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Effects of low levels of 2, 4-dichlorophenoxyacetic acid on the uptake, translocation, and incorporation of P32 by the bean plant (Phaseolus vulgaris) Etter, Harold MacDonald

Abstract

A study was carried out to gain information, which could be used to describe the mechanism whereby 2,4-D affects the growth of intact plants. Fourteen-day-old bean plants (Phaseolus vulgaris) growing in a phosphate-free or complete nutrient solutioi were sprayed to drip with 5 or 50 ppm 2,4-D. At various times up to 70 hours after spraying, the roots were immersed in a complete nutrient solution containing P³². Following an incorporation period of 2-9 hours, the plants were harvested, and separated into roots, stems plus petioles, and leaves. Acid soluble activity, acid insoluble activity, and the distribution of activity within the soluble fraction as revealed by paper chromatography were determined for each organ. In general the treatments had relatively small effects on phosphorus uptake, translocation, or incorporation into organic compounds. Initial increases in the rate of P³² uptake were not maintained and were followed by the same or less uptake as controls. The rate of upward translocation was reduced and less affected by the age of-the plants, as compared to controls, after treatment with 5 ppm 2,4-D. The higher concentration did not alter the distribution of activity among the organs. While the lower level did not change the rate of incorporation into the total acid insoluble fraction of the plant, the growth-inhibitory level reduced the activity in this fraction by 14-22%. At 5 ppm there was a trend toward more incorporation into insoluble compounds in the roots and less in the leaves, but no consistent change in the stems. The composition of the soluble fractions showed no uniform variations from controls at either concentration. The results indicated a pattern whereby the balance in P³² distribution between the leaves and roots was upset in favor of the roots by foliar application of 5 ppm 2,4-D, but not 50 ppm. Both stimulatory and inhibitory levels of 2,4-D produced responses which appear to be related to the synthesis of acid insoluble compounds in the roots and leaves. The major portion of the insoluble activity in these tissues was also ethanol insoluble and is believed to be RNA. Actigrapb scans of picric acid chromatograms indicated more incorporation of activity into organic compounds (nucleotides and sugar phosphates) in the leaf and stem tissues than in the roots. Plants grown in complete nutrient prior to exposure to P³² , took up, translocated, and retained less phosphate than those grown in phosphate-free nutrient,. Also, incorporation into soluble organic compounds was suppressed. The only radioactive compound in xylem exudates from plants 10 hours after their initial exposure to P³² was orthophosphate.

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