The senescence of the cut carnation (Dianthus caryophyllus L. cv. White Sim) flower.

Abstract

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.