Photo-Oxidation of Leaf Pigments in Cucumis Leaf Discs During Chilling

396

Chilling temperatures (5°C) and high irradiance (1000 microeinsteins per square meter per second) were used to induce photooxidation in detached leaves of cucumber (Cucumis sativus L.), a chilling-sensitive plant. Chlorophyll a, chlorophyll b, ,B carotene, and three xanthophylls were degraded in a light-dependent fashion at essentially the same rate.Lipid peroxidation (measured as ethane evolution) showed an 02 dependency. The levels of three endogenous antioxidants, ascorbate, reduced glutathione, and a tocopherol, all showed an irradiance-dependent decline. a-Tocopherol was the first antioxidant affected and appeared to be the only antioxidant that could be implicated in long-term protection of the photosynthetic pigments. Results from the application of antioxidants having relative selectivity for '02, 02-, or OH indicated that both '02 and 2-were involved in the chilling-and light-induced lipid peroxidation which accompanied photooxidation. Application of D20 (which enhances the lifetime of '02) corroborated these results. Chilling under high light produced no evidence of photooxidative damage in detached leaves of chilling-resistant pea (Pisum sativum L.). Our results suggest a fundamental difference in the ability of pea to reduce the destructive effects of free-radical and '02 production in chloroplasts during chilling in high light.

Section: Resultssupporting

confidence: 73%

“…The data do, however, provide strong supporting evidence that both '02 and 2-are involved in chilling-enhanced photooxidation in cucumber as has been suggested by Powles et al (25). It is quite possible that similar types of mechanisms are responsible for lipid and pigment breakdown in other systems exhibiting photooxidation (24,27,32 …”

Section: Resultsmentioning

confidence: 99%

Chilling-Enhanced Photooxidation

Wise¹,

Naylor²

1987

396

“…Observations to this effect rapid autooxida-have been made by many workers (2,12,14,28,30,32,36). be expected; the Heath and Packer (16,24) consider the process a photoabove 7, but at peroxidation forming malonic dialdehyde (14) but the pH s no effect in the of our experiment is close to that at which this reaction goes at minimum rate.…”

Section: Discussionmentioning

confidence: 62%

The Metabolism of Oat Leaves during Senescence

Choe¹,

Thimann²

1975

The changes in chlorophyll and protein in senescing chloroplasts isolated from the first leaves of 7-day-old oat (Avena sativa) seedlings have been investigated. In darkness the chlorophyll in these plastids is highly stable, losing only 5 to 10% of its content after 7 days at 26 C. This result contrasts with the behavior of chlorophyll in intact leaves, in which about 80% of the pigment would have disappeared in that time. The protein is less stable than the chlorophyll, though more stable than in the leaf; probably a small amount of protease is present in the plastids. Some protein is also being synthesized in the chloroplasts along with its breakdown; gains of up to 38% in protein and 13% in chlorophyll were observed under different conditions. L-Serine, which actively promotes senescence in the leaf, has only a very slight effect on the chloroplasts, and kinetin antagonizes it. Kinetin also has a small but significant effect in preserving the protein from breakdown. Acid pH somewhat promotes the breakdown, both of chlorophyll and protein. A loss of chlorophyll and protein comparable to that occurring in the senescence of the leaf could not be induced in the chloroplasts by suspending them in malate, in cytoplasmic extract, or in any of a number of enzymes tested alone. Incubation with a mixture of four enzymes was the only treatment which approximated the senescent process in the leaf, causing 34% loss of chlorophyll at pH 5 and 40% loss of protein at pH 7.4, both in 72 hours.In white light, the chlorophyll and the carotenoids, but not the protein, disappear rapidly. This disappearance was shown to be prevented in an atmosphere of nitrogen or in air by a number of reducing agents, of which ascorbic acid was the most effective. It is, therefore, ascribed to photooxidation rather than to normal senescence.The senescence of leaves, which has been studied in a number of laboratories over the years (3, 7, 10, 11, 18, 19, 21-23, 25, 26, 29, 31, 33-39), involves loss of Chl, decrease in carotenoids, enhanced proteolysis, hydrolysis of starch and other polysaccharides, and, in some cases, increase in respiration. In detached leaves, the hydrolyzed products accumulate and may be partly converted to organic acids (21, 23, 37), whereas if the leaves remain on the plant the amino acids, and perhaps 'This work was supported in part by National Science Foundation Grant GB35238 to K. V. T. other products, are re-exported to the stem or base (34). Both proteolysis and Chl loss can be effectively prevented by cytokinins and, in a few cases, also by gibberellins (e.g. 4, 13). The mechanism of this interference with hydrolytic reactions is under active study.In our own laboratory, detailed studies of the senescent process in the detached first leaves of 7-day-old oat seedlings have shown that senescence is largely prevented not only by cytokinins but also by anaerobiosis and by certain inhibitors of protein synthesis, i.e. it is dependent on some synthetic processes, probably formation of enzymes, at the start. There ...

