Effect of hydrogen peroxide on nitrate reductase activity in detached oat leaves in darkness

Nitrate reductase (NR, EC 1.6.6.1) is sensitive to O₂ concentration, and therefore it was of interest to study the action of H₂O₂, a normal substance in plant metabolism, on NR activity in segments of 7-, 14- and 17-day-old leaves of oat (Avena sativa L. ev. Suregrain). After 4 h of treatment in the dark, H₂O₂ decreased NR activity as measured with the in vivo assay. The effect was stronger in 14- and 17- than in 7-day-old leaves. Vacuum infiltration of cysteine did not prevent this decrease. When NR was determined with the in vitro assay, H₂O₂ did not seem to affect the activity after the 4 h treatment. but NR decreased when crude extracts prepared from untreated 14-day-old leaves were incubated directly with H₂O₂. This effect was prevented by addition of cysteine, ascorbate or reduced glutathione to the extracts. In order to study the possibility that low activity of the system for defense against oxidations could account for the age-dependent response of NR to H₂O₂ in the in vivo test, activities of catalase, ascorbate peroxidase and glutathione reductase were measured during leaf development and after a 4-h treatment with H₂O₂ in the dark. No clear correlation was found between the activities of those enzymes and changes in in vivo NR activity caused by H₂O₂. The results suggest that H₂O₂ might affect NR both directly by oxidizing SH-groups and indirectly by decreasing reductant availability as a result of NADH oxidation. The age-dependent response of NR to H₂O₂ treatment could also be explained in terms of decreased NADH availability in the tissues due to decreased NADH synthesis and/or increased degradation.     

Association of nickel versus transport of cadmium and calcium in tonoplast vesicles of oat roots

The plant vacuole has long been suspected of being a site for accumulation of Ni in plant roots, but testing this hypothesis directly by vacuole isolation is technically difficult and has not been reported. Here, we have attempted to determine if Ni can be transported into isolated oat (Avena sativa L.) root tonoplast vesicles as an alternative approach toward understanding the importance of the vacuole in Ni accumulation in roots. We found that, in contrast to Ca and Cd, Ni did not affect the proton gradient of vesicles (MgATP energized or artificially created), and further, that Cd/H antiport activity was not affected by the presence of Ni. Nickel was associated with vesicles, but relative rates of accumulation/association of metals with vesicles were Ca > Cd Ni. Protonophores and the potential Ni ligands citrate and histidine, and nucleoside triphosphates or PPi did not stimulate Ni association with vesicles. Comparison of Ni versus Ca and Cd associated with vesicles using various membrane perturbants indicated that while Ca and Cd are rapidly and principally antiported to the vesicle sap, Ni is only slowly associated with the membrane in a not-easily dissociated condition. Our results indicate the absence of an Ni/H antiport or Ni-nucleotide-dependent pump in oat root tonoplasts, and support the contention that the vacuole is not a major compartment for Ni accumulation in oat roots. Received: 2 June 1997 / Accepted: 17 July 1997     

CO2 alters water use,carbon gain,and yield for the dominant species in a natural grassland

Global atmospheric CO₂ is increasing at a rate of 1.5–2 ppm per year and is predicted to double by the end of the next century. Understanding how terrestrial ecosystems will respond in this changing environment is an important goal of current research. Here we present results from a field study of elevated CO₂ in a California annual grassland. Elevated CO₂ led to lower leaf-level stomatal conductance and transpiration (approximately 50%) and higher mid-day leaf water potentials (30–35%) in the most abundant species of the grassland, Avena barbata Brot. Higher CO₂ concentrations also resulted in greater midday photosynthetic rates (70% on average). The effects of CO₂ on stomatal conductance and leaf water potential decreased towards the end of the growing season, when Avena began to show signs of senescence. Water-use efficiency was approximately doubled in elevated CO₂, as estimated by instantaneous gas-exchange measurements and seasonal carbon isotope discrimination. Increases in CO₂ and photosynthesis resulted in more seeds per plant (30%) and taller and heavier plants (27% and 41%, respectively). Elevated CO₂ also reduced seed N concentrations (9%).     

