Physiological and Morphological Responses of Barley to 2,4-Dichlorophenoxyacetic Acid

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Abstract
An early and a late var. of barley were treated with 1 lb. acid equivalent of a butyl ester of 2,4-D at 9 stages of growth. The most susceptible period of growth occurred before the 5-leaf stage, when the differentiation of tiller buds was inhibited by the appln. of 2,4-D. The number of tillers was reduced, the number of spikes decreased and yield greatly depressed. A relatively tolerant 2d period of growth extends from the 5-leaf to the early boot stage. The heavy appln. of 2,4-D inhibited floral initiation, decreased the number of seeds per spike and lowered the yield. A relatively susceptible 3d period exists between the pre-heading and late-heading stages. Yield reductions were due to a decreased number of seeds, which may have been the result of an inhibition of embryo sac and gamete development, or an increased vegetative competition as the result of stunting the central culm. The most resistant period of growth in barley is a post-heading period when relatively heavy applns. of 2,4-D did not decrease the yield. The frequency of morphological malformations was increased by the appln. of 2,4-D. Seed viability was not impaired by a heavy rate of 2,4-D at 9 stages of growth.

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