Factors influencing the herbicidal activity of dimethyl 2,3,5,6-tetrachloroterephthalate

Studies were conducted to determine the site of DCPA toxicity on annual ryegrass. This was done by means of a technique which used pyrex tubing and a vapor barrier to separate exposure of coleoptile and roots of emerging grass seedlings to various concentrations of DCPA. Coleoptiles that emerged through treated soil were completely killed, but when only roots were exposed to treated soil, little shoot reduction was attained. The toxicity exerted through the roots was investigated by exposing only roots to DCPA-treated soil at concentrations ranging from 0 to 128 ppm. There was no significant shoot reduction when roots were exposed to concentrations as high as 128 ppm. Root growth, however, was greatly reduced when roots grew in soils treated with DCPA at concentrations of 4 ppm and greater. There was a direct correlation between phytotoxicity and the depth of treated soil through which the coleoptile emerged, which might indicate greater efficiency would result from shallow incorporation under field conditions. There was also a direct relationship between the extent of phytotoxicity and the concentration of DCPA in the soil through which the coleoptile emerged. These two correlations indicate that the toxicity exerted was directly related to a given quantity of DCPA absorbed. The lethal dosage could be accumulated either by long exposure to low DCPA concentrations or short exposure to high concentrations. DCPA seems to be an inhibitor of normal mitotic cell division. Enlargements occurred in all areas of meristematic tissue. The swollen area appeared to be due to excessive cell proliferation. DCPA at sub-lethal doses served as a growth stimulator causing in creased growth in both roots and shoots. Optimum growth stimulation occurred at a 72°F. temperature. The influence of soil temperature on DCPA toxicity to annual ryegrass was investigated by means of a temperature-controlled water bath in the greenhouse. DCPA was slightly more toxic at 84°F. and 60°F. temperature levels as compared to a medium temperature of 72°F. which would indicate a variation in plant-chemical response rather than chemical property differences. Plants growing in warm soil treated at 2 parts per million were able to overcome early evidence of toxicity better than those growing in cooler soils. Studies conducted on DCPA degradation in the soil indicated that DCPA was broken down faster at 90°F. than at 70°F. There was no detectable breakdown at the 50°F. temperature level. The halflife of DCPA at 90°F. was 105 days, and 155 days at 70°F