Nickel and rubidium uptake by whole oat plants in solution culture

Nickel and rubidium uptake by oat plants ( Avena sativa L. cv. Victory) were examined in relation to solution temperature, solution concentrations, metabolic inhibitors, anaerobic root conditions, transpiration and time. Over a 4-h period, uptake rates for both Ni²⁺ and Rb⁺ remained constant at 23°C. Decreasing temperatures to 2°C, 20 μ M concentrations of 2,4-dinitrophenol (DNP), or anaerobic root conditions decreased Ni²⁺ and Rb⁺ uptake rates by 97 to 86% in whole plants. Treatment of excised roots with 20 μ M DNP decreased Ni²⁺ uptake by 93%. Nickel and Rb⁺ uptake rates measured as a function of the external solution concentration followed a typical parabolic curve. K_m (0.012 m M ) and V_max [2.72 μmol (g dry weight)^-1 h^-1] values for Ni²⁺ were nearly 7 times lower than those for Rb⁺ [0.09 m M and 19.2 μmol (g dry weight)^-1 h^-1]. In all experiments, Ni²⁺ and Rb⁺ showed qualitatively similar uptake patterns, but Rb⁺ uptake was quantitatively more sensitive than Ni²⁺ to experimental manipulations.     

Association of two measures of vegetative growth rate with other traits in inter and intraspecific matings of oats

Summary F₂-derived oat lines from inter (Avena sativa L. x A. sterilis L.) and intraspecific (among A. sativa cultivars) matings were evaluated in the F₃, F₄, and F₅ generations for heading date, grain and straw yields, biomass, vegetative dry weight at anthesis, vegetative growth rates until anthesis (GRA) and until maturity (GRM), and harvest index. The associations of GRA and GRM with harvest index ranged from zero to slightly negative. The positive correlations of GRA and GRM with grain yield were stronger in inter than in intraspecific matings. Grain yield was positively associated with harvest index in both inter and intraspecific matings. The results suggest the use of A. sterilis x A. sativa matings to improve vegetative growth rate, grain yield, and, to a certain extent, harvest index simultaneously without affecting the growth duration of the crop.Journal Paper No. J 12130 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011, USA. Project No. 2447. Part of a study conducted by the senior author while he was a visiting scientist at Iowa State University     

A new theory of phototropism - its regulation by a light-induced gradient of auxin-inhibiting substances

Against the wholly indirect evidence of a lateral gradient of auxin as an explanation of phototropic curvature according to the Cholodny-Went theory, direct measurement of free. extractable or diffusable indoleacetic acid from phototropically curving hypocotyls and coleoptiles invariably shows an even distribution of auxin. On the contrary, growth inhibitors extracted or diffused from these organs turn out to be accumulated at the irradiated side, as proposed already by A. H. Blaauw (Z. Bot. 7: 465. 1915). the classical experiment by F. W. Went (Rec. Trav. Bot. Neerl. 25:1, 1928) has to be interpreted as evidence for a lateral gradient of substance(s) inhibiting auxin activity Phototropic curvature is thus a matter of differential auxin sensitivity across the unilaterally irradiated organ.     

A chlorate-hypersensitive, high nitrate/chlorate uptake mutant of Arabidopsis thaliana

N-ethylmaleimide (NEM) Lit 10-100 μ M led to a strong inhibition of the auxin-induced elongation growth of colcoptile segments, while fusicoccin-enhanced growth was not affected. Growth inhibition occurred only if NEM and auxin were allowed to act simultaneously. Preincubation of plant segments with NEM in the absence of auxin caused no inhibition of a subsequent growth stimulation by auxin, whenever NEM was removed before the application of IAA. However, preincubation with NEM plus auxin led to a remaining growth inhibition, which could not be reversed by a second auxin incubation in the absence of NEM. Fusicoccin added to NEM- plus auxin-treated segments was able to restore growth. It is suggested that auxin causes the unmasking of essential SH-groups of a protein to which NEM links covalently. thus inhibiting the growth process. This assumption was further supported by labeling experiments wish [¹⁴C]-NEM using membranes of maize ( Zea mays L. cv. Inraplus) coleoptiles. Two membrane fractions (S₂= 480-1900 g; S₄= 4300-15000 g) revealed a significantly higher [¹⁴C]-NEM labeling in the presence of auxin (2,4-diehlorophe-noxyacctic acid compared to 2,6 dichlorophenoxyacetic acid). This effect disappeared when the membranes were previously washed with EGTA [ethyleneglycolbis-(β-aminoethylether)-N,N,Nr',N'-tetraacetic acid]. The auxin-induced sensitization of coleoptilc segments against thiol-reagents and the auxin-induced expression of SH-groups of proteins of isolated membranes from coleoptiles arc suggested to be events involved in the primary action of auxins.     

