Interaction of polyamines and light on biochemical processes involved in leaf senescence
Abstract The rapid senescence of detached oat leaves in darkness is first manifested by a sharp rise in RNase activity (about 50% within 1 h), then by a rise in protease activity (indicated by an increase in non-protein α-amino nitrogen within 6 h) and ultimately by chlorophyll degradation (beginning after 18 h). These degradative changes are delayed or prevented by low concentrations (1–10 mM) of the naturally-occurring polyamines cadaverine, putrescine, spermidine and spermine. The tetraamine spermine is generally more active than the triamine spermidine, which is in turn more active than the diamines putrescine and cadaverine. All the polyamines are more active than kinetin or cycloheximide. As little as 10 min of exposure to 1 mM spermine, especially at the beginning of the dark period, produces a marked retardation of chlorophyll degradation over a 48 h period, and 60 min of exposure saturates the effect. 
 
In the light, all polyamines promote, rather than retard, the disappearance of chlorophyll but, as in the dark, they inhibit the rise in RNase and non-protein α-amino nitrogen. The photobleaching of chlorophyll in the presence of polyamines is proportional to the duration of exposure to high irradiance (16.5 Wm−2) fluorescent light. Such light is more effective toward the end of the 48 h post-excision test period than at the beginning. 
 
Calcium ion (1–10 mM) supplied together with the polyamines diminishes their action in dark and light, indicating probable involvement of an initial ionic attachment mechanism. 
 
The loss of chlorophyll from the leaves of four species of dicotyledonous plants (pea, bean, rape, tobacco) in the darkness is similarly retarded by 1–10 mM polyamines. In rape, the most rapidly senescing species, 1 mM spermine almost completely arrests chlorophyll degradation over a 96 h period. It is suggested that polyamine metabolism in plants may be related to normal physiological control mechanisms as in microorganisms and animals, and that polyamines could find use as anti-senescence agents for plants.