Studies in the Physiological Action of 2, 2-Dichloropropionic Acid: I.

The inter-relationships between time and concentration and the degree of inhibition of root elongation have been examined for Sorghum vulgare, Zea mays, Helianthus annuus, and Pisum sativum. For all species the inhibitory effect is cumulative but there is a tenfold difference in the concentration required to halve the elongation of the most sensitive (S. vulgare) and most resistant species (P. sativum). From a comparison of the growth of intacts roots and isolated segments, together with estimates of cell number, it has been established that the primary effect is to interfere with meristematic activity in the root tip, where the mitotic cycle is arrested at prophase. Using 2, 2-dichloropropionic acid, containing chlorine-36, the courses of uptake by both intact roots and isolated segments have been followes. In every instance uptake is cumulative with the greatest accumulation in the root tip. There are again large specific differences but of a reverse order; uptake is least for P. sativum and greatest for S. vulgare. For these two species and Z. mays, it is concluded that the magnitude of the equi-effective concentration required to halve root elognation is dependent on the level of accumulation rather than on the reaction at cell level: the cells of H. annuus are more sensitive.