A DESCRIPTIVE MODEL OF THE SENESCENCE OF THE CARNATION CARYOPHYLLUS) INFLORESCENCE (DIANTHUS R. Nichols, Glasshouse Crops Research Institute, Littlehampton, Sussex, United Kingdom. The object of this paper is to summarize in descriptive form some of the factors that contribute to wilting of the cut carnation flower. There are two distinguishable forms of petal wilting in the carnation flower. One is caused by a temporary loss of turgour (reversible wilting) which is usually the result of a water stress, whereas the other (irreversible wilting] precedes death of the petals. The former is recognised by flaccidity of the petals to which turgour can be restored by relieving the water stress, and the latter by inrolling of the outer petals followed by progressive wilting of the inner petals. Irreversible wilting is accompanied by changes in the physiology of the petals, for example, increased permeability of the cells, an increase in respiration, and a characteristic surge in the production of ethylene (Smith, et al. 1964; Nichols, 1968a; Maxie, et al. 1973; Mayak and Dilley, 1976a). If the ethylene surge occurs during reversible wilting it must be relatively small and transient. Broadly, the rise in ethylene signals the end of the display life (vase life) of the carnation flower. It can be caused or induced by exposure to exogenous ethylene, growth regulators, pollination (Nichols, 1968a, 1977), fungal infection (Smith, et al. 1964), abscisic acid (Mayak and Dilley, 1976b), or at the terminal phase as a result of natural ageing. In contrast, certain chemical and physical treatments, for example, carbon dioxide (Smith and Parker, 1966; Nichols 1968a; Uota, 1969; Mayak and Dilley, 1976b), hypobaric storage (Mayak and Dilley, 1976b) or low concentrations of oxygen (Nichols, 1968a) suppress or modify the ethylene surge. These treatments result in increased longevity of the flower and it is for this reason that endogenous ethylene has been assumed to play an important role in the senescence of the carnation flower. Accelerated senescence It is possible to summarize some of the proposed events that occur during, or lead to, senescence of carnation flowers. Figure 1; in this example it is proposed that the flowers have been subjected to treatments which cause accelerated senescence. Not all the intermediate stages of the model have been verified experimentally, that is the sequence of events leading to ovary swelling has been inferred from a number of independent but probably related treatments. For example, pollination causes accelerated wilting of carnation petals and ovary swelling, but we have not verified that sucrose is the translocated sugar in that system; this Acta Hort iculturae 71, 1977 Carnations 227
has been deduced from observations of the effects of ethylene on flower senescence, and from observations that pollination causes a surge of ethylene from petals and styles. However, since the ovary increases in dry weight after pollination, and because sucrose is the usual translocation sugar, it is probable that the translocation mechanisms are similar in the systems in which ovary growth occurs. The same consideration can be applied to the effects of auxins. In general, treatments which result in ethylene stimulation cause an accumulation of dry matter in the ovary. Natural ageing may be seen as an exception to this observation, but this phenomenon is further complicated by the effect of age on mobilisable substrate. During natural ageing, substrates are catabolised as shown by the depletion of total carbohydrate in senescing corollas (Nichols, 1973). Although there are still substantial amounts of carbohydrates present at wilting, they may be inadequate to elicit the ovary-swelling response. Delayed senescence If ethylene is the trigger to accelerated wilting as suggested in the model, Figure 1, it follows that suppression of ethylene should lead to delayed senescence, that is longer vase life. The best evidence that this can be the case is provided by the data of Mayak and Dilley (1976b). They used hypobaric storage to lower the internal cell ethylene levels. The longevity of the flowers was increased by this procedure; or, conversely, if ethylene was introduced into the system, longevity was predictably decreased. The most commonly used treatment for increasing carnation flower longevity is by means of a solution containing a respiratory substrate. The substrate is usually glucose or sucrose, to which a germicide is added to retard growth of microorganisms. Many commercial preservatives are based on this principle. It seems likely that a solution containing a metabolically active substrate is required rather than a solution isotonic with it (Aarts, 1957); most workers agree that the sugar is respired because respiration is increased by