AbstractsChemistry

Penetration has long been recognized as an important factor in the action of chemicals on plants. The effectiveness of herbicides on such plants as blackberries is often thought to be limited by the failure to obtain adequate penetration through the cuticular barrier. A number of factors, among them relative humidity, temperature, polarity of the compound, additives and physiological factors of the plant are known to be important. This study was concerned with obtaining more precise estimates of the quantitative relationships of temperature, humidity and other factors in the penetration of picloram and 2, 4, 5-T into plants. Measured quantities of chemicals were placed on the adaxial surface of the leaves of beans held under carefully controlled conditions of temperature, humidity, and light intensity and the amount of chemical penetrating the plant measured. Radio labeled compounds were used and the difference between the amount applied and that removed by washing at predetermined intervals, was taken as the amount penetrating the plant. It was found that the more nonpolar derivatives of these acids penetrated most readily. The addition of surfactants also enhanced the rate of penetration. Substantial differences in the effectiveness of different surfactants in enhancing penetration were observed. These differences can be accounted for in part by the lowering of surface tension but other chemical factors relating to the surfactant appear to be important. Further the mixture of four parts of 2, 4, 5,-T and one part picloram resulted in a significant increase in the rate of penetration of 2, 4, 5-T. The quantitative relationship of relative humidity and temperature in penetration was also determined. As either temperature or relative humidity or both increase, the rate of penetration for all chemical derivatives increased. Analysis of the quantitative relation of penetration revealed the overall process to follow a first order rate law. There is in fact, two or more rate processes each appearing to follow first order rate laws and in overall obeys a first order rate model. Application of Arrhenius equation to these data yields reproducible values of energy of activation. The energy of activation may be derived either directly from thermal energy available or an increase of the entropy of the system. The data suggest involvement of the process of diffusion and also the process of partitioning. Diffusion coefficients in waxes of various substances were determined and partition coefficients calculated. The understanding provided by this study should be of value in overcoming problems of penetration encountered under field conditions. Application of this type of data would permit more intelligent selection of the proper adjuvants or a more suitable mixture of chemicals to enhance penetration.