LOW-TEMPERATURE SWEETENING IN POTATO-TUBERS - THE ROLE OF THE AMYLOPLAST MEMBRANE

Properties of the amyloplast membrane were examined for potato cultiva rs susceptible (Norchip) and resistant (ND860-2) to low temperature sw eetening following storage for 4 months at either 12 degrees C or 4 de grees C. Chips from Norchip stored at 4 degrees C were darker and the tissue contained higher levels of glucose, fructose and sucrose as com pared to ND860-2 tubers stored at 12 degrees C and 4 degrees C and Nor chip tubers at 12 degrees C. Membrane lipid phase transitions of intac t amyloplasts assessed using differential scanning calorimetry were hi gher for Norchip than ND860-2 tubers after storage at both 12 degrees C and 4 degrees C (45 vs. 38 degrees C and 35 vs. 27 degrees C, respec tively). Fluorescence depolarization measurements using the probes 1,6 -diphenyl-1,3,5-hexatriene (DPH) and trans-parinaric acid (t-PA) indic ated that the structural order parameters of Norchip amyloplasts membr anes at 4 degrees C were higher than at 12 degrees C (0.655 vs. 0.459 with DPH and 0.658 vs. 0.531 with t-PA) while the opposite was true fo r ND860-2 (0.402 vs. 0.549 with DPH and 0.481 vs. 0.566 with t-PA). Sm all differences in the SDS-PAGE protein patterns of amyloplasts were o bserved between Norchip and ND 860-2. Degradation of amyloplast unsatu rated fatty acids was greater for Norchip membranes (93% decrease in d ouble bond index) than for ND860-2 (70% decrease in DBI). The above re sults suggested that amyloplast membranes from Norchip potatoes senesc ed to a greater extent than ND860-2 amyloplast membranes upon exposure to low temperature and that this event may be associated with low-tem perature sweetening.