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Carbohydrates and Postharvest Leaf Blackening of Proteas

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Carbohydrates and Postharvest Leaf Blackening of Proteas HORTSCIENCE 40(1):181–184. 2005. protea ‘Sylvia’ but a 25 g·L–1 glucose holding solution significantly reduced leaf blackening (Stephens et al., 2001b). Carbohydrates and Postharvest Leaf We report on the changes in the concentra- tion of glucose, fructose, sucrose and starch Blackening of Proteas from harvesting flowers until the onset of leaf blackening. Changes in the glucose and starch 1 Iain A. Stephens, Celeste Meyer, Deirdre M. Holcroft, and content caused by pulsing with glucose and Gerard Jacobs after cold storage is reported for protea ‘Sylvia. Department of Horticultural Science, University of Stellenbosch, Stellenbosch, Results on the efficacy of glucose, applied as South Africa 7600 a pulse or in the holding solution, in reducing leaf blackening is presented for a number of Additional index words. glucose, fructose, sucrose, starch, pulsing, proteaceae protea cultivars. Abstract. Glucose, fructose, sucrose, and starch concentrations were determined in leaves Materials and Methods 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 Plant material. Flower bearing shoots of lower in both inflorescences and leaves. Proportionately, sugars and starch decreased ‘Brenda’ (P. compacta × P. burchellii), ‘Car- more in leaves than in inflorescences. Flower-bearing shoots of ‘Sylvia’ were pulsed indi- dinal’ (P. eximia × P. susannae), ‘Carnival’ (P. vidually with 5% glucose solution until each shoot had taken up 10 mL solution. Water compacta × P. neriifolia), ‘Ivy’ (P..lacticolor served for control treatment. Flowers were then stored for 21 days at 1 °C. After puls- selection), ‘Pink Ice’ (P. compacta × P. susan- ing and after cold storage groups of flowering shoots were separated into inflorescence, nae), ‘Sheila’ (P. magnifica × P. burchellii), ‘Su- leaf and stem components and glucose and starch content determined. Glucose content, sara’ (P. magnifica × P. susannae) and ‘Sylvia’ determined upon completion of pulsing treatments, was significantly greater in all shoot (P. eximia × P. susannae) were obtained from components of shoots pulsed glucose compared with nonpulsed control shoots. Glucose commercial protea farms near Stellenbosch content of leaves was significantly greater after storage for shoots pulsed than control (33°55'S; 18°50'E), South Africa. The area has shoots. Starch content of leaves determined upon completion of pulsing treatments was a Mediterranean climate with hot, dry sum- significantly greater in shoots pulsed with glucose than that of controls. There was a sig- mers and a rainfall of 600 to 700 mm, falling nificant decrease in starch content for all tissue types during 21 days of storage. Pulsing mainly in winter. Shoots were harvested at the flower stems of seven protea cultivars before 3 weeks cold storage significantly reduced soft tip stage and brought to our laboratories the incidence of leaf blackening when assessed both on day 1, and again on day 7 after 3 within 1 h, or nonpulsed flowers packed for weeks of cold storage. Supplementing holding solutions with 1% or 2% glucose reduced export were collected within 1 d of delivery leaf blackening of proteas pulsed with glucose and cold stored for 3 weeks. from Bergflora, a flower exporting company, at Cape Town International Airport. Starch and sucrose have been identified as blackening has also been correlated with re- Experiment 1: Carbohydrate use. After the main nonstructural metabolic carbohydrates duced leaf carbohydrate content (Bieleski et harvest, the lower leaves were cut off flower- in protea (Bieleski et al., 1992; McConchie and al., 1992; Jones and Clayton-Greene, 1992; bearing shoots of ‘Cardinal’, ‘Carnival’, ‘Pink Lang, 1993a, 1993b; McConchie et al., 1991). McConchie and Lang, 1993b; McConchie et Ice’, ‘Sheila’ and ‘Susara’ leaving 16 distal However, in ‘Sylvia’ leaves both fructose and al., 1991, 1994; Newman et al., 1990). Under leaves. The shoots were divided into two glucose were present in higher concentrations lighted conditions carbon assimilates and re- groups for each cultivar. Inflorescences and than sucrose (Stephens et al., 2001a). Several serves in P. neriifolia shoots were converted to leaves of one group of stems were processed Protea species produce significant nectar vol- transport carbohydrates during inflorescence for freeze-drying soon after harvest, while umes that contain glucose, fructose, sucrose and development (McConchie et al., 1991). the other group of stems were placed in water xylose (Cowling and Mitchell, 1981; Mostert Carbohydrate supplementation is a recog- and kept at room temperature (19 ± 2 °C)and et al., 1980; van Wyk and Nicholson, 1995; nised practice in storage and vase life extension natural light. At the onset of leaf blackening Wiens et al., 1983). The nonstructural meta- of many cut flower crops (Goszczyfiska and inflorescences and leaves were processed for bolic carbohydrate