REEXAMINING POLYPHENOL OXIDASE, PEROXIDASE, AND LEAF-BLACKENING ACTIVITY IN PROTEA

Premature leaf blackening in Protea severely reduces vase life and mar ket value. The current hypothesis suggests that leaf blackening is ind uced by a sequence of events related to metabolic reactions associated with senescence, beginning with total depletion of leaf carbohydrates . It is thought that this carbohydrate depletion may induce hydrolysis of intercellular membranes to supply respiratory substrate, and subse quently allow vacuole-sequestered phenols to be oxidized by polyphenol oxidase (PPO) and peroxidase (POD) (Whitehead and de Swardt, 1982). T o more thoroughly examine this hypothesis, leaf carbohydrate depletion and the activities of PPO and POD in cut flower Protea susannae x P. compacta stems held under light and dark conditions were examined in r elationship to postharvest leaf blackening. Leaf blackening proceeded rapidly on dark-held stems, approaching 100% by day 8, and was tempora lly coincident with a rapid decline in starch concentration. Blackenin g of leaves on light-held stems did not occur until after day 7, and a higher concentration of starch was maintained earlier in the postharv est period for stems held in light than those held in dark. A large co ncentration of the sugar alcohol, polygalatol, was maintained in dark- and light-held stems over the postharvest period, suggesting that it is not involved in growth or maintenance metabolism. Polyphenol oxidas e activity in light- and dark-held stems was not related to appearance of blackening symptoms. Activity of PPO at pH 7.2 in light-held stems resulted in a 10-fold increase over the 8-day period. Activity in dar k-held stems increased initially, but declined at the onset of leaf bl ackening. There was no significant difference in POD activity for dark - or light-held stems during the postharvest period. Total chlorophyll and protein concentrations did not decline over the 8-day period or d iffer between light- and dark-held stems. Total phenolics in the dark- held stems increased to concentrations approximate to 30% higher than light-held stems. Consequently, the lack of association between membra ne collapse, leaf senescence, or activities of oxidative enzymes (PPO or POD) with leaf blackening does not support the hypothesis currently accepted by many Protea researchers. An alternative scenario may be t hat the rapid rate of leaf starch hydrolysis imposes an osmotic stress resulting in cleavage of glycosylated phenolic compounds to release g lucose for carbohydrate metabolism and coincidentally increase the poo l of free phenolics available for nonenzymatic oxidation. The physiolo gy of such a carbohydrate-related cellular stress and its manifestatio n in cellular blackening remains to be elucidated.