Studies on the Absorption, Translocation, and Action of CMU

Studies with 3-(p-chlorophenyl)-1, 1-dimethylurea (CMU) demonstrated that the action of this chemical is very different from that of 2,4-dichlorophe-noxyacetic acid (2,4-D). In the form used (80% wettable powder) CMU was much less volatile than the isopropyl and butyl esters of 2,4-D and less selective. At relatively low concentrations CMU killed young corn plants as readily as velvet bean. No cellular proliferation or modifications were induced except for the collapse of the softer tissues which was apparently associated with wilting of the stem. The action of this chemical is very slow, no visible responses being obtained for several days after application. 2. Entry of CMU took place readily through leaves, stems, or roots. Entry into the leaves was slower through surfaces covered with a thick cuticle, but the presence of stomates had no apparent effect. Movement of CMU in the leaf was most rapid between the veins and appeared to be blocked by the larger veins. Velvet bean plants were killed by an immersion of one-half of the root system in 200-p.p.m. CMU solution for 15 minutes, demonstrating that entry and translocation were very rapid in the roots. Movement of the CMU was primarily toward the shoot apex, and experimental evidence indicated that the path was through the xylem and the transpiration stream. Conditions favoring reduced transpiration resulted in less CMU injury. Excision of the xylem prevented passage of CMU, but excision or burning of the phloem did not prevent the movement toward the apex. 3. Excised roots were grown successfully for 3 months in sterile nutrient cultures containing CMU in amounts sufficient to kill intact plants in 2 weeks, indicating that the toxic action of CMU is primarily on the aerial portion of the plant.