Physiological Studies on Postharvest Leaf Blackening in Cut Flower Protea Species.

To develop an understanding of the physiology which results in postharvest leaf blackening, cut flower Protea species were studied in relation to changes in leaf carbohydrate metabolism, membrane deterioration, and oxidative enzyme activity. Rapid development of leaf blackening in the dark was correlated with low starch and sucrose concentrations. Postharvest decline in leaf carbohydrate status appears to be associated with inflorescence sink demand. Leaf blackening was reduced in the light and carbohydrate pools were maintained due to photosynthesis. In floral stems of P. neriifolia, P. Susannae x compacta and P. eximia (which have differing susceptibilities to leaf blackening), leaf blackening was highest in P. eximia and lowest in P. Susannae x compacta. Preharvest sucrose concentrations were inversely related to leaf blackening, however preharvest starch concentrations were not related to leaf blackening. Low daytime photosynthetic rates and stomatal closure suggest some Protea species may be CAM plants. Leaf blackening may be related more to inflorescence sink demand after harvest than preharvest carbohydrate reserves. In P. neriifolia, postharvest addition of 0.5% exogenous sugar or decapitation of the flowerhead did not delay leaf blackening. A 24-h 20% sucrose pulse reduced blackening to that of vegetative stems. Starch concentrations dropped ca. 70% within 24-h of harvest. Leaves with reduced leaf blackening maintained higher levels of starch and sucrose early in the postharvest period. Low levels of lipid peroxidation and oxidized glutathione concentrations indicated that membrane damage (reduced leaf antioxidant status) did not precede appearance of blackening symptoms. Polyphenol oxidase (PPO) and peroxidase (POD) activities in leaves of P. Susannae x compacta indicated that PPO activity in dark-held stems may not be related to leaf blackening. POD and phenolic concentrations related more closely with appearance of leaf blackening. Chlorophyll and protein concentrations suggested that blackening is not related to senescence, but may result from a wounding response