Effect of Spray Applications of 2,4-Dichlorophenoxyacetic Acid on Subsequent Growth of Various Parts of Red Kidney Bean and Soybean Plants

Abstract

1. Plots containing young red kidney bean and soybean plants were sprayed with 2,4-dichlorophenoxyacetic acid at rates of 0.1, 0.01, and 0.001 gm. per sq. yd. The effects of the agent on the growth of various plant parts were studied. Linear, and fresh and dry weight measurements were obtained at several intervals following treatment. 2. Hypocotyls and first internodes increased greatly in diameter. The increase in fresh weight was due mostly to increased water content. 3. Primary leaves of treated plants were much heavier than those of controls. Those of red kidney beans remained green and attached to the plants long after those of controls had withered and fallen. 4. Total weights of trifoliate leaf blades, trifoliate leaves, and weight of plants above second node were decreased by the growth regulator. There was good correlation between decrease in weight and increase in amount of compound applied. These criteria would be excellent in studying the relative inhibitory or stimulating actions of growth regulators on red kidney beans. Ten days after treatment there were highly significant differences between controls and treated plants, and between treated plants, when fresh weights of trifoliate leaves or leaf blades were used as criteria. The differences were significant when the fresh weight of kidney bean plants above the second node was used as a criterion. With soybeans, only fresh weight of trifoliate leaf blades gave significant differences between controls and treated plants and between plants that received various rates of compound when plants were harvested 21 days after treatment. 5. The growth in length and width of certain leaflets which developed after treatment was inhibited by the growth regulator. The two higher levels of compound (0.1 or 0.01 gm. per sq. yd.) inhibited the linear growth of certain petioles. The lowest level (0.001 gm. per sq. yd.) stimulated growth in length of a soybean petiole but had little effect on a bean petiole. 6 The fresh weight of tops was decreased by the growth regulator. Heights were usually decreased but differences caused by various treatments were sometimes small. 7. The compound at rates of 0.01 and 0.1 gm. per sq. yd. decreased dry weight of roots of kidney bean, but 0.001 gm. had little effect. With soybeans the two lower rates of application significantly increased weight of roots at 16 days after treatment. However, by 21 days the weight of control roots was about the same as that of the treated plants. 8. With kidney beans there was little effect on the shoot-to-root ratio when two lower amounts of the compound were used. With soybeans, the ratio was reduced by all treatments. 9. The growth regulator delayed the onset and decreased the amount of pod formation.