Initial events and inter-organ relations during senescence of orchid (Cymbidium) flowers.

<TT>During storage and transportation, harvested crops are continuously exposed to all kinds of stress, such as desiccation and mechanical damage. These conditions are known to shorten shelf life and this is often associated with an earlier appearance of the sudden upsurge in ethylene production. It is doubtful whether this so-called autocatalytic ethylene production, although presumably necessary for the coordination and integration of the senescence process, is the trigger of senescence. Rather, changes in membrane properties or in ethylene sensitivity at an early stage of senescence may be responsible. An increase in the sensitivity to ethylene may, in turn, be an effect of the synthesis of so-called ethylene-sensitivity factors or the disappearance of inhibiting substances (chapter 1).</TT><TT>In Cymbidium flowers, as in other orchids, the stigma, style and stamens are united in an organ called the central column. Two pollinia, covered by an antherious tissue called the anther cap, are positioned on top of the central column. Removal of the pollinia and/or the anther cap (emasculation) is known to dramatically advance the senescence process. In nature, emasculation is carried out by insects, while searching for nectar in the flower, or by mice that feed on these apparently tasteful flower parts. As a result of suboptimal conditions during cultivation the anther caps may be abscised spontaneously, while during postharvest life the flowers may lose their anther cap due to handling.</TT><TT>Emasculation generally leads in about one day to red coloration of the labellum (lip), a modified petal differing in shape and color from the others. As the early effect of emasculation is very reproducible and easy to assess visibly, the Cymbidium flower was taken as a model system to study the biochemical changes that appear during stress(emasculation)-induced senescence. The purpose of the work described in this thesis, was to determine the factor(s) responsible for the advanced senescence and to analyse the interorgan relations during the senescence process.</TT><TT>From the literature it was derived that the negative effects of emasculation may be due to wounding. The disruption of a presumed cytokinin flow from the pollinia and the anther cap to the central column may also be involved. Concerning the communication between the floral parts, the biological precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC) as well as so-called ethylene-sensitivity factors ( e.g. short-chain, saturated fatty acids) were suggested to be involved.</TT><TT>By using a very sensitive laser-driven photoacoustic detection system for ethylene we were able to show the existence of a small peak in ethylene production almost immediately after emasculation (chapters 2 and 3). This small and short-lasting increase in ethylene production appeared well before the visible symptoms ( e.g. coloration of the lip), indicating a causal relationship.</TT><TT>Red coloration of the lip as a result of anthocyanin accumulation becomes visible within approximately one day after emasculation. This process is preceded by a pronounced increase in the activity of the key enzyme in phenylpropanoid metabolism, phenylalanine ammonia-lyase (PAL). A similar effect is apparent after treatment of the flower or the isolated lip with ethylene and inhibitors of ethylene synthesis and ethylene action greatly inhibit coloration. It is therefore concluded that lip coloration is regulated by ethylene (chapters 4 and 5).</TT><TT>Desiccation of the rostellum, an anther-derived tissue that is uncovered by emasculation, was found to be the primary factor in emasculation-induced ethylene production (chapter 6). Desiccation leads, possibly through the release of "endogenous elicitors", to an increase in the levels of ACC and malonyl-ACC in the top of the central column, which means that the endogenous elicitor locally stimulated the ACC-synthase activity (chapter 7). Determination of the ethylene production in different flower parts isolated at different times after emasculation, revealed that only the production of the central column shows a significant increase. The other flower parts, including the lip, have only very low productions (chapter 8).</TT><TT>Following emasculation, the lip does not show an increase in ethylene production but does show an ethylene effect (coloration). It may therefore be concluded that the sensitivity to ethylene has increased. Similar arguments were used by other authors to introduce a role for unknown sensitivity factors in senescence of Petunia and carnation flowers.</TT><TT>However, in vivo measurement of the ethylene production in the central column and the remaining portion of the flower showed that the major part of the ethylene (ca. 80%) is produced by the remaining portion and only a small amount by the central column i.e. at the site of ACC synthesis. In this way it was shown that ACC is rapidly translocated within the flower and that measurements in isolated flower parts do not yield valuable information concerning the in vivo ethylene production (chapter 8). The same is true for the translocation of ACC. Although ACC is rapidly translocated from the central column to the perianth, it is largely immobile in isolated columns (chapter 9). With ACC as a transported intermediate, localized stress affects the senescence processes in all the flower parts. Ethylene Itself was also found to be translocated within the flower and an additional role in the coordination of the senescence process is suggested (chapter 8). No indications for the existence of (mobile) ethylene-sensitivity factors were found and also no biological activity of the wilting and ethylene-sensitivity factors mentioned in the literature was apparent in this system (chapter 9).</TT><TT>The emasculation-induced ethylene production has a significant effect on the internal ethylene concentration in all the floral parts. As a result, an increase in the activity of different senescence-related enzymes, e.g. PAL and the ethylene forming enzyme (EFE), was observed (chapter 7). In addition, emasculation leads to an increase in membrane permeability (chapter 3). Although the ethylene production and the internal concentration both decrease to the initial level within one day, the EFE-activity is maintained at a high level for a longer period of time. This higher EFE-activity may stimulate, through negative feedback control rather than by its ethylene production, the ACC-synthase activity, leading to an earlier appearance of the more pronounced upsurge in ethylene production that accompanies senescence (chapter 10).