Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

J. P. Snow

Second Advisor

J. B. Baker


The phytotoxicity of haloxyfop-methyl $\{$methyl 2- (4- ( (3-chloro-5-(trifluoromethyl)-2-pyridinyl) oxy) phenoxy) propanoate$\}$ is reduced when applied to grasses growing under moisture stress, but the reasons for this reduction are not known. Glasshouse studies were conducted to determine the effect of imposed moisture stress on the phytotoxicity of haloxyfop-methyl; the absorption, translocation, and metabolism of $\sp{14}$C-haloxyfop-methyl; and the partitioning of $\sp{14}$C-photoassimilates in johnsongrass (Sorghum halepense (L.) Pers.) and large crabgrass (Digitaria sanguinalis (L.) Scop.). Foliar applications of haloxyfop-methyl at 30 and 25 g ai/ha to crabgrass and johnsongrass, respectively, resulted in 92% regrowth of moisture-stressed plants and less than 12% regrowth of nonstressed plants. Foliar absorption of $\sp{14}$C-haloxyfop-methyl was reduced by moisture stress at 1, 3, 5, and 24 h after treatment (HAT) in crabgrass and at 1, 3, and 5 HAT in johnsongrass. Absorption was complete in both species by 48 HAT. Movement of the radiolabel out of the treated leaf, as well as acropetal translocation, were inhibited by moisture stress in both species. Basipetal translocation was inhibited between 24-96 HAT in johnsongrass, but no reductions were observed in crabgrass. Recovery of the radiolabel from treated plants was not affected by moisture stress treatments. Metabolism of $\sp{14}$C-haloxyfop-methyl did not appear to be altered by moisture stress. Distribution of the $\sp{14}$C-photoassimilates was affected by moisture stress, but did not closely follow the changes in $\sp{14}$C-haloxyfop-methyl translocation. In a second set of experiments, intermediate water stress treatments were imposed by watering stressed plants 24 or 48 h prior to completion of the moisture stress treatment. These plants were intermediate in susceptibility to a foliar application of haloxyfop-methyl with at least 29 and 42% regrowth, respectively. Absorption of the radiolabel was affected consistently by all 3 moisture stress treatments while translocation and metabolism were less sensitive. Moisture stress clearly reduced the phytotoxicity of haloxyfop-methyl in the two grasses, and the reduction of herbicide activity appears to be partly related to changes in herbicide absorption and translocation patterns.