Responses of Plants to the Herbicide 3-(p-Chlorophenyl)-1,1-Dimethylurea (CMU)

Abstract

1. Plots of Moore barley and Manchu soybean were treated with aqueous suspensions of CMU at four rates and at five stages of plant development for barley and at three stages for soybean. Tomato plants were treated under green-house conditions with CMU-lanolin paste mixtures applied to various inter-nodes. Roots of sprouting onion bulbs were exposed to aqueous suspensions of CMU. Gross responses were observed, and samples of plant organs were removed at several intervals after treatment for morphological and cytological studies. 2. Gross responses of barley to CMU toxicity involved loss of turgor, chlorosis, and progressive die-back of older leaves. CMU at the rate of 2 pounds per acre killed about 30% of the plants in a plot sprayed at any stage of plant development through early jointing but not thereafter. 3. In treated barley, mitosis appeared to be retarded in apical meristems of the shoot. Fruiting heads were not produced by many plants sprayed at any of the first three stages of development with 2 pounds of CMU per acre. Malformations induced by spraying at these times included tubular leaves, incomplete heading, and abnormally twisted flag leaves. Barley was found to be highly susceptible to CMU at early jointing and at booting, as measured by the formative effects of the chemical. Responses included stunting and retardation in spike development and emergence. Plants treated at early jointing gave rise to spikes with only a few mature grains. 4. Shoot reduction in barley treated postemergence with CMU was accompanied by root reduction, both in length and in dry weight. 5. Soybean was very susceptible to pre-emergence treatment with CMU at rates above 1/2 pound per acre. Stunting was marked at the 1-pound rate, while 2 pounds proved lethal to all plants treated. In soybean and tomato treated postemergence with CMU there were chlorosis and progressive development of necrotic areas in the leaves as well as reduction in growth. When CMU-lanolin mixtures were applied to the internodes of tomato, toxicity symptoms always appeared above the point of application, demonstrating acropetal translocation to leaves. The higher the inter-node to which CMU was applied, the less the total damage to the tomato plant. 6. Morphologically, the toxic response of barley appeared first in the epidermis of leaves at the margins and then progressed to the mesophyll. Mature vascular tissue was relatively undamaged, although "plugging" of xylem vessels was common. 7. In the leaves of tomato and soybean, epidermal cells collapsed first, then palisade tissue became disorganized, and finally the remainder of the mesophyll became necrotic. There was marked reduction of mature xylem and phloem in stems of treated soybean and tomato, indicating less cambial activity. Phloem disorganization and collapse of interfascicular cambium was noted in tomato and soybean, particularly in the latter. 8. Onion root-tip sections from bulbs treated with CMU showed a lack of normal differentiation of the meristem, nuclear breakdown, and disruption of the epidermis in the region of elongation. Mitotic activity in such root tips appeared reduced as determined by examination of Feulgen squashes of the meristems of roots growing in aqueous suspensions of CMU.