Nonphotosynthetic Retardation of Chloroplast Senescence by Light

Abstract

Abstract. Excised apical portions of green wheat leaf sections were treated with amino. triazole to prevent formation of new ohloroplasts. Illumination retarded the decline in chlorophyll content per leaf section, the disintegration of chloroplast ultrastruoture, and the loss of capacity for photosynthetic carbon fixation. We interpret these 3 effects of illumination as facets of a single light effect in retarding chloroplast senescence. This light effect in retarding chloroplast senescence has features differing from characteristics of photosynthetic carbon fixation. For example, A) application of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea did not decrease, and may have even slightly increased, the effectiveness of light; B) although the action spectrum contains peaks in the blue and red regions, it differs from the action spectrum for photosynthetic CO2 sssimilation in wheat; C) in nonphotosynthesizing tissue, application of sugars did not retard chloroplast senescence; D) light saturation was achieved by only a few hundred microwatts/cm2. Considered together with the well-known light requirement for chloroplast formation, our results indicate that light has a dual, photomorphogenetic control in maintaining the green status of the plant by also exerting a second effect: retarding of senescence of chloroplasts already present.It is common knowledge that leaves are greelner in the light than in the dark and that in most higher plants light is necessary for the formation of the photosynthetic apparatus. The work described in this paper was begun to see to what extent, if any, light could control the photosynthetic capacity of leaves by also controlling functional and ultrastructural aspects of senescence of chloroplasts already formed.In general, when iboth chloroplast formation and chloroplast disintegration can occur, it may be difficult'to unravel the 2 processes, formation and disintegration, in the maintenance of numbers of intact, mature chloroplasts. A study of chloroplast disintegration in light and in darkness is greatly simplified by use of aminotriazole (AT) 2, a chemical that prevents normal chloroplast formation even in the light. AT has no other pronounced effects on overall metabolism in leaves of young wheat seedlings (1). When chloroplast formation is absent in wheat leaf tissue in darkness, chloroplast disintegration is indicated by the decline in chlorophyll content (6,16). We now report and describe the effects of light in retarding chloroplast senescence in leaf chlorenchyma of young wheat seedlings.

Photocontrol of Leaf Senescence

“…The existing reports suggest that light primarily checks the rapid breakdown of leaf organelles during senescence. This has been clearly demonstrated during plastid senescence of wheat (Haber et al, 1969). The authors have shown that the effects of light in retarding senescence of chloroplasts is independent of its formation because light response with regard to senescence has been demonstrated in 3-amino-1.2.4-triazole (AT) treated leaves, in which plastid formation is prevented.…”

Xenobiotic Metabolism in Plants: In Vitro Tissue, Organ, and Isolated Cell Techniques

“…However, evidence indicates that light retardation of senescence is not linked to CO2 fixation or photochemical activity of PS II. Diuron [3-(3,4-dichlorophenyl)-l,l-dimethylurea] did not eliminate or reduce the effectiveness of light in retarding chlorophyll loss (16). Addition of sucrose also had no effect on chlorophyll loss when CO2 fixation was inhibited by diuron (16).…”

“…Diuron [3-(3,4-dichlorophenyl)-l,l-dimethylurea] did not eliminate or reduce the effectiveness of light in retarding chlorophyll loss (16). Addition of sucrose also had no effect on chlorophyll loss when CO2 fixation was inhibited by diuron (16). Glucose inhibited significantly the loss of chlorophyll in the dark (14).…”

Control of Senescence in Marchantia by Phytochrome

“…A number of investigations (12,13,15,(17)(18)(19) have confirmed the initial report by Vickery et al (20) that light retarded senescence in tissue from higher plants, but (12) reported that the action of light in retarding senescence in bean leaf discs was mediated by photosynthesis since DCMU prevented the action of light, while Haber et al (13), working with excised tips from young wheat leaves, concluded that the action of light was nonphotosynthetic because DCMU did not prevent the retarding effect of light on senescence. Goldthwaite and Laetsch (12) looked for but could not find any evidence for the involvement of phytochrome even though Sugiura (18) had previously reported that the loss of protein and chlorophyll in tobacco leaf discs was controlled by phytochrome.…”