The Metabolism of Oat Leaves during Senescence

Abstract

The senescence of the first leaves of light-grown Avena seedlings when detached and placed in the dark is inhibited by a, a' -dipyridyl and a,a', a"-tripyridyl at concentrations between 10-and 104 M. Five other chelating agents exert similar inhibiting effects at concentrations 3 to 30 times higher. The senescence of etiolated leaves, as shown by loss of carotenoid and protein, is similarly inhibited. Ethylenediaminetetraacetate has a similar effect in the dark, though only at 10 mM and above, but in the light it causes bleaching of chlorophyll. It is deduced that an iron-containing system plays an essential part in the initiation of the senescence process.Occasional reports in the literature have indicated that the metal chelators 8-quinolinol and EDTA are capable of retarding senescence in darkened leaves (2,7,14). The Chl retention effect of 8-quinolinol was thought to resemble that of a cytokinin (2), but the compound does not cause cell division in tissue culture bioassays (8,14 (7) showed that relatively high EDTA concentrations (50 mM) caused the bleaching of barley leaves in the light but improved the maintenance of Chl in the dark. These experiments were repeated with Avena leaf segments and a similar phenomenon was observed (Table I). EDTA clearly promotes senescence in the light, but inhibits it in the dark. Control experiments with NaCl substituted for EDTA (Table I) show that the salt alone can cause an effect similar to that of EDTA in the light, but only at higher concentrations.Several other chelators are much more powerful. Those showing strong preservation of Chl in the dark at concentrations of 0.05 to 0.3 mm were: 8-hydroxyquinoline and its sulfonic acid derivative and o-phenanthroline (Fig. 1); those effective at well below 0.1 mm were: a, a'-dipyridyl and a, a , a'-tripyridyl (Fig. 2). The following chelators showed little or no activity at these relatively low concentrations: diethyldithiocarbamate, dimethyldithiocarbamate, salicylic acid, salicylhydroxamic acid, m-chlorobenzhydroxamic acid, 2-mercaptopyridine-N-oxide, EDTA, ethylenediamine, oxalate, citrate, and pyridine. The maximal activity of 8-OH-Q3 was not as great as that of the di-and tripyridyl compounds at their concentrations of maximum activity. Of the concentrations of di-and tripyridyl tested, small but clear effects were observed at 0.05 and 0.01 mM, respectively, while the maximal activity for Chl retention was at 1 and 0.1 mm respectively (Fig. 2). These values are approximately 100 times lower than the EDTA concentration that gave the best response (Table I). In the case of o-PH and 8-OH-Q, the concentrations giving maximal Chl retention also resulted in slight toxic effects such as moderate loss of turgor or localized areas of Chl retention. However, at the optimal dipyridyl concentration, the leaf segments remained turgid and the entire leaf 3 Abbreviations:

Metabolism of Oat Leaves during Senescence

“…Victory) being grown for 7 days under cool white fluorescent light (200 ft-c at the base of the plants), as used in the preceding papers (22)(23)(24). In most experi-ments, the leaves or leaf segments were floated on the test solutions as described in ref.…”

“…The four preceding papers of this series (4,(22)(23)(24) have been concerned with the senescence of the first leaf of the 7-day-old oat plant, held in darkness, either detached or attached to the whole plant. However, as a practical matter, leaves do senesce even in full light, and a few investigators (e.g.…”

“…In rice, red light was effective in delaying senescence and far red had the opposite effect (16). In the presence of EDTA, however, white light was reported to promote bleaching (11), and although this effect has twice been confirmed (9,23), its relationship to normal senescence in light is not clear. Recently, Takegami (20) has found light to prevent the decrease in total RNA which occurs in senescing tobacco leaves.…”

Relation between Respiration and Senescence in Oat Leaves

“…To determine whether a delay of senescence is necessarily associated with delay in the respiratory rise, several reagents other than cytokinins were studied. These include chelators, some polyamines, and several other compounds that were earlier found active on dark senescence (17,24,29). Table I (22).…”

Interaction between Senescence and Wounding in Oat Leaves

“…But stomatal resistance (21). A wound-induced agent nearer to the senescence process than the known plant hormones is the PIIF brought to light by Ryan and his co-workers (18). In that work, it was shown that a proteinase inhibitor is produced as a result of wounding and that it is formed in a leaf adjacent to the leaf that was actually wounded (9).…”