Changes In Carbon Fluxes Research Articles

Ecological Benefits of the Conservation Reserve Program

The Conservation Reserve Program was initiated in 1985 to reduce soil loss on highly erodible agricultural land. This stated objective of the program has been quite successful. However, there are other unintentional yet significant ecological benefits to the program that merit evaluation. These benefits include the reversal of landscape fragmentation, maintenance of regional biodiversity, creation of wildlife habitat, and favorable changes in regional carbon flux. These and other benefits should be used by policy makers and federal officials to maintain the program even after enrollment expectations have been achieved.

The effects of oxygen on fermentation in Giardia lamblia.

Detailed study of the effects of oxygen on the carbohydrate metabolism of Giardia lamblia revealed that low concentrations of oxygen (< 0.25 microM) produced profound alterations in the carbon balance of this organism. Although this concentration of oxygen could not be detected by mass spectrometry, a marked stimulation of ethanol production was observed. Associated with this was an inhibition of alanine production and oxidation of the intracellular NAD(P)H pool. Higher concentrations of oxygen inhibited ethanol production and further reduced levels of alanine. These results suggest that this stimulation is due to changes in carbon flux. Analysis of cell and medium hydrolysates after the growth of trophozoites in [U-14C]glucose suggests that G. lamblia does not synthesise detectable levels of labelled amino acids, except alanine and to a lesser extent valine, from this sugar. Trophozoites of G. lamblia have both glutamate dehydrogenase and alanine aminotransferase activity. As glutamate is taken up from the medium, it is suggested that glutamate dehydrogenase and alanine aminotransferase cooperate to convert pyruvate to alanine, with the concomitant oxidation of NAD(P)H.

Banana Ripening: Implications of Changes in Glycolytic Intermediate Concentrations, Glycolytic and Gluconeogenic Carbon Flux, and Fructose 2,6-Bisphosphate Concentration

In ripening banana (Musa sp. [AAA group, Cavendish subgroup] cv Valery) fruit, the concentration of glycolytic intermediates increased in response to the rapid conversion of starch to sugars and CO(2). Glucose 6-phosphate (G-6-P), fructose 6-phosphate (Fru 6-P), and pyruvate (Pyr) levels changed in synchrony, increasing to a maximum one day past the peak in ethylene synthesis and declining rapidly thereafter. Fructose 1,6-bisphosphate (Fru 1,6-P(2)) and phosphoenolpyruvate (PEP) levels underwent changes dissimilar to those of G 6-P, Fru 6-P, and Pyr, indicating that carbon was regulated at the PEP/Pyr and Fru 6-P/Fru 1,6-P(2) interconversion sites. During the climacteric respiratory rise, gluconeogenic carbon flux increased 50- to 100-fold while glycolytic carbon flux increased only 4- to 5-fold. After the climacteric peak in CO(2) production, gluconeogenic carbon flux dropped dramatically while glycolytic carbon flux remained elevated. The steady-state fructose 2,6-bisphosphate (Fru 2,6-P(2)) concentration decreased to (1/2) that of preclimacteric fruit during the period coinciding with the rapid increase in gluconeogenesis. Fru 2,6-P(2) concentration increased thereafter as glycolytic carbon flux increased relative to gluconeogenic carbon flux. It appears likely that the initial increase in respiration in ripening banana fruit is due to the rapid influx of carbon into the cytosol as starch is degraded. As starch reserves are depleted and the levels of intermediates decline, the continued enhancement of respiration may, in part, be maintained by an increased steady-state Fru 2,6-P(2) concentration acting to promote glycolytic carbon flux at the step responsible for the interconversion of Fru 6-P and Fru 1,6-P(2).

Photosynthesis, Respiration, and Carbon Assimilation in Water-Stressed Maize at Two Oxygen Concentrations

Photosynthesis decreased with decreasing leaf water potential as a consequence of stomatal closure and possibly non-stomatal effects of severe stress. Assimilation ceased at c. 16 x 105 Pa. Photo respiration, in 21% 02, was small in relation to assimilation in unstressed leaves and decreased as leaf water potential fell but it was much larger in proportion to photosynthesis at severe stress. Decreasing the 02 content to 1-5% increased photosynthesis slightly and decreased photo respiration but did not change the stress at which assimilation stopped. Dark respiration was insensitive to both 02 and stress. Less 14C accumulated in stressed leaves but in 21% 02 a greater proportion of it was in amino acids, particularly glycine and serine. 1-5% 02 decreased the l4C in glycine to 10% and in serine to 50% of their levels in 21% 02. In both 02 concentrations the proportion of 14C in serine increased only at the most severe stress. Gas exchange measurements and changes in the 14C flux to glycine are interpreted as the result of glycolate pathway metabolism increasing as a proportion of assimilation in stressed leaves in high 02. The small absolute rate of photorespiration in high 02 and at low leaf water potential may be due to slow rates of glycine decarboxylation as well as efficient fixation of any C02 produced. Serine is synthesized by an (In sensitive pathway and an 02-insensitive pathway, which is most active at severe stress. Synthesis of alanine competes with that of glycine and serine for a common precursor supplied by the photo synthetic carbon reduction cycle. The relative specific radioactivities of aspartate and alanine suggest that they are derived from a common precursor pool, probably pyruvate from 3-PGA. The amounts of 3-PGA, aspartate, malate, alanine, and sucrose decreased with increasing water stress as a consequence of slower assimilation and pool filling. Other amino acids, glycine, serine, glutamate, and proline, accumulated at low water potential possibly due to increased synthesis and slower rates of consumption. Changes in pool sizes, carbon fluxes, and specific activities of metabolites are related to the mechanism of C4 photosynthesis and current concepts of glycolate pathway metabolism.