Wheat mitochondria: oxidative activity and membrane lipid structure as a function of temperature.

Mitochondrial oxidative activity and membrane lipid structure of two wheat (Triticum aestivum L.) cultivars were measured as a function of temperature. The Arrhenius activation energy for the oxidation of both succinate and alpha-ketoglutarate was constant over the temperature range of 3 to 27 C. The activation energy for succinate-cytochrome c oxidoreductase activity was also constant over the same temperature range. The concentration of mitochondria in the reaction, the degree of initial inhibition of state 3 respiration, and the time after isolation of mitochondria were each shown to be capable of causing a disproportionate decrease in the rate of oxidation at low temperatures which resulted in an apparent increase in the activation energy of oxidative activity. Using three spin-labeling techniques, wheat membrane lipids were shown to undergo phase changes at about 0 C and 30 C. It is concluded that the membrane lipids of wheat, a chillingresistant plant, undergo a phase transition similar to the transition observed in the membrane lipids of chilling-sensitive plants. For wheat, however, the transition is initiated at a lower temperature and extends over a wider temperature range.

Feeding, energy flow and soil turnover in the desert isopod, Hemilepistus reaumuri.

The desert woodlouse Hemilepistus reaumuri is one of the most abundant macroscopic invertebrates in North Africa, the Arabian steppes, semi-desert and desert, and the Negev desert Israel.The main purposes of our study were: 1. To investigate the feeding behaviour of H. reaumuri in the field and the laboratory, with special attention to the importance of soil and perennials in the isopods' diet. 2. To estimate annual energy flow and soil turnover by the isopods and to relate it to the role of isopods in the desert ecosystem. The results of field observations and feeding experiments in the laboratory show: a) That desert isopods are saprovores, herbivores and microbivores. b) That in order to survive the isopod should include soil particles and or soil minerals when feeding on vegetation. c) Annual ingestion was 10.3-38.6 kcals/m2, 3-12% of the available dead organic matter, and soil turnover 28.5-105.7 g/m2. It was hypothesized that by ingestion and defaecation of organic matter and inorganic soil particles, H. reaumuri alters the structure of the decomposition substrate and increase the rate of decomposition in the desert ecosystem.

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves: III. A Comparison between Some C(3) and C(4) Plants.

The chlorophyll-based specific activity of cytochrome oxidase and three exclusively mitochondrial enzymes of the tricarboxylic acid cycle showed little variation between leaves of C(3) and C(4) plants or between mesophyll and bundle sheath cells of Atriplex spongiosa and Sorghum bicolor. However, a large, light-dependent transfer of label from intermediates of the tricarboxylic acid cycle to photosynthetic products was a feature of leaves of C(4) plants. This light-dependent transfer of label was barely detectable in leaves of C(3) plants and in leaves of F(1) and F(3) hybrids of Atriplex rosea (C(4)) and Atriplex patula spp hastata (C(3)). The light-dependent transfer of label to photosynthetic products in leaves of C(4) plants was inhibited by the tricarboxylic acid cycle inhibitors malonate and fluoroacetate. The requirement for continued tricarboxylic acid cycle activity was also indicated in experiments with specifically labeled succinate-(14)C. These experiments, together with the distribution of (14)C in glucose prepared from sucrose-(14)C formed during the metabolism of succinate-2,3-(14)C, confirmed that the photosynthetic metabolism of malate and aspartate derived from the tricarboxylic acid cycle, and not the refixation of respiratory CO(2), was the main path of carbon from the cycle to photosynthesis.

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves: II. Intermediary Metabolism and the Location of Control Points.

Long term feeding of acetate-2-(14)C, (14)CO(2), citrate-1,5-(14)C, fumarate-2,3-(14)C, and succinate-2,3-(14)C to mung bean (Phaseolus aureus L. var. Mungo) leaves in the dark gave labeling predominantly in tricarboxylic acid cycle intermediates. Kinetics of the intermediates during dark/light/dark transitions showed a light-induced interchange of (14)C between malate and aspartate, usually resulting in an accumulation of (14)C in malate and a decrease of it in aspartate. (14)C-Phosphoenolpyruvate also showed a marked decrease during illumination. Changes in other intermediates of the tricarboxylic acid cycle were relatively minor. The kinetic data have been analyzed using the Chance crossover theorem to locate control points during the dark/light/dark transitions. The major apparent control points are located at malate and isocitrate dehydrogenases, and less frequently at citrate synthase and fumarase. These findings are explained in terms of the light-induced changes in adenine nucleotides and nicotinamide adenine dinucleotides.

The effect of light on the tricarboxylic Acid cycle in green leaves: I. Relative rates of the cycle in the dark and the light.

Excised green leaves of mung bean (Phaseolus aureus L. var. Mungo) were used to determine the effect of light on the rate of endogenous respiration via the tricarboxylic acid cycle. Illumination with white light at an intensity of 0.043 gram calories cm(-2)min(-1) (approximately 8600 lux) of visible radiation (400-700 nm) gave a rate of apparent photosynthesis, measured as net CO(2) uptake, of 21 mg CO(2) dm(-2)hr(-1) which was about 11-fold greater than the rate of dark respiration. The feeding of (14)CO(2) or (14)C-labeled acids of the tricarboxylic acid cycle in the dark for 2 hours was established as a suitable method for labeling mitochondrial pools of cycle intermediates.At a concentration of 0.1 mm 3-(3,4-dichlorophenyl)-1,1-dimethylurea, apparent photosynthesis was inhibited 82%, and the refixation of (14)CO(2) derived internally from endogenous respiration was largely prevented. In the presence of this inhibitor endogenous respiration, measured as (14)CO(2) evolution, continued in the light at a rate comparable to that in the dark. Consequently, under these conditions light-induced nonphotosynthetic processes have no significant effect on endogenous dark respiration. Inhibitors of the tricarboxylic acid cycle, malonate and fluoroacetate, were used to determine the relative rates of carbon flux through the cycle in the dark and in the light by measuring the rate of accumulation of (14)C in either succinate or citrate. Results were interpreted to indicate that the tricarboxylic acid cycle functions in the light at a rate similar to that in the dark except for a brief initial inhibition on transition from dark to light. Evidence was obtained that succinate dehydrogenase as well as aconitase, was inhibited in the presence of fluoroacetate.