KINETIC-MODEL FOR CARBON PARTITIONING IN SOLANUM-TUBEROSUM TUBERS STORED AT 2-DEGREES-C AND THE MECHANISM FOR LOW-TEMPERATURE STRESS-INDUCED ACCUMULATION OF REDUCING SUGARS

Exposure to low but nonfreezing temperatures induces the breakdown of starch and the accumulation of sucrose, glucose and fructose in potato tubers, a complex phenomenon known as low-temperature sweetening (LTS ). A kinetic model for the degradation of starch to sucrose, fructose, glucose, hexose phosphates and carbon dioxide in TC-stored mature Sol anum tuberosum cv. Norchip (LTS-sensitive) and Solanum tuberosum seedl ing ND860-2 (LTS-tolerant) tubers is presented in this work. Analysis of sugar accumulation data in tubers grown in 1993 and 1994 showed no significant differences in the rates of conversion of starch to hexose phosphates and hexose phosphates to sucrose for both cultivars (P > 0 .05). The rate constant corresponding to invertase activity was 2.3 da y(-1) for Norchip tubers and 1.1 day(-1) for ND860-2 tubers grown in 1 993 (P I 0.05); however, no significant differences were observed in i nvertase activity for 1994-grown tubers (P > 0.05). The accumulation o f the reducing sugars fructose and glucose was found to be dependent o n the relative difference in rate constants corresponding to invertase activity and glycolytic/respiratory capacity. This difference was 3-4 fold greater for Norchip in 1993, and 4-6 fold greater for Norchip in 1994, than for ND860-2 (P less than or equal to 0.05). Results from t he analysis also suggest that the amount of available starch for degra dation was greater in Norchip tubers than ND860-2 tubers (P less than or equal to 0.05). Our analysis suggests that tubers with decreased in vertase activity coupled to increased glycolytic/respiratory capacity should be more tolerant to low-temperature stress.