Formative Effects of Certain Substituted Chlorophenoxy Compounds on Bean Leaves

Abstract

The development of normal leaves of bean was compared with the development of leaves of plants treated with substituted phenoxy compounds. In control trifoliate leaves, the lamina was developed by the activity of a single row of sub-epidermal cells at the lateral margin of the midrib. Four internal layers of plate meristem were produced. The adaxial layer developed into the palisade; the other 3 layers produced spongy mesophyll. Veins of the minor vascular network were initiated by divisions of a row of cells in the layer beneath the embryonic palisade. Additional cells and layers became involved. Treatment of young plants with 0.5% 2-chloro-phenoxyacetic acid in Carbowax 1500 inhibited the formation of intercellular spaces in the mesophyll but vascular tissue retained the normal histologieal appearance. Laminae of fully developed leaves were darker green and smaller than controls, and had more prominent major veins, less prominent minor veins. Application of 0.5% 4-chlorophenoxyacetic acid inhibited the activity of plate meristem. As a result, veins were approximate and the parenchyma surrounding them became continuous although vascular elements remained discrete. Fully developed leaves were small and had narrow straplike leaflets. The fasciated veins formed a wide stripe in the center of the leaflets and chlorenchymatous tissue was confined to the narrow rolled margin. The earlier leaves produced on plants treated with 0.5% 2,4-dichlorophenoxy-acetic acid, in either lanolin or Carbowax as the carrier, were similar to leaves induced by treatment with 2-chlorophenoxy -acetic acid. In later leaves, the external form and structure resembled that found in leaflets of plants treated with 4-chlorophenoxyacetic acid.