ISHS Acta Horticulturae 545: V International Protea Research Symposium
LEAF BLACKENING IN PROTEA FLOWERS: RECENT DEVELOPMENTS |
| Author: | W.G. van Doorn |
| Keywords: | Protea spp. leaf blackening, physiology, biochemistry, carbohydrate depletion, sink-source relations, water stress, ethylene, phytochrome, polyphenol oxidase, peroxidase, beta-D-glucosidase |
| Abstract:
The main reason for leaf blackening in Protea flowers is a low level of carbohydrates in the leaf cells. Recent research has attempted to further elucidate the biochemistry of blackening and to find practical treatments to prevent the problem. Leaf blackening is known to be due mainly to the carbohydrate demand by the flower head. Nectar production, which is high in several Protea species, may be one reason for this demand. The presence of water on the leaves also stimulates blackening. The possible roles of ethylene and phytochrome in the onset of blackening are also discussed. It has been suggested that blackening were due to mixing of the vacuolar contents (including phenols) with enzymes located either in the cytoplasm or in various organelles. This would imply that the vacuolar membrane becomes ruptured. However, little evidence has been found for membrane degradation during blackening. It is as yet unclear in which cellular compartment blackening starts. It has also been held that both peroxidase and polyphenol oxidase are involved, although enzyme activities, measured in vitro, were not always related to the degree of blackening. In contrast, the activity of beta-D-glucosidase, also measured in vitro, correlated well with blackening. However, in vitro measurement of enzyme activity, in particular that of polyphenol oxidase, seems of little relevance of the processes in vivo. It is therefore as yet not known which enzymes are involved. Ethanol, which is an unspecific inhibitor of beta-D-glucosidase, delayed blackening. Both a dip in 30-50% ethanol, or the use of 1% ethanol as a vase solution were effective. Ethanol vapour also delayed blackening in stems that were stored in darkness in plastic bags. Girdling, sucrose pulsing, and maintaining the flowers cold (close to 1°C) during transport are currently used in to delay the problem. | |
Download Adobe Acrobat Reader (free software to read PDF files) | |
