Ascorbic Acid and the Illuminated Chloroplast

Abstract

The chloroplasts in the leaves of higher plants produce several damaging oxygen-derived species in the light, namely, hydrogen peroxide, singlet oxygen, lipid peroxides, superoxide, and the hydroxyl radical. The high concentra tion of ascorbic acid often present in the chloroplast helps to protect them against these species. Toxic Effects of Oxygen on Illuminated CbloroplastsOxygen is, by definition, essential for the life of all aerobic organisms, including higher plants, but oxygen has long been known to be toxic to aerobic organisms at concentrations greater than those in normal air. High oxygen concentrations inhibit chloroplast development, decrease seed viability and root growth, damage the membranes of leaves and roots, stimulate leaf abscission, and increase the incidence of growth abnormalities ( 1-3 ). Exposure to oxygen at a pressure of 6 atm is lethal to a wide variety of plants (4).Oxygen toxicity is seen not only in whole plants but also in cell cultures and even in isolated organelles. For example, high oxygen con centrations inhibit carbon dioxide fixation by isolated, intact chloroplast fractions (the so-called "Warburg effect"). Chloroplasts are especially prone to oxygen-toxicity effects because their internal oxygen concen tration during photosynthesis is probably somewhat greater than that in the air surrounding the leaf (5). The effects of elevated oxygen concen trations on chloroplasts are attributable to a number of factors (6), two of which are considered in the next sections.Generation of Superoxide Radicals and Hydrogen Peroxide in Chloroplasts. Isolated, illuminated chloroplast thylakoids slowly take up oxygen in the absence of added electron acceptors. This phenomenon was first observed by Mehler (7) and is often known as the "Mehler reaction." The reaction appears to result from the reduction of O 2 to the 0065-2393/82/0200-0263$06.00/0

Ureide Catabolism of Soybeans

“…Reactions (1), (2), (3), and (4) are well documented for plants (4). Although reactions (6), (7), and (8) (Table I).…”

“…were incubated with [4,5- (1), Glutamate-glyoxylate aminotransferase; (2), glycine decarboxylase; (3), serine hydroxymethyltransferase; (4), serine-glyoxylate aminotransferase; (5), D-glycerate dehydrogenase; (6), nonenzymic oxidation of glyoxylate; (7), NADPH-glyoxylate reductase; (8), glyoxylate oxidase; (9), tartronate semialdehyde synthase; (10), isomerase; (11), tartronate semialdehyde reductase. Reactions (1), (2), (3), and (4) are well documented for plants (4).…”

Physiological constraints on bromoform (CHBr3) production by Ulva lactuca (Chlorophyta)

“…The inactivation of SOD results in the reaction of O with H 2 O 2 to form the highly reactive hydroxyl Ϫ 2 radical, which causes the peroxidation of membranes (Halliwell 1982). As a means of coping with internally generated H 2 O 2 , a suite of enzymes have evolved to catalyze its removal.…”

“…Other enzymes that degrade H 2 O 2 include catalase and other peroxidases. Catalase is found in the peroxisome of all higher plants and most green algae (Gross 1993) but not in the chloroplast (Halliwell 1984). U. lactuca has been reported to have less than 10% the catalase activity of those algae with glycollate oxidase (Tolbert 1980), whereas no catalase activity has been detected in Ulva rigida .…”

Interactive effects between supplemental ultraviolet-B radiation and heavy metals on the growth and biochemical characteristics of Spinacia oleracea L.

“…Ascorbic acid acts as an in vitro biological antioxidant, and functions as a co-substrate of plant peroxidases (Halliwell, 1982). Singh (1996) also reported significant reductions in the ascorbic acid content of three leguminous plants (Vigna radiata L., Phaseolus mungo L., and Glycine max L.) after exposure to enhanced UV-B radiation.…”