sugar feeding. Furthermore, 14 C-labelled sugars fed into cut flowers evolve cc>2 (Nichols, 1973; Nichols and Ho, 1975). However, conversion of the sugars to C0 2 or by secretion from nectaries may be ways of preventing excessive accumulation of sugars in the petals; a non-metabolic sugar might accumulate extra-cellularly and either become toxic or induce reverse osmosis. The respiratory CO2 may well act as an endogenous competitive inhibitor of ethylene (Mayak and Dilley, 1976b) and the competition between CO2 and ethylene is well recognised in fruit physiology. Carbon dioxide concentrations in excess of 4% suppress or delay the natural surge of ethylene (Nichols, 1968a; Mayak and Dilley, 1976b), whereas lower concentrations mitigate effects of exogenous ethylene (Smith and Parker, 1966) depending on their respective concentrations. It follows that the role of sugars is complicated by their influence on osmotic potential, carbon dioxide production and energy status of the petal tissue. 228
INTACT FLOWER CUT FLOWER style abscisic acid (1) \ / cell metabolism endogenous _ ethylene. exogenous growth regulators ageing / 24D (2) ethylene PETALS WILT I I I _ membrane ^disorganisation (cell leakage (3) ) J substrate mobilised (4 ' translocation OVARY SWELLS (1) Mayak and Dilley 1976b (2) Nichols, 1971 (3) Nichols, 1968a (4) Nichols and Ho, 1975. and DRY WEIGHT INCREASES (2) ' (4) stem pollination»-auxingynaecium translocation 229
Kinin-like materials, (Smith, 1967» Heide and Oydvin, 1969» Mayak and Dilley, 1976a; Jeffcoat, 1977} delay senescence of carnations, which can be seen as an effect on cell metabolism and substrate mobilisation. Gibberellin-like and auxin-like materials have been reported in carnations (Jeffcoat and Harris, 1972). Gibberellic acid increases petal sire of cut carnations (Nichols, 1968b). Cycloheximide, an inhibitor of protein synthesis, delays senescence of carnations (Dilley and Carpenter, 1975). It seems likely that these compounds affect senescence through their regulation of cell metabolism and thus the maintenance of cell membrane structure. In this context it is relevant that relatively fresh, mature flowers can produce endogenous ethylene (Dilley and Carpenter, 1975; Nichols, 1977). On the other hand, the relative insensitivity of immature buds to exogenous ethylene (Barden and Hanan, 1972) may be due to their low capacity for producing endogenous ethylene (Camprubi and Nichols, 1977). In certain circumstances the end of petal life is not necessarily associated with enhanced ethylene production asj for example, in flowers undergoing senescence at low temperatures (* 2 C), high concentrations of carbon dioxide (> 4%) or low concentrations of oxygen (< 2%). The enzymic production of ethylene is low or suppressed, and the tissue slowly loses turgour and finally succumbs to infection by microorganisms. Thus in air at ambient temperatures, ethylene hastens petal wilting so that the whole process is over in a few days. There appears to be a similar mechanism in morning glory (Ipomoea tricolor) although for this flower wilting is completed in a matter of hours (Kende and Baumgartner, 1974). Returning to a consideration of Figure 1, the involvement of ethylene as the link between petal wilting and pollination is analogous to the "fertilisation-stylar ethylene-flower fading" hypothesis proposed by Burg and Dijkman (1967) for the orchid. Pollination of 'White Sim' flowers leads to accelerated petal wilting and increased ethylene production from the style and the petals (Nichols, 1977). In the unpollinated flower, although both style and petals increase ethylene production at the end of natural senescence, they can do so independently of each other; for this reason the pathways for ageing and pollination are shown separately in Figure 1. In the scheme, Figure 1, no indication has been made of the influence of water conductivity at the cut stem on flower senescence, instead emphasis has been given to metabolic activities of the corolla. It is assumed that the flower stem is intact or, if cut, the stem is placed in water or a suitable solution. Concerning the cut flower, the resistance of the stem conferred by accumulation of microorganisms and/or physiological blockages imposes a constraint to water flux. The constraint can be partly overcome by certain chemical additives. But it is the forces of transpiration, matric and osmotic potentials that promote water uptake, and it is these which are affected, directly or indirectly, by the metabolic activities of the tissues. This argument seems reasonable as far as the cut carnation is concerned, but it cannot be extended to all cut flowers; in some, such as the cut chrysanthemum, water uptake may be the first consideration. 230
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