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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 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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Steven Dupee Dr. and Peter Goodwin

The strategy of this study was to determine the period of floral initiation for both species and then to determine the critical regulator(s) of flower initiation and floral development. Plants grown under different temperature regimes gave best shoot extension and flower initiation at temperatures with 10°C night and 15 to 25°C day. Field data from four locations showed a correlation of time of flower initiation and temperatures over the same range.

Temperature is an important determinant of the vegetative flush period of both species. The stem diameter of all shoots is a consequence of the vegetative flush growth and in turn is well correlated with flower initiation. Plants given day temperatures of 20°C or above remain in the vegetative phase. Flower abortions in Protea neriifolia and reversions from floral to vegetative shoots in Protea cynaroidesresult from high day temperatures.

Daylength was not found to be critical for flower initiation. A cool temperature period acts as a control to change shoots from the vegetative to reproductive phase.

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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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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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Eleanor W. Hoffman, Dirk U. Bellstedt, and Gerard Jacobs

The cytokinin concentration in the xylem sap of Protea L. cv. Carnival (Protea compacta R. Br. × Protea neriifolia R. Br.) shoots was determined at regular intervals from 11 weeks before until 10 weeks after spring budbreak. Cytokinin levels were high during the early phases of spring shoot growth. Benzyladenine (BA) at 50, 250, or 500 mg·L−1 was applied to entire shoots on 22 Feb., 12 Apr., and 22 May 2001 (fall in the southern hemisphere) or only to terminal buds on 22 May 2001 at 500 mg·L−1. Most of the terminal buds sprouted and initiated an inflorescence when BA application at 500 mg·L−1 in May was directed only to terminal buds, whereas lower flowering percentages (0%–35%) were achieved when the entire shoot was treated. After whole shoots were treated with BA in Apr. 2001, between 5% and 45% floral reversion was observed. High flowering percentages of 87% to 93% were recorded when BA was applied at 500 mg·L−1 to the terminal bud in the dormant state or up to the stage when sprouting buds reached the green point development stage. Later applications were less effective, inducing 42% to 43% inflorescence initiation. The flowering time of BA-induced inflorescences was advanced by more than 2 months compared with flowers that initiated naturally on the spring flush.

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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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Eugenie-Lien Louw, Eleanor W. Hoffman, Karen I. Theron, and Stephanie J.E. Midgley

two flushes are considered essential for flowering in Protea ( Coetzee and Littlejohn, 2001 ). The minimum number of flushes is most likely species or cultivar specific. Protea ‘Carnival’ ( Protea neriifolia × P. compacta ) initiates