Analysis of respiratory chain regulation in roots of soybean seedlings

Changes in the respiratory rate and the contribution of the cytochrome (Cyt) c oxidase and alternative oxidase (COX and AOX, respectively) were investigated in soybean (Glycine max L. cv Stevens) root seedlings using the 18O-discrimination method. In 4-d-old roots respiration proceeded almost entirely via COX, but by d 17 more than 50% of the flux occurred via AOX. During this period the capacity of COX, the theoretical yield of ATP synthesis, and the root relative growth rate all decreased substantially. In extracts from whole roots of different ages, the ubiquinone pool was maintained at 50% to 60% reduction, whereas pyruvate content fluctuated without a consistent trend. In whole-root immunoblots, AOX protein was largely in the reduced, active form at 7 and 17 d but was partially oxidized at 4 d. In isolated mitochondria, Cyt pathway and succinate dehydrogenase capacities and COX I protein abundance decreased with root age, whereas both AOX capacity and protein abundance remained unchanged. The amount of mitochondrial protein on a dry-mass basis did not vary significantly with root age. It is concluded that decreases in whole-root respiration during growth of soybean seedlings can be largely explained by decreases in maximal rates of electron transport via COX. Flux via AOX is increased so that the ubiquinone pool is maintained in a moderately reduced state.

Biodegradation and sorption of polyaromatic hydrocarbons by Phanerochaete chrysosporium

The ability of the white-rot fungus Phanerochaete chrysosporium (INA-12) to degrade various polynuclear aromatic hydrocarbons (PAH) was investigated. Under static, non-nitrogen-limiting conditions, P. chrysosporium mineralized both phenanthrene and benzo[a]pyrene. Total mineralization, based on radioactive tracing, was limited to 1.8%-3% for phenanthrene and benzo[a]pyrene respectively. In both cases the pattern of mineralization did not correlate temporally with the production of lignin peroxidase activity. Sorption of radiolabelled material to the biomass was very significant with 22% and 40% of the total radioactivity being sorbed for benzo[a]pyrene and phenanthrene respectively. A number of models were examined to predict the sorption isotherms, the best performance being obtained with a three-parameter empirical model. It is apparent that lignin peroxidase is not necessarily involved in the biodegradation of all PAH and that a significant factor in PAH biodegradation and/or disappearance in cultures with the intact fungus may be attributed to sorption phenomena.

Modelling the growth kinetics of Phanerochaete chrysosporium in submerged static culture

The potential commercial application of Phanerochaete chrysosporium requires methods for quantitatively predicting growth and substrate utilization. The growth kinetics of P. chrysosporium INA-12 (CNCM I-398) were investigated and modelled under nonlimiting nitrogen and carbon conditions in submerged static culture. This strain, unlike other strains, does not require nutrient limitation for induction of lignin peroxidase. Maximum levels of lignin peroxidase activity were reached 7 days after culture initiation, when almost 80% of the initial glycerol and 70% of the initial nitrogen were still present. Lignin peroxidase levels then decreased, while biomass levels increased until about day 14. The ratio of cell dry weight to wet weight was constant until the maximum biomass concentration was achieved, after which there was a decrease in the water content. The change in this ratio reflects cell lysis as it correlated with increased concentrations of nitrogen in the media, arising from cell leakage. The suitability of four growth models to predict growth, and in some cases glycerol consumption, was evaluated. A simple linear model and the Emerson model performed poorly for the early stages of growth, while a modified Williams model and the Monod model predicted substrate and biomass concentrations equally well. All models will predict biomass concentrations during the active growth phase, but they should not be used to predict biomass concentrations after the stationary growth phase, when cell lysis becomes significant.

Modelling the isotope enrichment of leaf water

Farquhar and Gan have proposed a model for the spatial variation in the isotopic enrichment of H(2)(18)O across a leaf, which is specifically formulated for monocotyledoneous leaves. The model is based on the interaction between mass fluxes longitudinally within the xylem, and fluxes laterally through veinlets into the lamina mesophyll, where moisture leaves the leaf through transpiration. The lighter, more abundant, molecule H(2)(16)O escapes preferentially with the evaporating water, resulting in the enrichment of H(2)(18)O at these sites. Enriched water diffuses throughout the leaf, and it is this spatial distribution of enriched water which the model seeks to capture. In this paper we present a general formulation of the model in terms of mass flux, extending it to include variable transpiration rates across the leaf surface, as well as a tapering xylem. Solutions are developed for the general case and, since the solutions present in the form of Kummer functions, properties are established as well as methods for estimating the solutions under certain conditions relevant to the biology. The model output is compared with Gan's data collected from maize plants.

Control analysis of photosynthetic CO2 fixation

The potential of control analysis to aid our understanding of regulation and control of photosynthetic carbon metabolism is investigated. Methods of metabolic control analysis are used to determine flux control coefficients of photosynthetic reactions from enzyme elasticities. Equations expressing control coefficients symbolically by enzyme elasticities are derived, and general properties of these expressions are analysed. Suggestions for experimental determination of flux control coefficients from enzyme elasticities are given. A simplified model of the Calvin-Benson cycle is used to illustrate interrelations between patterns of photosynthetic metabolites and that of control coefficients.