Growth-regulating properties of picloram, 4-amino-3,5,6-trichloropicolinic acid

Abstract

Picloram, 2,4-D (2,4-dichlorophenoxyacetic acid) and IAA (indole-3-acetic acid) cause similar stimulations of polarized cell elongation in stem and coleoptile sections floated on solutions containing the growth regulator. Picloram, like 2,4-D and IAA, supports growth of tissue explants in culture, inhibits root growth, induces cell wall loosening, produces stem curvature and other formative effects, promotes loss of chlorophyll, and regulates abscission and rooting responses. A kinetic analysis of root growth inhibition indicates a common mode of action for picloram and 2,4-D. Picloram meets the minimal structural requirements for auxin activity. The greater herbicidal effectiveness of picloram is attributed to increased mobility and resistance to breakdown within the plant. The ability of picloram to increase markedly the growth of intact shoot cuttings is not shared by either 2,4-D or IAA and neither picloram nor 2,4-D are readily transported in a polar manner in the Avena curvature test. We conclude that picloram functions as a growth regulator of the auxin type with a herbicidal action similar to that of 2,4-D.