Postharvest leaf blackening in Protea neriifolia R. Br.

Field: Agricultural and Biological Sciences (Life Sciences)

Cut flowers of Protea spp. have 20 to 30 leaves attached to their woody stem. The leaves of some species usually turn black within 4 to 8 days after harvest. Protea neriifolia R. Br., an important cut flower in the Protea industry, is a species highly susceptible to this problem, and was used in this study of leaf blackening. Different postharvest stresses lead to different leaf symptoms. Browning and drying of leaves detached from the flower stem, starting from the midrib, was observed under water stress conditions. This leaf browning occurred after 3 days without water supply, following 30% loss of fresh weight. Complete browning occurred over the next 2 days, when the leaf had lost 50% of its fresh weight. A marginal leaf blackening spreading inward, as well as black spots on the leaf surface were observed when individual leaves were held in the dark. Leaf exudate collected from 10% to 30% blackened leaves in the dark showed a 3 to 5 times higher ion concentration than healthy green leaves, indicating cellular membrane damage and leakage of cell contents. Soluble polyphenol oxidase (PPO) activity also increased in the dark, due probably to the disruption of the chloroplast membrane. Protea neriifolia inflorescence was 2/3 of the total stem fresh weight and significantly influenced leaf blackening. Flower head removal or girdling of the stem immediately below the flower head significantly delayed leaf blackening. Flowers at five stages of flower opening were characterized for flower head diameter, fresh and dry weight, respiration and nectar production. Flower head diameter increased 2.5 fold from 2.7 cm at stage 1 (very tight bud) to 7.0 cm at stage 5 (bracts reflexed). A parallel increase occurred in flower head fresh and dry weight during opening. Nectar production began at stage 4 (open, cylindrical flower). As the flower opened, the rate of nectar production increased from 2.7 ml to 9.8 ml per flower. Sugar content in the nectar varied from 15% to 23.5%. When sucrose-14C was applied to a flower stem leaf for 24 hr, more than 50% of the radioactivity was found in the nectar. The results suggest that flower head growth and nectar production make the flower a strong sink for available carbohydrates. Carbohydrate withdrawal from the leaves remaining on a cut flower stem may lead to a carbohydrate depletion in the leaf, disruption of leaf metabolism and subsequent leaf blackening. Sucrose (2.5% to 5% w/v) significantly delayed or even prevented leaf blackening. The respiration rate of individual leaf and flower head on stems held in sucrose solution was 60% higher than those held in water. No ion leakage was found in leaves held in sucrose solution and soluble PPO activity remained low. Leaf susceptibility to blackening differs between P. neriifolia R. Br. and Leucospermum 'Rachel', members from two different genera of the family Proteaceae. Leaf discs from Leucospermum did not turn brown in the dark, while Protea leaf discs browned within 48 hours in the dark. Protea had a higher dry weight percentage and protein content, as well as a greater total phenolic content. Polyphenol oxidase (PPO) activity was high in Protea, while none was detected in Leucospermum leaf tissue. A significantly higher pH was observed in Protea leaf homogenate. No significant difference was observed in ascorbic acid content in leaf tissues of the two genera. However, a subsequent decrease in the ascorbic acid content was observed in Protea leaf extracts as they turned brown, then black. Leucospermum leaf extract remained green and appeared to slow the rate of extract browning when mixed with Protea leaf extract. An inhibitory effect on Protea PPO activity was found in Leucospermum leaf extract. The present studies suggest that PPO activity plays an important role in Protea neriifolia postharvest leaf blackening. Depletion of leaf carbohydrates, or continued darkness, probably cause a loss of chloroplast membrane integrity and release and activation of membrane bound PPO. Leucospermum is not susceptible to the blackening due to lack of, or the presence of inhibitors of PPO activity.