Carbohydrates and postharvest leaf blackening of proteas
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.