concentrations in protea Rudnicki, 1988; Halevy and Mayak, 1981). Use freeze-drying. Leaves were removed from the leaves declined rapidly after harvest (Bieleski of exogenous sugars in Protea cut flowers has stem by cutting and separated into the eight et al., 1992; McConchie and Lang, 1993a, only been partially successful. Sucrose holding upper and eight lower leaves. Inflorescences 1993b; McConchie et al., 1991; Stephens et al., solutions (~2 g·L–1) have been reported to ef- were cut in half longitudinally and one-half 2001b), particularly at elevated temperatures fectively reduce postharvest leaf blackening of discarded. Leaves and inflorescences were (Stephens et al., 2001a). The report that 24 h P. compacta ( Haasbroek et al., 1973; Ireland et lyophilised before being milled to a fine powder after the application of 14C sucrose to P. neri- al., 196), P. eximia (Bieleski et al., 1992; Ireland for carbohydrate analyses. Two flower stems ifolia stems more than 50% of the radioactivity et al., 1967), P. cynaroides and P. magnifica were used per treatment and treatments were was found in the nectar (Dai, 1993) supported (Ireland et al., 1967) and P. neriifolia (Brink replicated five times. the hypothesis that carbohydrate depletion in and de Swardt, 1986; McConchie et al., 1991; Experiment 2: Glucose supplementation leaves, caused by the demand of the developing Mulder, 1977; Paull and Dai, 1990). Holding and use. Flower-bearing shoots of ‘Sylvia’ inflorescence and nectar production, initiate solutions with sucrose at higher concentra- were pulsed individually with 5% glucose leaf blackening (Ferreira, 1986; Paull and Dai, tions exacerbated P. neriifolia leaf blackening solution until each shoot had taken up 10 1990). Further support for this notion came (Jones, 1991). In contrast, a holding solution mL solution (500 mg glucose/shoot). Con- from findings that inflorescence removal and with 30 g·L–1 of sucrose significantly sup- trol treatments were placed in water alone. girdling significantly reduced or delayed leaf pressed leaf blackening of P. eximia (Akamine Treatments were held at 25 °C under lights blackening (Brink and de Swardt, 1986; Dai et al., 1979). Sucrose pulsing solutions (200 (140 µmol·m–2·s–1 PAR). Upon completion and Paull, 1995; Paull et al., 1980; Reid et al., g·L–1, 24 h) significantly reduced leaf black- of pulse treatments flowers were packed into 1989; Paull and Dai, 1990; Stephens, 2001a; ening of P. cynaroides during long-term dry polyethylene lined and enclosed SAPPEX S14 Tranter, 1989). The onset of postharvest leaf storage (1 °C) (Jones, 1991). A similar benefit fibreboard mini-cartons, and cold stored for 21 was found in P. neriifolia pulsed with sucrose d at 1 °C. After pulsing and after cold storage –1 Received for publication 21 May 2004. Accepted (200 g·L , 24 h, 25 °C) before seven days of groups of shoots were separated into flower for publication 18 July 2004. dark, wet storage at 25 °C (McConchie and head, leaf and stem components. Samples were 1Dole Fresh Vegetables, P.O. Box 1759, Salinas, Lang, 1993b). In contrast, sucrose holding Iyophilized and dry mass determined, before CA 93902. solutions did not reduce leaf blackening in being milled to a fine powder for carbohydrates HORTSCIENCE VOL. 40(1) FEBRUARY 2005 181 analyses. Twelve single shoot replications pulsing, the fl owers were randomly packed placed in a controlled temperature room at 19 per treatment were used. Glucose and starch into cartons, cooled to 4.5 °C before the lids ± 2 °C and natural light conditions. Flowers contents were determined by multiplying the were closed and kept over night. The next day were evaluated 1 and 10 d after removal from concentration values with the dry weight of the cartons were wrapped in polyethylene fi lm storage as described earlier. Ten single shoot the different shoot parts. and placed in a 12-m integral container for 3 replicates (8 in the case of ‘Brenda’) were Carbohydrate analysis. A 0.5-g sample weeks at 1 °C to simulate sea transport. After used per treatment. of the dried tissue described was taken for cold storage fl ower stems were recut, randomly Statistical analysis. Analyses of variance glucose and starch analysis. Samples were placed in buckets with tap water and held at 19 (one-way classifi cation) were performed on the extracted by shaking in 5% acetic acid for ± 2 °C under natural light. The fl owers were data using the SAS program (Statistical Analy- 18 h and centrifuged (4,000 gn, 10 min). The evaluated after 1 and 7 d. Ten single shoots sis Systems Institute, 1996) and LSD values supernatant was fi ltered and made up to 100 were used per treatment except for ‘Cardinal’, calculated for the 5% level of signifi cance. In mL with 5% acetic acid. Thereafter, the pel- ‘Carnival’ and ‘Susara’ where 5, 6, and 8 shoots Experiments 3 and 4 analyses of variance were let was resuspended in an acetate buffer (pH were used, respectively. performed on logit transformed data. 4.8) and gelatinized in a boiling steam bath Evaluation of vase life.
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