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Jinqwei Dai and R. E. Paull

Leaf blackening in Protea neriifolia R. Br. is influenced by sink demand and nectar production. Maximum nectar production occurred when the flower was in the cylindrical shape. C14-sucrose applied to a postharvest flower stem leaf moved preferentially into the nectar (65%). Darkness increased the rate of leaf blackening. Covering an individual leaf on a postharvest flower stem with aluminum foil lead to leaf blackening in 3 days while removing the inflorescence or girdling the stem just below the inflorescence prevented this leaf blackening. Girdling the stem around a leaf base and covering this leaf with foil resulted in leaf blackening in 5 days, but removal of the inflorescence influence did not prevent blackening. Sucrose 2.5% (w/v) in the vase solution prevented leaf blackening in both girdled and non-girdled leaves covered with foil. Polyphenol oxidase apparently plays an important role in protea leaf blackening. Leucospermum, another genera in the family of Proteaceae, did not show any PPO activity and did not blacken in the dark, while high PPO activity was detected in leaf extract of P. neriifolia.

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Iain A. Stephens, Celeste Meyer, Deirdre M. Holcroft, and Gerard Jacobs

Glucose, fructose, sucrose, and starch concentrations were determined in leaves and inflorescences of protea cutflower cultivars soon after harvest and at the onset of leaf blackening while standing in water. At the onset of leaf blackening sugars and starch were lower in both inflorescences and leaves. Proportionately, sugars and starch decreased more in leaves than in inflorescences. Flower-bearing shoots of `Sylvia' were pulsed individually with 5% glucose solution until each shoot had taken up 10 mL solution. Water served for control treatment. Flowers were then stored for 21 days at 1 °C. After pulsing and after cold storage groups of flowering shoots were separated into inflorescence, leaf and stem components and glucose and starch content determined. Glucose content, determined upon completion of pulsing treatments, was significantly greater in all shoot components of shoots pulsed glucose compared with nonpulsed control shoots. Glucose content of leaves was significantly greater after storage for shoots pulsed than control shoots. Starch content of leaves determined upon completion of pulsing treatments was significantly greater in shoots pulsed with glucose than that of controls. There was a significant decrease in starch content for all tissue types during 21 days of storage. Pulsing flower stems of seven protea cultivars before 3 weeks cold storage significantly reduced the incidence of leaf blackening when assessed both on day 1, and again on day 7 after 3 weeks of cold storage. Supplementing holding solutions with 1% or 2% glucose reduced leaf blackening of proteas pulsed with glucose and cold stored for 3 weeks.

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Robyn McConchie and N. Suzanne Robbins

Leaf blackening of Protea neriifolia is a common postharvest problem which renders flowers unsalable. Previous reports suggest that depletion of carbohydrates in source leaves caused by transfer of carbohydrates to the strong flower sink may be a major cause. Flowering stems of P. neriifolia were harvested in California under standard conditions and shipped to Baton Rouge, La. Upon arrival, the stems were re-cut (1 cm.), the number of leaves counted and the diameter and height of the flowers measured. Stems were transferred to 1 liter deionized distilled water containing 50 ppm hypochlorite, and 0.5% sucrose or no sucrose, and placed in a growth chamber (25°C) either with 12 hrs light (120 μmol/m2/s), or 24 hrs darkness. Number of leaves 10% black, flower diameter and height, and carbon exchange rates were measured every two days over a 16 day interval. Soluble and insoluble nonstructural carbohydrates were determined and assimilate export rate was estimated for each sampling day. Stems placed in the light maintained healthy foliage while those in the dark had 77-l00% of their leaves 10% black by day 8. Flower and leaf quality in the fight treatment were superior with addition of sucrose to the vase solution. Influence of treatments on carbohydrate metabolism in relation to leaf blackening and flower development will be discussed.

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Robyn McConchie and N.Suzanne Lang

A major postharvest problem of Protea neriifolia is premature leaf blackening. Carbohydrate stress, due to floral sink demand, may lead to cellular disorganization and leaf blackening. Leaf blackening, nonstructural carbohydrates, ethylene, carbon exchange rates, stomatal conductance and lipid peroxidation were measured on leaves of vegetative and floral stems preharvest, and during a 7 day dark postharvest period. Postharvest treatments were: 0 or 0.5% sucrose in the vase solution, 20% sucrose pulse, or floral decapitation. Leaf blackening was significantly reduced in vegetative stems and floral stems in the 20% pulse treatment, in comparison to all other treatments. Ethylene production and lipid peroxidation were not associated with leaf blackening in any treatment and leaf respiration rates declined for all treatments over time. The magnitude and rate of leaf blackening was inversely related to leaf starch concentrations, with greatest carbohydrate depletion occurring within 24 h of harvest (by 75-85%). Leaf starch from the 20% pulse treatment increased by 300%, in contrast to declining starch concentrations in all other treatments. The data suggest that the flowerhead functions as the major sink for carbohydrate depletion leading to subsequent leaf blackening.

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Robyn McConchie, N. Suzanne Lang, and Ken C. Gross

Leaf blackening on cut flower Protea nerii[olia R. Br. stems was dramatically reduced under a 12-hour photosynthetic light period (120 μmol·m-2·s-1) at 25C for 15 days compared with stems kept in the dark. In the light, addition of 0.5% exogenous sugar to the vase solution resulted in a maximum of 2.5% leaf blackening, while stems with no exogenous sugar had a maximum of 16.5%. Continuous darkness resulted in 94% leaf blackening by day 7, irrespective of sugar treatment. Starch and sucrose concentrations were markedly lower in leaves on dark-held stems than in leaves on stems held in the light; thus, carbohydrate depletion could be the primary stress that initiates leaf blackening. In the light, rates of carbon exchange and assimilate export were similar, indicating that the amount of carbon fixed maybe regulated by sink demand. The pattern of carbon partitioning changed in light-held leaves of the 0% sugar treatment during rapid floral expansion and senescence. Inflorescence expansion appears to influence partitioning of photoassimilates and storage reserves into transport carbohydrates; under decreased sink demand, the assimilate export rate decreases and photoassimilates are partitioned into starch. The data suggest that sink strength of inflorescences held in darkness may be responsible for the depletion of leaf carbohydrates and. consequently, blackening.

