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Distribution, metabolism, and localisation of cyclohexanecarboxylic acid, a naphthenic acid in Phaseolus vulgaris L. Padmanabhan, Usha

Abstract

Naphthenic acids, obtained from crude petroleum, have been known for the past two decades, to act as plant growth stimulants. The present investigation deals with several aspects of the fate of cyclohexanecarboxylic acid (CHCA), a component of the naphthenic acid mixture, in bush bean plants, Phaseolus vulgaris L. cultivar "Top Crop" following application to the primary leaves. A foliar spray of 0.01 M CHCA applied to the primary leaves of 14-day old bush bean plants increased the vegetative and reproductive growths. The glucose conjugate of CHCA was the first metabolite to be formed (in 1/8 hr) in the leaves treated with labelled CHCA as a K salt (KCHC-7-¹⁴C). The aspartate conjugate of CHCA and an unknown metabolite 'Y’ were detected about an hour after treatment. These three metabolites were present in the plant until four weeks after treatment. No free CHCA was detected 8 hr after application. This suggests that the metabolites, rather than the free acid, were responsible for the growth stimulation observed. The metabolites of CHCA hence cannot be merely detoxication products. The major fraction of the ethanol soluble ¹⁴C activity remained in the treated primary leaves. The lowest amounts of activity were found in the fraction composed of buds, flowers and pods. This pattern of distribution of activity was observed at 1, 2, 3 and 4 weeks after application of KCHC-7-¹⁴C. The glucose and aspartate conjugates of CHCA and the unidentified compound 'Y’ were present in the plant at the times mentioned above. At the end of each week, an amount of activity equal to 0.4% or less of the total ethanol-soluble activity was found in ethanol-insoluble plant residues. CHCA was decarboxylated by bean plants. The ¹⁴CO₂ released during a period of seven days accounted for 33% of the activity absorbed. The distribution of the activity in the plant, following application of KCHC-7- ¹⁴C to primary leaves, involved both basipetal and acropetal movements. The basipetal movement occurred in the phloem and the acropetal, in the xylem and phloem. The translocation of CHCA was favoured by light. Evidences obtained suggest that energy in the form of ATP was required for translocation. In the dark, the provision of glucose favoured the translocation, perhaps by serving as a source of ATP via respiration. A supply of aspartate in the dark favoured the translocation slightly. The radioactivity from CHCA was localized in the chloroplasts of the cell. An ethanol extract of these organelles contained both glucose and aspartate conjugates of CHCA. It is suggested that the chloroplasts represent an important site of action of CHCA. The effects of the naphthenate mixture and the individual naphthenate, cyclohexanecarboxylate on growth show marked similarities. It is possible that the cyclohexanecarboxylic acid may be the chief growth promoting component of the naphthenic acid mixture.

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