Effects of direct contact of Pinus resinosa seeds and young seedlings with 2,4-D or picloram on seedling development

Abstract

Effects of 2,4-D and picloram at 50 and 100 ppm on early development of Pinus resinosa seedlings were studied. Both herbicides caused abnormal development of pine seedlings. After seeds germinated, the first conspicuous effect of 2,4-D treatment was cessation of root elongation. Proliferation and expansion of parenchyma cells in the stem and cotyledons were also stimulated by 2,4-D, causing abnormal thickening of seedlings. Disorganization and collapse of parenchyma cells in the upper stem were followed by callus formation. Cotyledons of 2,4-D-treated plants developed abnormally. Formation of vascular strands was retarded. Division and expansion of mesophyll cells were stimulated by 2,4-D causing decrease in intercellular spaces. Numbers of stomata and chloroplasts were lower in cotyledons of treated plants than in controls. Cotyledons of treated plants were fused to primary needles. Expansion of early formed primary needles and initiation and expansion of additional primary needles were inhibited by 2,4-D. Effects of picloram on pine seedlings were somewhat similar to effects of 2,4-D but picloram appeared to be more toxic at comparable dosages.