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Robyn McConchie, N. Suzanne Lang, Alan R. Lax, and Gregory A. Lang

Premature leaf blackening in Protea 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 Protea susannae × P. compacta 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 Protea 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.

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Jingwei Dai and Robert E. Paull

The inflorescence of Protea neriifolia B. Br. was two-thirds of the total cut floral stem fresh weight and significantly influenced blackening of the attached 20 to 30 leaves. Floral stems harvested at five developmental stages were characterized for inflorescence diameter, fresh and dry weights, respiration, and nectar production. Inflorescence diameter and fresh and dry weights increased from stage 1 (very tight bud) to stage 5 (bracts reflexed). Respiration rate was high in stages 1 and 3. Nectar production began at stage 4 (open, cylindrical flower) and increased from 2.7 to 9.8 ml per flower with 15% to 23.5% total soluble solids as the flower opened. Postharvest inflorescence diameter, respiration rate, and nectar production increased and leaf blackening decreased when floral stems were placed in 5% (w/v) sucrose solution. Application of 14C-sucrose to a leaf subtending the inflorescence lead to >50% of the radioactivity being found in the nectar within 24 hours. These data indicate that leaf blackening in protea is the result of depletion of carbohydrate by the inflorescence, and that this depletion is primarily due to the sugar demand for nectar production.

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Robyn McConchie and N. Suzanne Lang

During a 7-day dark postharvest period, Protea neriifolia R.Br. leaf blackening 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 vegetative stems was similar to that on the 20% sucrose-pulsed floral stems. Leaf starch and sucrose concentration profiles demonstrated that stems with reduced leaf blackening maintained higher levels of those carbohydrates during the early postharvest period. Conversely, leaf starch and sucrose reserves were quickly depleted in stem treatments that resulted in early blackening symptoms. Starch concentrations in all treatments of stems dropped 70% to 82% within 24 h of harvest, suggesting that leaf blackening may be initiated during shipping. Ethylene production was not associated with leaf blackening in any treatment. Lipid peroxidation did not differ among floral treatments nor did it increase over the postharvest interval. Oxidized glutathione (GSSG) concentration increased only with the 20% pulsed stems and was not related to leaf blackening. After an initial decrease, leaf respiration rate was generally maintained regardless of treatment. Collectively, these data are consistent with the hypothesis that carbohydrate depletion is the initiating factor in leaf blackening and is accelerated by inflorescence sink demand. We suggest that membrane degradation does not necessarily precede leaf blackening.

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Robyn McConchie and N. Suzanne Lang

Protea neriifolia R. Br., P. susannae E.P. Phillips × compacta R. Br., and P. eximia (Salis. ex Knight) Fourcade cut flower stems were examined to determine the relationship between postharvest leaf blackening rate and preharvest carbohydrate status. Postharvest leaf blackening was highest (83% by day 4) in P. eximia floral stems, which had the lowest preharvest sucrose concentrations. In contrast, P. susannae × compacta had <5% leaf blackening by day 4 and the highest preharvest leaf sucrose concentrations. Starch concentrations were highest in P. neriifolia; however, leaf blackening was intermediate between P. susannae × compacta and P. eximia and reached 52% at day 4. Preharvest carbon-exchange rate and stomatal conductance in all three species were extremely low, despite high photosynthetically active radiation and apparent lack of water stress. Comparing preharvest carbohydrate profiles in vegetative and floral stems suggests that vegetative stems may have a sink-to-source transition zone between the second and third divisions, while most leaves on floral stems may have transferred carbohydates to source leaves at harvest. While preharvest floral stem sucrose concentrations can be linked to leaf blackening rate, the high starch reserves in P. neriifolia reduced leaf blackening little in this species. We conclude that leaf blackening may be related more to inflorescence sink demand after harvest and oxidative substrate availability than preharvest reserve carbohydrate concentrations in each species.

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R.L. Bieleski, J. Ripperda, J.P. Newman, and M.S. Reid

We tested the hypothesis that premature leaf blackening in cut flower stems of Protea eximia (Salisb. ex Knight) Fourcade may be brought about by a low leaf carbohydrate status. Leaves on cut flower stems held in darkness blackened within 4 days, whereas those on stems held in a greenhouse remained healthy for 5 days. Leaf blackening was also retarded by supplying 1% sucrose in the vase solution; but other additives (hypochlorite, silver thiosulfate, bisulfite) were not effective. The hypothesis was further explored by examining postharvest carbohydrate changes in the leaf of cut flower stems held in light or darkness. At harvest, leaves contained very little hexose (< 1 mg·g-1 fresh weight), comparatively small concentrations of sucrose (≈ 5 mg·g-1 fresh weight) and starch (≈ 6 mg·g-1fresh weight), but high concentrations (≈ 30 mg·g-1fresh weight) of the polyol polygalatol. Starch and sugar contents of leaves held in darkness fell rapidly, to one-third of their initial level after only 1 day and to one-sixth after 3 days. In contrast, starch and sugar contents increased slowly in leaves of stems held in light to three times the initial level after 3 days. Polygalatol content was unaffected by any treatment. Removal of the inflorescence did not delay blackening of leaves held in darkness and did not affect their carbohydrate changes.