A review of the literature pertaining to cut carnation flower
senescence and the regulatory role of plant hormones in this process
revealed the value of this system in physiological studies. Carnation
flower senescence is a good example of correlative senescence and
therefore this final development stage involves an interaction between
flower parts dying at the expense of the development of others. Due
to the survival value of the seed, ovary growth occurs to the detriment
of the surrounding flower parts especially the petals, the flower part
that determines vaselife. This senescence strategy occurs, although
at a later stage, even when pollination is unsuccessful. Additional
ethylene applied using 2-chloroethyl phosphonic acid, which when
incorporated into plant tissue produces ethylene, accelerated carnation
flower senescence. If the carnation flowers are treated with silver
thiosulphate,which prevents ethylene action,and ethanol,which inhibits
ethylene biosynthesis,petal longevity is extended to the detriment of
ovary growth. Correlating the physical appearance of the flowers in
the presence and absence of ethylene with dry mass and labelled sucrose
analyses, carbohydrate movement appeared to be a major event during
the senescence of this cut flower. Such a conclusion could not be
reached on dry mass analyses alone as the photosynthetic organs of the
carnation flower contribute to the carbohydrate pool in the first days
following harvest. Furthermore the respiratory pattern of the
flower is not a steady decline. Concomitant with the natural ethylene
emanation as the petals irreversibly wilt, so the respiratory rate increases. On the other hand, the respiratory rate is greatly reduced
with silver thiosulphate and ethanol treatment. In the presence of
ethylene, together with the growth of the ovary there is an influx
of carbohydrates from all the flower parts including the petals into
the ovary. With silver thiosulphate and ethanol treatment the petals
become the dominant carbohydrate sink. It thus appears that insufficient
carbohydrates moving to the ovary may be the cause of the lack of ovary
development. However , an experiment with isolated cultured ovaries on
a modified MILLER'S (1965) medium lacking in plant hormones but with a
range of sucrose concentrations showed that sucrose alone cannot
stimulate ovary growth. The mechanism by which this source-sink
relationship is determined appears to be controlled from the sink.
The source organs contribute carbohydrates that are in excess of their
metabolic needs. Acid invertase activity, maintaining the sucrose
gradient into the sink, was considered as a mechanism by which sink
strength could be controlled due to the parallel in other plant systems
between the activity of this enzyme and sink strength. On investigation
the levels of acid invertase activity are higher in the ovaries of
senescing carnations than in the petals. This balance of invertase
activity was reached mainly due to a decline in petal invertase
activity. However, as silver thiosulphate treatment lowered the level
of acid invertase activity in the ovary and this flower part was not
the dominant sink with this treatment, acid invertase activity appears
to contribute to sink activity in the senescing carnation flower.
Nevertheless due to the immobility of sucrose through membranes, for the
passive movement of sucrose down a concentration gradient, membrane
permeability to sucrose would have to be altered. This is a possible role of the plant hormones and specific ions. Furthermore, this
ovary growth was correlated with chloroplast development in the ovary
wall. In the presence of ethylene 'greening' or an increase in
chlorophyll content during flower senescence was measured. This increase
in the chlorophyll content did not occur in the silver thiosulphate and
ethanol treated carnations. Relating this to chloroplast development,
an electron microscope study showed that in the presence of ethylene
the original amyloplast present at harvest developed into a chloroplast
with thylakoids stacked into grana. With the ethylene inhibitory
treatments, although thylakoids developed in the ovary wall chloroplasts,
grana did not form. As chlorophyll is synthesised in the thylakoids,
this chloroplast structure correlated with the chlorophyll measurements.
The results of the parameters measured during the senescence of the cut
carnation flower suggested that the other plant hormones besides
ethylene were involved in this process. Endogenous cytokinin measurements
showed that, overall, the level within the cut flower declined as the
flower senesced. The ovary cytokinin levels did not steadily decline
but increased as the petals irreversibly wilted. This peak of cytokinin
activity was common to ovaries of flowers treated with 2-chloroethyl
phosphonic acid and naphthalene acetic acid, treatments that accelerated
senescence. Previous workers showed that a silver thiosulphate treatment
prevented this increase in cytokinin activity in the ovary. This, together
with the lack of ovary development, suggests that the ovary cytokinin
activity may be a crucial event in the regulation of carnation flower
senescence. To confirm such a hypothesis zeatin was injected into the
ovary but was found ineffective in mobilising sucrose and accelerating
petal senescence. It was only when both zeatin and indoleacetic acid were applied to the ovary that sucrose mobilisation and accelerated
petal senescence occurred. Thus auxins together with cytokinins appear
important in ovary development. The importance of the presence of auxin
in ovary development was further recognised by a naphthalene acetic acid
treatment being far more effective in ~timulating the growth of isolated
cultured ovaries than kinetin. Auxin treatment increased the size of the
cells within the ovary wall and the development of the chloroplasts
within these cells to a greater extent compared to control and kinetintreated
ovaries. It was thus hypothesised that the auxin levels in
the ovary were protected against conjugation by the presence of
adequate levels of cytokinins. When the cytokinin levels dropped,
as in the petals, ethylene could then accelerate auxin conjugation
resulting in a retardation of growth. Sink tissues, such as the ovary,
with a higher cytokinin and hence auxin content, may utilise mobilised
assimilates from the petals thus contributing to petal senescence. To
further prove this hypothesis an investigation into the site of
ethylene action using the silver ion as a tool was initiated. A
review of the histochemical and histological literature revealed that
common silver binding sites in plants included sulphydryl groups,
chloride ions, ascorbic acid and invertase. Each was considered as
potential channels via which ethylene could effect its physiological
response but no conclusion was reached. Because of this a decision
on the importance of the translocatory path of a ten minute silver
thiosul phate pulse within the flowerhead and its accumulation within
the receptacle could not be reached. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1985.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10529 |
| Date | 26 March 2014 |
| Creators | Cook, Elizabeth Louise. |
| Contributors | Van Staden, Johannes. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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