Suppression of ammonium uptake by nitrogen supply and its relief during nitrogen limitation

Barley ( Hordeum vulgare , cv. Triumph), wheat ( Triticum aestivum , cv. Kleiber) and oat ( Avena sativa , cv. Tarok) were grown until day 20 in nitrate-containing solutions or in nitrogen-free solutions for periods up to 8 days immediately prior to day 20. They then were exposed for 4 h to complete, nitrate-free solutions containing 0.5 or 2.0 mM ammonium (98 atom%¹⁵N). In all 3 species in 2 experiments, net ammonium uptake was low in plants grown continuously in nitrate, and increased 3 to 4-fold with increasing nitrogen deprivation. Charge balance during net ammonium uptake was largely maintained by the sum of net potassium and net proton efflux. Variations in root ammonium concentration at the time of exposure to the ammonium solutions revealed no consistent pattern with net ammonium uptake, implying that a product of ammonium assimilation may serve as a negative effector for the uptake process. In nitrogen-replete plants, and in those deprived of nitrogen for 2 days, the amounts of endogenous ¹⁴N-ammonium recovered in the ambient ¹⁵N-ammonium solution during the 4-h uptake period were greater than the initial amounts of ¹⁴N-ammonium present in the root tissue. Significant generation of ¹⁴N-ammonium from endogenous organic nitrogen sources was thus evident in all 3 species.     

Changes in ribulose-1,5-bisphosphate carboxylase and its translatable small subunit mRNA levels during senescence of mustard (Sinapis alba) cotyledons

During the senescence of detached first leaves of oat ( Avena sativa L. cv. Victory) seedlings (grown in continuous light) the protein is hydrolyzed and the proteases increase, but the expected simple relation between these two factors is not always realized. The present experiments examine the timing, the influence of light and darkness and the action of the protein synthesis inhibitors cycloheximide (CHI) and cordycepin. Transfer from dark to light delays the breakdown of both chlorophyll (Chl) and protein, but some residual proteolysis is ascribed to the enzyme initially present. Transfer to CHI resembles transfer to light, while the action of cordyceptin is similar but much weaker. Repeated determinations of the acid protease, which is the most active one and the first to appear, show that this enzyme is formed in the light about as rapidly as in the dark, though with different kinetics. In spite of this there is little proteolysis in light in the first 5 days. One possible explanation of that could be that protein is rapidly resynthesized in light, but treatment with [¹⁴C]-leucine shows that such resynthesis is no faster in light than in darkness. It is therefore concluded that the protease initially does not have access to its substrates and, as a corollary, that the senescence process must be controlled by the gradual impairment of the vacuolar membrane, allowing protease to enter the cytosol and attack the proteins there and in the organelles. This concept is supported by many observations on the timing and on the known changes in membrane permeability during senescence.     

Growth and development of embryo parts during the germination of caryopses of the wild oat (Avena fatua L.)

Wild oat (Avena fatua L.) caryopses were germinated on moist filter paper and under water in the presence and absence of hydrogen peroxide (H₂O₂). The sequential growth and development of embryo parts were studied. Germination, as indicated by radicle emergence, was least and slowest in caryopses submerged in deoxygenated water. The coleorhiza in such caryopses elongated much earlier than the root, in contrast to the other treatments where the coleorhiza and the root emerged at about the same time. In caryopses incubated on moist filter paper all embryo parts showed considerable growth. In H₂O₂ treated caryopses only the epicotyl showed substantial growth over the experimental period. In all treatments the first mitotic peaks were noticed at the same period. The occurrence of these early nuclear divisions may be due to release of 4 C nuclei from inhibition by the uptake of water during caryopsis imbibition. The mitosis continued in the radicle of the embryo in those caryopses germinating on moist filter paper, indicating occurrence of DNA synthesis. In the other two treatments, however, few divisions were detected. Here the early growth of the root, causing caryopsis germination, was due to cell elongation, especially in the proximal part of the root.