Die invloed van fenoliese verbindings op sommige ensieme en metaboliese prosesse in loofblare van Protea nerlifolia R. Br.

Abstract

The aim of this study was to determine the influence of phenolic compounds on selected enzymes and metabolic processes in the leaves of Protea neriifolia R. Br. over a period of 10 days after cutting the inflorescences. Although the phenolic browning of protea leaves is regarded as a physiological abnormality, an attempt has been made to determine the relationship between the appearance of phenolic compounds and senescence. As a criterium for the determination of internal changes in the foliage of Protea neriifolia , quantitative changes in the total soluble protein content, lipase, chlorophyllase, malate dehydrogenase activity, as well as changes. in chlorophyll and differential permeability were determined. The respiration rate was used as a criterium for the determination of the degree of senescence. Senescence of protea leaves was characterized by a typical climacteric sequence. A considerable increase in the persentage apparent free space (%AFS) occurred during the preclimacteric stage. This initial increase was followed by a decline. The final increase in permeability corresponded with the onset of the climacteric rise. Since there was a slight increase in the total soluble protein content during the preclimacteric stage, this temporary synthetic reaction which preceded the climacteric, probably caused the apparent recovery of the membrane permeability. Decompartmentation as a consequence of the degradation of the tono-

plast and surrounding membranes of certain organelles occurred during the climacteric. Ultrastructural degradation as well as a changeover in metabolic pathways, which caused biochemical changes, may have been responsible for the in= crease in %AFS and respiration climacteric. Although there was a decrease in the total soluble protein content during the climacteric rise, it is possible that an increase in hydrolytic activities may have been responsible for an increase in catabolic versus anabolic reactions. The activity of lipase increased during the senescence of the foliage of Protea neriifolia. Senescence of the foliage was characterized by an irreversible loss of chlorophyll, during which chlorophyllase seemed to have been responsible for the initial degradation of the chlorophyll. Subsequently the synthesis of chlorophyll was stimulated by chlorophyllase during the climacteric phase. It is possible that two different enzymes were responsible for this phenomenon. One enzyme probably catalysed the hydrolysis of chlorofil a while the other enzyme probably catalysed the transesterification of methyl chlorophyllide and phytol. The decline in malate dehydrogenase activity probably resulted from an apparent changeover in metabolic pathways during the senescence of the foliage of Protea neriifolia. Tannins, the polymeric compounds which cause browning of tissues of various plant species, were mainly found in cytoplasmic vacuoles, plastids and in membranes of vesicels which occurred in vacuoles of the cells of P. neriiifoZia leaves.

Tannin-containing cytoplasmic vacuoles fused with the central vacuole and deposited their contents in the central vacuole. It is possible that tannins occur in starch grains in chloroplasts. This indicates some relation between the dissolution of starch and the increase of tannins in cells. Flavonoid containing extracts from the leaves of Pr o t ea nerii-fo Zia R. Br. inhibited the chlorophyllase activity while the lipase- and malate dehydrogenase activities were activated, as well as inhibited. Changes in the Rf-values, chromatographic colour intensities, intensity of leuco-anthocyanin reactions as well as the different activation and/or inhibition reactions of the different flavonoid compounds, suggest that the flavonoid composition of the leaves of Protea neriifolia changed considerably during the ten days after cutting. Browning of protea leaves is a complex process during which flavonoid components affect metabolic processes, which probablyaccelerates senescence of the leaves. Although the browning phenomenon is not associated directly with phytogeronto logy, is it obvious that tannins in the cells cause ultra- structural changes which in turn results in accelerated senescence.