Carbon dioxide fixation in barley roots, with special reference to its relationship to mineral ion absorption

Abstract

Carbon dioxide fixation in excised barley roots has been investigated, particularly in relation to mineral ion absorption, using carbon-14 dioxide and radiochromatography. As unsatisfactory results were obtained in the determination of radioactivity were obtained in the determination of radioactivity in the ethanol-insoluble root residues, the ethanol-soluble activity was taken as the basis for carbon-14 assimilation. A linear zonation of fixation was demonstrated within the roots, the greatest uptake occurring in the region of the spices. Maintaining the root material in distilled water, rather than a nutrient solution, prior to exposure to carbon-14 dioxide, led to a marked increase in the carbon-14 incorporated in the eathol-soluble fraction. This increase was principally in malic acid. Mineral deficiency experiments, involving the omission of potassium, nitrogen, and phosphorus in turn from the pretreatment medium, resulted in differences in the level of carbon dioxide fixation compared with that for the complete mineral solution. Results for the radioactivity of the 80% ethanol extracts expressed as percentages of that found in the complete mineral solution were 70.6, 137.33, and 96.11 respectively. Addition of glucose to the mineral solution resulted in a small increase in the level of carbon dioxide assimilation. The increase was in glutamic acid. Experiments using single salt solutions provide support for the hypothesis that carbon dioxide fixation is largely determined by organic acid formation in response to excess cation absorption. It is suggested that the relative activity of the labelled products may be related to the metabolic status of the roots. Malonate reduced carbon dioxide fixation, but the same range of labelled ethanol-soluble compounds was formed as in uninhibited roots, and there was no accumulation of labelled succinate. An experiment with cyanide was inconclusive, due to unsatisfactory experimental conditions.