Nature of Photooxidative Events in Leaves Treated with Chlorosis-Inducing Herbicides

Abstract

Leaves of rye seedlngs (Secalk cerealk L.) (15,18,23,27). To characterize the photooxidative events and the forms of activated 02 involved in the herbicide-induced bleaching we have compared the photodestruction of Chl occurring when intact leaf segments or isolated chloroplasts from plants grown under dim light were exposed to a high light intensity. Several scavengers for free radicals or for specific species of activated 02 were tested for their ability to prevent, and D20 that prolongs the lifetime of 102 (28) and H202 for their ability to promote, the degradation of Chl. Superoxide dismutase was assayed to study the influence of the herbicides on the activity of one of the protective mechanisms against O2 toxicity and malondialdehyde was estimated to elucidate whether photooxidation of Chl was related to lipid peroxidation in herbicide-treated leaves. Petkus 'Kustro'). The seeds were surface-sterilized by a 5-min vacuum infiltration and about 30 min soaking in a freshly prepared, filtered solution of 3% calcium hypochlorite, thoroughly washed with demineralized H20, and placed in plastic boxes on filter paper (Schleicher and Schull, No. 598) moistened with either distilled H20, or a herbicide solution, as indicated. The seedlings were grown for 6 d at 22°C. MATERIALS AND METHODSContinuous irradiation with white light was provided by fluorescent tubes (Astra, 40 w, Tageslicht and Warmton de Luxe in alternating sequence) giving an incident intensity of either 5,000 ± 500 lux or 10 lux (Warmton de Luxe only).The following herbicides and concentration were used: 0.25 and 0.3 mm 3-amino-1,2,4-triazole (amitrole), 0.05 and 0.15 mm 3,5-dichloro-2,6-difluoro-4-hydroxypyridine (haloxidine), 0.02 mm 4-chloro-5-(dimethylamino)-2-a,a,a (trifluoro-m-tolyl)-3-(2H)-pyridazinone (Sandoz 6706, metflurazon), 0.05 and 0.1 mm 5-dimethylamino-methylene-2-oxo-4-phenyl-2,5-dihydrofurane-carbonitrile-(3) (difunone, EMD-IT-5914).For growing seedlings in the presence of D,L-a-tocopherol acetate, 500,umol each were dissolved in ethanol and applied to filter paper discs in glass boxes (20 cm diameter). After evaporation of the solvent the filter paper was moistened with the herbicide solution (50 ml).Isolation of Chloroplasts. Leaves of 6-day-old rye seedlings grown at 10 lux in the presence of either 0.3 mM aminotriazole or 0.02 mm Sandoz 6706 were finely minced with razor blades in 2 to 3 volumes of ice-cold grinding medium and then briefly and gently ground in a mortar in the presence of a small quantity of sea sand. The grinding medium of Feierabend and Beevers (9) was used, except that Ficoll was omitted. The homogenate was pressed through four layers of muslin, and two layers of Miracloth. The sediment obtained after 20-s centrifugation at 270g was discarded. The supernatant was centrifuged for 5 min at 3,000g. The resulting sediment was resuspended in grinding medium without ascorbate and the suspension adjusted to a Chl concentration of 0.12 mg ml-n.

Control of the appearance of ascorbate peroxidase (EC 1.11.1.11) in mustard seedling cotyledons by phytochrome and photooxidative treatments

“…In NF-treated seedlings, R produces strong photooxidative stress because of the absence of carotenoids, while in FR no photooxidative stress occurs. To decrease the amount of toxic oxygen species the seedlings were grown in some experiments in the presence of the singlet-oxygen-quenching substances p-benzoquinone (10 mM in the medium) or ct-tocopherol, according to Feierabend and Winkelhfisener (1982). To raise seedlings in the presence of ct-tocopherol, 100 ~tmol of D,L-ct-tocophero1 acetate per box was dissolved in ethanol and the solution applied to four layers of germination paper (8 9 8 cm 2, for details see Mohr 1966).…”

The Fate of Chloroplast Proteins during Photooxidation in Carotenoid-Deficient Maize Leaves

“…Growth of carotenoid-deficient seedlings in very low light permits accumulation of normal LHCP II mRNA levels (29). The evidence thus far suggests that photooxidative damage to plastid components is responsible for most, if not all, of the pleiotropic effects of a deficiency in carotenoid pigments (16)(17)(18). This is true regardless of the cause of the carotenoid deficiency, herbicide or mutational lesion.…”

Regulation of Plastid Gene Expression during Photooxidative Stress

“…considerable evidence that supports a model in which carotenoids play a direct role in quenching triplet states of Chl, thus blocking the formation of oxygen radicals and the photodestruction of photosynthetic complexes (13). When seedlings, in which carotenoids do not accumulate either due to a mutational or chemical block in the synthesis pathway, are transferred from permissive low light to high light, rapid bleaching occurs in the green leaf tissue (2).…”