Metabolic regulation of leaf respiration and alternative pathway activity in response to phosphate supply

In this study the question whether the alternative respiratory pathway acts as an electron bypass for the cytochrome pathway under conditions of growth on limited phosphorus in leaves of bean (Phaseolus vulgaris L.), tobacco (Nicotiana tabacum L.) and Gliricidia sepium Walp was investigated. The oxygen isotope fractionation technique was used to assess the in vivo activities of the cytochrome and alternative respiratory pathways in the absence of added inhibitors. The response of respiration to low phosphorus supply varied among species. Growth at low phosphorus reduced cytochrome pathway activity in bean and tobacco. Alternative pathway activity increased only in bean leaves in response to low phosphorus and not in tobacco. In the case of G. sepium, cytochrome pathway activity remained unchanged whereas the alternative pathway activity increased with low nutritional phosphorus. At low phosphorus, alternative oxidase protein levels increased in the leaves of bean and G. sepium but not in tobacco, suggesting a dependence of alternative pathway activity on protein level. Alternative pathway activity was also not correlated with soluble carbohydrate concentration in bean or tobacco at any phosphorus level. These results show that the alternative pathway does not always act as an electron bypass in response to the downstream restriction of the cytochrome pathway imposed by low phosphorus supply. These results suggest that factors in addition to cellular carbohydrate level and adenylate control can act to regulate alternative pathway activity.     

Potential of Allyl Isothiocyanate to Control Rhizoctonia solani Seedling Damping Off and Seedling Blight in Transplant Production

The synthetic mustered flavouring essential oil, allyl isothiocyanate (AITC), was evaluated for its effect on suppression of Rhizoctonia solani growth in vitro, and in field soils for reducing inoculum density, saprophytic substrate colonization and seedling damping off and blight using snap bean and cabbage as indicator plants. In vitro growth was completely inhibited at the concentration of 50 μl/l. Inoculum density and saprophytic substrate colonization by the fungus in soil were not affected by AITC concentrations of 50 or 75 μl/kg soil. The inoculum density estimation by the use of soil‐drop technique created an artefact leading to an erroneous conclusion that the fungus was eradicated from soil within 1–3 days after AITC treatment at 150 or 200 μl/kg soil. The saprophytic substrate colonization showed that although the activity of R. solani was greatly reduced, the fungus still colonized 45% of the substrate units at these concentrations, and up to 100% at lower concentrations within 1 day after treatment. At higher concentrations the recovery rate from the substrates gradually declined over time to <6%. Drenching R. solani infested sandy‐loam or silty‐clay‐loam soil with water containing the emulsified AITC to provide 150 or 200 μl/l soil, a few days prior to planting, gave over 90% disease control in snap bean and cabbage, with no apparent phytotoxic effect. The effect of AITC was not influenced by the physical soil texture. AITC appears to have a good potential to replace methyl bromide fumigation of the substrate used for transplant production.     

Iron stress-induced redox reactions in bean roots

Iron stress-induced and constitutive redox activity of bean ( Phaseolus vulgaris L. cv. Delinel) roots was measured on intact plants using FeEDTA and ferricyanide as electron acceptors. The presence of the translation inhibitor cycloheximide caused a decrease in the reduction of both oxidants. However, a differential decline in the reduction rates of FeEDTA and ferricyanide was observed, suggesting enzyme heterogeneity. In the presence of the H⁺ -ATPase inhibitor vanadate, the reduction of FeEDTA was nearly completely suppressed in both Fe-deficient (–Fe) and Fe-sufficient (+Fe) plants, providing evidence for an involvement of plasma membrane-bound ATPase activity in the regulation of the reduction process. The inhibition of the ferricyanide reduction by vanadate was restricted to –Fe plants.
The data are interpreted in terms of simultaneous operation of distinct redox systems in roots of iron-deficient bean plants. The role of proton extrusion in iron stress-induced electron transfer is discussed.     

