CARBOHYDRATE-METABOLISM AND POSSIBLE MECHANISMS OF LEAF BLACKENING INPROTEA-NERIIFOLIA UNDER DARK POSTHARVEST CONDITIONS

During a 7-day dark postharvest period, Protea neriifolia R.Br. leaf b lackening was significantly reduced on floral stems treated with a 24- h 20% sucrose pulse compared with continuous holding in a 0.5% sucrose vase solution or removal of the flowerhead. Leaf blackening on vegeta tive stems was similar to that on the 20% sucrose-pulsed floral stems. Leaf starch and sucrose concentration profiles demonstrated that stem s with reduced leaf blackening maintained higher levels of those carbo hydrates during the early postharvest period. Conversely, leaf starch and sucrose reserves were quickly depleted in stem treatments that res ulted in early blackening symptoms. Starch concentrations in all treat ments of stems dropped 70% to 82% within 24 h of harvest, suggesting t hat leaf blackening may be initiated during shipping. Ethylene product ion was not associated with leaf blackening in any treatment. Lipid pe roxidation did not differ among floral treatments nor did it increase over the postharvest interval. Oxidized glutathione (GSSG) concentrati on increased only with the 20% pulsed stems and was not related to lea f blackening. After an initial decrease, leaf respiration rate was gen erally maintained regardless of treatment. Collectively, these data ar e consistent with the hypothesis that carbohydrate depletion is the in itiating factor in leaf blackening and is accelerated by inflorescence sink demand. We suggest that membrane degradation does not necessaril y precede leaf blackening.