“…Chlorosis is a common symptom of chilling injury in chilling-sensitive species such as cucumber, sorghum, and corn (10,23,26). This measure of chilling injury was used to assess the chilling sensitivity of PM 11 by measuring the Ch 1 content of plants grown at 220C and of plants shifted to 1 3IC for one to 6 d (Fig.…”

Section: Leaf Chlorosismentioning

confidence: 99%

A Chilling Sensitive Mutant of Arabidopsis with Altered Steryl-Ester Metabolism

Hugly

McCourt²,

Browse³

et al. 1990

A chilling-sensitive mutant of Arabidopsis thaliana was isolated and subjected to genetic, physiological, and biochemical analysis. The chilling-sensitive nature of the mutant line is due to a single recessive nuclear mutation at a locus designated chsl. In contrast to wild-type plants, which are not adversely affected by low temperatures, the chsl mutant is killed by several days of exposure to temperatures below 180C. Following exposure to chilling temperatures, the mutant displays two common symptoms of chilling injury-leaf chlorosis and electrolyte leakage. In these respects, the physiological response of the mutant to low temperatures mimics the response observed in some naturally occurring chilling sensitive species. The biochemical basis of chilling sensitivity was explored by examining the pattem of incorporation of 14C02 into soluble metabolites and lipids in wildtype and mutant plants. The only difference observed between the mutant and wild type was that following low temperature treatment, the mutant accumulated 10-fold more radioactivity in a specific class of neutral lipids which were identified by a variety of criteria to be steryl-esters. The accumulation of radioactivity in the steryl-ester fraction occurs 24 hours before there is any visible evidence of chilling injury. These results suggest one of two possible explanations: either the mutation directly affects sterol metabolism, which in tum leads to chilling sensitivity, or the mutation affects another unidentified function and the accumulation of radioactivity in steryl-esters is a secondary consequence of chilling injury. temperature fitness, the molecular basis of chilling resistance or sensitivity has not been established.The ideal experimental material for investigating the mechanisms associated with chilling tolerance would be a pair of isogenic lines in which quantitative or qualitative differences in one or a few gene products were responsible for a significant difference in chilling sensitivity. Therefore, we have explored the possibility that variability in chilling tolerance could be induced by mutagenic treatment. Our working hypothesis was that survival at low temperature requires gene products that are not required at higher temperatures. Thus, we sought to convert the chilling-resistant species Arabidopsis thaliana into a chilling-sensitive species by the inactivation ofgene products that were specifically required for growth at low temperature. We isolated a number of mutants of Arabidopsis that exhibited symptoms associated with chilling sensitivity when exposed to low nonfreezing temperatures, but that have no visible phenotype at standard growth conditions. Here, we describe the genetic, biochemical, and physiological characterization of one such chilling-sensitive mutant ofArabidopsis designated PM 1 1. MATERIALS AND METHODS Plant Material and Growth ConditionsMany plants of tropical origin are injured or killed by exposure to low temperatures below 20°C (6), whereas chilling-resistant species can survive and maint...