The influence of black bean aphid, Aphis fabae Scop., and its honeydew on the photosynthesis of sugar beet

The effect of black bean aphids on the photosynthesis of sugar beet plants was studied under glasshouse and field conditions. The presence of up to several hundred aphids per leaf had no significant effect on CO₂ exchange rates over a range of light intensities between complete darkness and light saturation. Artificially prepared honeydew, sprayed onto leaves in the same amounts and composition as was found on severely aphid-infested plants, covered 30% of the stomata on the upper epidermis but did not significantly alter the rate of photosynthesis of these leaves in the light or the rate of respiration in the dark. The stomata on the lower epidermis were uncovered and functional. High pressure liquid chromatography of aphid-produced honeydew detected 20 different amino-acids. Three amino-acids, aspartic acid, glutamic acid and gluta-mine, made up the bulk of the amino-acid weight in the honeydew produced on young plants, up till the 8 leaf-stage. In the 10 to 12 leaf-stage, several different amino-acids occurred in substantial amounts. The amino-acids to sugars ratio of the honeydew produced by the aphids decreased strongly as the sugar beet plants aged: from 1:6 in plants with 3 or 4 leaves to 1:25 in plants having 10 to 12 leaves.     

PCR amplification of the 3'' external transcribed and intergenic spacers of the ribosomal DNA repeat unit in three species of Saccharomyces

Abstract Desulfotomaculum thermobenzoicum TSB converted 4 mol pyruvate to 5 mol acetate in the absence of sulfate. The cells grown on pyruvate without sulfate showed both carbon monoxide dehydrogenase (CODH) and methylmalonyl-CoA: pyruvate transcarboxylase activities. However, considering the fermentation products, the acetogenesis from pyruvate might be conducted by CODH pathway rather than methylmalonyl-CoA pathway. Contrary to this finding, Desulfobulbus propionicus MUD fermented 3 mol pyruvate to 2 mol acetate and 1 mol propionate stoichiometrically via methylmalonyl-CoA pathway. Desulfovibrio vulgaris Marburg, which has neither the CODH pathway nor the methylmalonyl-CoA pathway, converted pyruvate to acetate, H₂ and CO₂ as the main products. These results indicate that the fermentation pattern of pyruvate depends on the metabolic characteristics of each sulfate-reducing bacterium.     

Effects of removal of organic matter on the productivity of heathlands

Abstract. Effects of milling, mowing and sod cutting on productivity in a Deschampsia grass heath and a Molinia-Deschampsia grass heath were studied from 1977 to 1986. The sum of above-ground biomass, litter and accumulated humified matter (TOM) in both types was ca. 70 ton/ha and the primary productivity 3–5 ton ha^-1yr^-1. Mulching did not affect the annual production. Sod cutting reduced the productivity to 1–2 ton ha^-1yr^-1; on the Molinia-Deschampsia site this reduction lasted for at least a decade, while the Deschampsia heath started to recover from sod cutting soon and increased in productivity again. Annual mowing both with and without removal of hay reduced the production as well, particularly on the Molinia-Deschampsia heath. Lower production was either the result of phosphorus depletion - Molinia-Deschampsia site - or nitrogen depletion - Deschampsia site. In the mowing treatments there was also a depletion of carbohydrate reserves.     

Photosynthetic photon flux effects on bean response to nitrogen dioxide

Phaseolus vulgaris L. cv. Kinghorn Wax seedlings, supplied with nutrient solution containing either 0 or 5 mM nitrate as sole N source, were exposed to 0.25 μl/l NO₂ for 6 hr each day for 10 days at continuous photosynthetic photon flux (PPF) of 100, 300, 500 or 700 μmol m⁻² sec⁻¹. There was a significant interaction of PPF and nitrate. Shoot and root dry weights increased with increasing PPFs only when nitrate was supplied. The main effects of NO₂ on plant growth were significant; none of the interactions involving NO₂ were significant. Exposure to NO₂ decreased shoot and root dry weight in both the presence and absence of nutrient N and at all PPF levels. All interactions were significant for in vitro leaf nitrate reductase activity (NRA), which increased markedly at PPFs above 100 μmol m⁻² sec⁻¹ when nitrate was supplied. Treatment with NO₂ strongly inhibited enzyme activity in the presence of nitrate, particularly at the 300 μmol m⁻² sec⁻¹ PPF level. These experiments demonstrated that PPF level does not modify the effect of NO₂ on growth but does have a major effect on NRA and on NO₂ effects on NRA in the presence of nutrient nitrate.     

