All of the major hormones can influence senescence of leaves and other parts (Nooden and Leopold, 1988). Cytokinin seems particularly important in retarding senescence, and ethylene may be an important promoter, even in leaves. Recently, brassinosteroids have also been implicated as senescence promoters (Section VII below; Chory and Li, 1997). Light has been shown to influence positively or negatively the levels of most of these hormones (Wareing and Thompson, 1976; Hart, 1988), but there are relatively few reports showing a direct linkage between the light effects on senescence and those on the hormone concentrations, i.e., that light influences senescence through an effect on the hormones. Simple correlations, e.g., light increasing the hormone along with retardation of senescence, are a good first step but do not themselves establish causality. Likewise, the ability of exogenous applications to substitute for or duplicate the action of light are good first steps, but again more is needed to establish causality.
A related question is: Do the hormones and light act through the same pathway? One simple approach is to determine whether or not light can exert effects above those produced by saturating doses of hormones or vice versa with hormones acting on saturating light treatments, but data of this type are very limited. For example, R or B pulses could retard Chl loss in papaya leaf disks already treated with saturating concentrations of cytokinin (Biswal and Choudhury, 1986). In alstroemeria cuttings, R is able to further delay Chl loss when applied to cuttings treated with doses of gibberellins that maximally delay Chl loss (Kappers et al., 1998). These data suggest that hormones and light may act on different pathways when they delay senescence. On the other hand, sometimes light does not produce an additional delay in senescence when combined with a saturating dose of cytokinin (Sugiura, 1963), and cytokinin is sometimes effective in delaying senescence in darkness but not in light (Cuello et al., 1987). These data suggest light and cytokinin act through the same pathway in some cases. Since light has been shown to act differently on different species or even on the same species under different conditions, the role of hormones may likewise differ between species and even with the circumstances.
Apex senescence in the G2 pea line has been probed through photoperiod manipulations, and it appears to be linked to gibberellins which retard apex senescence (Proebsting and Davies, 1978).
Although the data are still quite limited, they do suggest that light effects on senescence, particularly retardation of senescence, may sometimes be mediated by hormones with cytokinin and gibberellin being prime candidates.
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