Reexamining Polyphenol Oxidase, Peroxidase, and Leaf-blackening Activity in Protea

Abstract

<jats:p>Premature leaf blackening in <jats:italic>Protea</jats:italic> severely reduces vase life and market value. The current hypothesis suggests that leaf blackening is induced 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 subsequently allow vacuole-sequestered phenols to be oxidized by polyphenol oxidase (PPO) and peroxidase (POD) (Whitehead and de Swardt, 1982). To more thoroughly examine this hypothesis, leaf carbohydrate depletion and the activities of PPO and POD in cut flower <jats:italic>Protea susannae</jats:italic> × <jats:italic>P. compacta</jats:italic> stems held under light and dark conditions were examined in relationship to postharvest leaf blackening. Leaf blackening proceeded rapidly on dark-held stems, approaching 100% by day 8, and was temporally coincident with a rapid decline in starch concentration. Blackening of leaves on light-held stems did not occur until after day 7, and a higher concentration of starch was maintained earlier in the postharvest period for stems held in light than those held in dark. A large concentration 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 oxidase 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 dark-held stems increased initially, but declined at the onset of leaf blackening. 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 differ between light- and dark-held stems. Total phenolics in the dark-held stems increased to concentrations ≈30% higher than light-held stems. Consequently, the lack of association between membrane collapse, leaf senescence, or activities of oxidative enzymes (PPO or POD) with leaf blackening does not support the hypothesis currently accepted by many <jats:italic>Protea</jats:italic> researchers. An alternative scenario may be that the rapid rate of leaf starch hydrolysis imposes an osmotic stress resulting in cleavage of glycosylated phenolic compounds to release glucose for carbohydrate metabolism and coincidentally increase the pool of free phenolics available for nonenzymatic oxidation. The physiology of such a carbohydrate-related cellular stress and its manifestation in cellular blackening remains to be elucidated.</jats:p>

Phenolic and Oxidative Metabolism as Bioindicators of Nitrogen Deficiency in French Bean Plants (Phaseolus vulgaris L. cv. Strike)

“…The metabolism of phenolic compounds involves the activation of oxidative enzymes, such as PPO and POD, which catalyse the oxidation of phenols to quinones (Vaughn and Duke, 1981 [60] ; Thipyapong et al, 1995 [57] ). Numerous researchers have demonstrated the activity of both enzymes to be affected by different types of biotic and abiotic stress (McConchie et al, 1994 [35] ; Cheng and Crisoto, 1995 [15] ; Thipyapong et al, 1995 [57] ; Söderhäll, 1995 [52] ; Kwak et al, 1996 [30] ; Thipyapong and Stellens, 1997 [58] ), notable among the latter being the nutritional status of elements such as B and Ca (Kawai et al, 1995 [25] ; Söderhäll, 1995 [52] ; Kwak et al, 1996 [30] ; Cakmak and Römheld, 1997 [10] ; Tomasbarberan et al, 1997 [59] ; Kim and Heinrich, 1997 [29] ; Ruiz et al, 1998 [46] , 1999 [47] ). In our experiment, the N dosage significantly affected the activities of PPO and POD (p < 0.001; Table 1), N2 and especially N1 presenting the highest activities, while N3 gave the lowest, with a reduction of 42 and 58 %, respectively.…”

Effect of Time of Harvest on Postharvest Leaf Abscission in Lantana (Lantana camara L. ‘Dallas Red’) Unrooted Cuttings

“…1-MCP treatment completely suppressed leaf abscission in all cuttings, including those harvested at 0800 HR that had the lowest leaf preharvest TNC concentration but developed black shoot apices. Cuttings harvested at the onset of the photoperiod with a lower preharvest carbohydrate status, which was further depleted during dark storage (Rapaka et al, 2005) for metabolic processes, may have been subject to hydrolysis of phenolic glycoside esters by glycosidase enzymes (McConchie et al, 1994;Stephens et al, 2005). The cleavage of phenolic glycoside esters results in the production of a free sugar and a reactive phenolic moiety (Dey and Dixon, 1985), which can undergo nonenzymatic oxidation resulting in leaf blackening.…”

Carbohydrates and Postharvest Leaf Blackening of Proteas

“…Phenolic glycoside esters hydrolysed by glycosidase enzymes result in a free sugar and a reactive phenolic moiety (Dey and Dixon, 1985), which can undergo nonenzymatic oxidation resulting in leaf blackening. McConchie et al (1994) suggested that glycosalated compounds may be cleaved under periods of carbohydrate stress. It is therefore possible that high glucose levels in protea leaves may Table 3.…”

“…Further support for this notion came from findings that inflorescence removal and girdling significantly reduced or delayed leaf blackening (Brink and de Swardt, 1986;Dai and Paull, 1995;Paull et al, 1980;Reid et al, 1989;Paull and Dai, 1990;Stephens, 2001a;Tranter, 1989). The onset of postharvest leaf blackening has also been correlated with reduced leaf carbohydrate content (Bieleski et al, 1992;Jones and Clayton-Greene, 1992;McConchie and Lang, 1993b;McConchie et al, 1991McConchie et al, , 1994Newman et al, 1990). Under lighted conditions carbon assimilates and reserves in P. neriifolia shoots were converted to transport carbohydrates during inflorescence development (McConchie et al, 1991).…”