Salinity response of a freshwater charophyte, Chara vulgaris

Abstract. Chara vulgaris L. growing in an oligohaline lake was adapted to laboratory conditions and subjected to long-term salinity treatments ranging from 0 to 350 mol m ³ NaCl added to the lake water (40–680 mosmol kg ¹). Osmotic potential and concentration of the main osmotically active solutes (K⁺, Na⁺, Mg²⁺, Cl and sucrose) in the vacuolar sap of the central internodal cells were estimated. C. vulgaris did regulate turgor but incompletely. Turgor decreased from 335 mosmol kg ¹ under control conditions to 52–111 mosmol kg ¹ at 350 mol m ³ NaCl. The enhancement of πⁱ was achieved by increase in both ions and sucrose. Sterile and fertile plants differed in their response to osmotic stress. In sterile plants, the ions accounted for about 87% of the vacuolar osmotic potential. The increase of πⁱ under osmotic stress was exclusively due to an accumulation of Na⁺ and Cl^-. In fertile plants, sucrose accounted for about 35% of πⁱ and ions for about 51% Under osmotic stress, sucrose content increased together with the ionic content of Na⁺ and Cl^-.     

Survivorship and fecundity of the radiate and non-radiate morphs of Groundsel,Senecio vulgaris L., raised in pure stand and mixture

British populations of Senecio vulgaris frequently contain two common capitulum morphs (radiate and non-radiate) and one rare intermediate morph. The radiate morph shows a higher maternal rate of intermorph outcrossing than the non-radiate morph and due to the ‘cost of outcrossing’ should decline in frequency, ultimately to be lost from a population. To determine whether the radiate morph exhibits some inherent advantage in fitness to offset the ‘cost of outcrossing’, a comparison was made of the survivorship and fecundity of the radiate and non-radiate morphs raised in pure stands and 1 : 1 mixture at three planting dates (autumn 1983, and spring and autumn 1984). Plants in stands established in spring 1984 were harvested in late August 1984, while plants in stands established in autumn overwintered before being harvested the following summer. In spring planted stands, the two morphs exhibited equivalent survivorships, while the fecundity of the non-radiate morph tended to be greater than the radiate morph. In autumn planted stands, survivorship and Net Reproductive Output (survivorship × fecundity) of the non-radiate morph was greater than that of the radiate morph in mixture, and also in pure stands established in 1983. In no instance was the Net Reproductive Output of either morph significantly greater in mixture than in pure stand. Density had a contrasting effect on morph survivorship and fecundity in the spring and autumn 1984 planted stands. Whereas, in spring stands, fecundity was subject to compensating density dependent regulation while survivorship was density independent, the opposite trend was observed in autumn planted stands. It is concluded that under the conditions of the experiments, the radiate morph exhibited no fitness advantage which might offset the inherent disadvantage it suffers in natural polymorphic populations due to the ‘cost of outcrossing’.     

Changes in the levels of enzymes involved in ammonia assimilation during the development of Phaseolus vulgaris seedlings.Effects of exogenous ammonia

Seeds of Phaseolus vulgaris L. cv. White Kidney were germinated and grown either in a nitrogen-free or in an ammonia-supplied medium. The changes in the soluble protein concentration and in the levels of glutamine synthetase (GS, EC 6.3.1.2), NADH–glutamate synthase (NADH-GOGAT, EC 1.4.1.14), ferredoxin-glutamate synthase (Fd-GOGAT, EC 1.4.7.1) and glutamate dehydrogenase (GDH, EC 1.4.1.2), both NADH- and NAD⁺-dependent, were examined in cotyledons and roots during the first 10 days after sowing. Soluble protein declined rapidly in the cotyledons and increased slightly in the roots. GS activity was initially high both in cotyledons and roots but subsequently decreased during seedling growth. Exogenous ammonia hardly affected GS activity. High levels of NADH-GOGAT were present both in cotyledons and roots during the first days of germination. The activity then gradually declined in both organs. In contrast, Fd-GOGAT in cotyledons was initially low and progressively increased with seedling development. In roots, the levels of Fd-GOGAT were higher in young than in old seedlings. Supply of ammonia to the seedlings increased the levels of NADH-GOGAT and Fd-GOGAT both in cotyledons and roots. NADH-GDH (aminating) activity gradually increased during germination. In contrast, the levels of NAD⁺-GDH (deaminating) activity were highest during the first days of germination. Exogenous ammonia did not significantly affect the activities of GDH.