OPTIMUM TIME OF HARVEST FOR THE CONTROL OF LEAF BLACKENING IN PROTEA 'SYLVIA'

Abstract

Leaf blackening is a major postharvest disorder that reduces vase life in Protea 'Sylvia' and limits export of this product, especially when exposed to long-term cold storage as required in sea freight to reach distant markets. Despite established recommendations for other floricultural crops on the optimum harvesting time to ensure high product quality, no clear guidelines exist for protea, especially as pertaining to the control of leaf blackening. The development of leaf blackening on flowering stems of Protea cultivar 'Sylvia' harvested at various times during the day was evaluated. 'Sylvia' stems were harvested at the commercial soft-tip stage on 18 February 2010 at 17h00 and at 08h00, 10h00, 13h00 and 15h00 the following day. At the time of harvest, non-structural carbohydrate content (reducing sugars, sucrose and starch) and water status of the foliage of the flush subtending inflorescence was determined to correlate these physiological parameters of stem quality to the incidence of leaf blackening. Stems were either evaluated fresh or stored for 21 days at 4°C, where after stems were evaluated for a 10-day vase life period. Leaf blackening was assessed daily during the vase life period, by recording the percentage leaves having ≤ 10% blackening per stem. Results showed that stems harvested in the morning had the highest incidence of leaf blackening by day 10 of vase life, both when evaluated fresh (± 60%) or when stored (± 90%). Both sucrose content and the levels of reducing sugars, fructose and glucose, were the lowest early in the morning compared to stems harvested later in the day, supporting the theory that postharvest leaf blackening is linked to carbohydrate stress. In contrast, the water content within leaves harvested in the morning was significantly higher compared to the other harvest times. However the starch content remained relatively constant, irrespective of harvest time. Bi-plot analysis identified sucrose content as the variable that was the best associated with the respective harvest times. The time of harvest may be an important consideration in a management strategy to control postharvest leaf blackening in Protea 'Sylvia'.