Absorption and Translocation of Ammonium 2,4-Dichlorophenoxyacetate by Bean Plants

Abstract

1. Young Red Kidney bean plants were treated by applying a known amount of ammonium 2,4-dichlorophenoxyacetate [NH4(2,4-D)] to the base of the midrib on the upper surface of one primary leaf. The treated leaves were cut off at the bases of the petioles at various intervals (4, 10, 24, 48, and 72 hours) after application so as to interrupt further translocation of the growth-regulator from the leaf to other parts of the plant. Fresh weights of the expanded trifoliate leaf blades on treated and control plants 9 days after application of the salt were used in evaluating the relative amounts of growth inhibition which resulted for the various time intervals during which its translocation from the leaf was possible. The average degree of stem curvature at the time of clipping the treated primary leaves was also used as a supplementary indication of the amount of such translocation. To determine how much NH4(2,4-D) was apparently absorbed by each leaf, the treated area was washed volumetrically with distilled water. The washings were tested for content of the salt by a slight modification of the method developed by Bandurski (1) The amount left on the surface of a treated leaf at any time and thus measured was subtracted from the amount applied to give an approximate measure of the amount absorbed. 2 Much of the absorption of NH4(2,4-D) from a wholly aqueous solution by the leaf occurred within the first 4 hours after application. The addition of 0.5% by weight of Carbowax 1500 to the solution resulted in the salt being absorbed continuously during the period of treatment (72 hours). 3. The amount of the salt absorbed was positively correlated with temperature values during treatment, both with and without Carbowax 4 There was no significant difference between the amounts of the salt absorbed at 100 foot-candles of light and at 900 foot-candles at the same air temperature, the amount absorbed in the dark was significantly greater. 5. The fresh weight of the trifoliate leaf blades of plants treated in the dark was essentially the same as that of the corresponding controls, and no stem curvature occurred. In addition, the treated primary leaves folded upward along the midrib and became wavy. These facts suggest that, though there may have been movement of NH4(2,4-D) within the treated leaf, no translocation from the treated leaf occurred in complete darkness. 6. The relative fresh weights (expressed as a percentage of the average of the controls) of the trifoliate leaf blades of plants treated at 100 foot-candles of light were considerably greater than those of plants treated at 900 foot-candles, even though the amounts of the growth-regulator absorbed were the same The average degree of stem curvature was considerably higher in the latter series. Moreover, the primary leaves treated at the lower intensity tended to become wavy, similar to those in the dark but in lesser degree, while those at 900 foot-candles were not visibly affected. All these facts suggest that the rate of translocation of NH4(2,4-D) from the leaf in the kidney bean is positively correlated with light intensity prevailing during treatment, at least for the lower intensities. 7. The available data do not indicate any effect of Carbowax 1500 on translocation of NH4(2,4-D) from the treated leaf. 8. Data concerning the effects of temperature on the rate of translocation of the growth-regulator were inconclusive.