The subcellular distribution of ascorbate peroxidase and glutathione reductase (EC 1.6.4.2) in pea leaves was compared with that of organelle markers. Enzyme distribution was found to be similar to that of the chloroplast enzyme NADPH-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). Isolated chloroplasts showed a close correlation between intactness and the percentage of enzyme activity recovered. Chloroplasts of 85% intactness were found to contain a high proportion of leaf dehydroascorbate reductase activity (EC 1.8.5.1), 10% of leaf glutathione and 30% of leaf ascorbate. These results are discussed in relation to the potential role of chloroplast antioxidant systems in plant resistance to environmental and other stress conditions.
The effect of acifluorfen and oxyfluorfen on chlorophyll bleaching, lipid peroxidation and photosynthesis in pea leaf discs was studied. Both her‐ bicides induced light‐dependent bleaching and lipid peroxidation, the level of damage being greater at higher light intensities. Photosynthetic carbon dioxide fixation was only partially inhibited in treated leaf discs incubated in darkness, thus indicating that these herbicides did not inhibit photo‐ synthesis as a primary mode of action. Leaf discs maintained in darkness showed no visible signs of injury, and light‐dependent herbicide‐induced damage was reduced by incubating discs under nitrogen, orpre‐incubating them with the electron‐transport inhibitor monuron. It is suggested that acifluorfen and oxyfluorfen are activated by a light‐dependent process, which requires photosynthetic electron transport.
The nitrogen content of cell wall preparations from normal tomato (cv Ailsa Craig) fruit remained constant during ripening, whereas salt-soluble protein increased throughout this process. Tomato polygalacturonase released about twice as much protein from the preparations as salts did, with a maximum at the orange stage of development. Polygalacturonase-solubilized protein from the tomato mutant 'ripening inhibitor' (rin) was less, and that from the mutant 'Never ripe' (Nr) cell wails was more than that from normal wail preparations. Release of protein by fungal cellulase was limited, but was increased by the addition of polygalacturonase from the same source. Salt-solubilized protein contained a range of enzymic activities but these were distributed between fewer multimolecular forms than is the case for whole ceUl preparations. The results suggest that metabolicaly active protein, removable by strong salt solutions, celulase, or polygalacturonase, remains attached to the cell wals of tomato fruit until late in ripening. The unusual amounts of protein attached to the cell walls of mutant fruit appear to be a reflection of the absence of some or all of the isoenzymes of polygalacturonase that are associated with normal ripening.
Changes in photosynthetic activity, leaf pigments and the activities of enzymes that scavenge damaging oxygen species in chloroplasts were followed during the greening of 8‐day‐old etiolated pea (Pisum sativum L. cv. Meteor) seedlings. Accumulation of chlorophyll and carotenoids was accompanied by development of photosynthetic activity. Carotenoids present in etiolated leaves, and the high ratio of carotenoid to chlorophyll detected during the early hours of greening are suggested to provide important protection against singlet oxygen. Superoxide dismutase, ascor‐bate peroxidase and glutathione reductase, which scavenge superoxide and hydrogen peroxide in chloroplasts, are present at high activities in etiolated leaves and throughout greening. The mechanisms by which developing chloroplasts may generate damaging oxygen species, and the role of these scavengers during greening is discussed.
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