| dc.description.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. | en_US |
