ABSORPTION AND TRANSLOCATION OF 2 , 4 - DICHLOROPHENOXYACETIC ACID AND p 32 BY LEAVES " 12

Foliar applications of nutrient minerals and the increasing use of systemic pesticides have served to reemphasize the importance of absorption and translocation in plant physiology. Studies of the movement of these materials have an immediate application value; at the same time they can aid in the solution of basic problems in plant behavior. A number of workers (5, 12, 14, 24, 30) have stressed the importance of surfactants in increasing the effects of herbicides applied to leaves. Bryan et al (5) and Hauser (12) showed that the rate of absorption of 2,4-D as well as the total qualtity absorbed was increased by lowering the surface tension of sprays. Some of this effect is due to the better wetting leaves (12, 21) by solutions of low surface tension. However, leaves without surface wax deposits (23) show greater absorption of herbicides when surfactants are added, and additions far beyond those giving minimum surface tension give continued increases in toxicity (24). Weintraub et al (30), for example, found that surfactants increased 2,4-D absorption from alcoholic solutions, even though surface tension was not affected. Bryan et al (5) and Hauser (12) showed that absorption of 2,4-D was accelerated by rising temperatures, indicating that chemical reactions are limiting the process at some point. The work of Bennett (2) suggests types of membrane changes that might be concerned in this response. Biddulph (3) and Swanson and Whitney (25) showed that p32 could be absorbed through leaf surfaces. The latter workers found, however, that this absorption was reduced by the single surfactant tried in their work. The older work on translocation is reviewed by Curtis (8). Mlore recent work, particularly as it applies to the mass-flow hypothesis, is reviewed by Crafts (6). Translocation of organic molecules, up or down, is normally limited to the phloem, while inorganic substances may move either in the xylem or phloem (17). Many workers find that translocation in the phloem is accelerated by temperature (4, 13, 25, 26), although complicating factors of respiration, fixation, etc., enter the picture at temperatures above about 25° C. Leonard (15) and others (18, 29) have shown that sugars may be exported from leaves against steep concentration gradients. In the present research we have rechecked earlier experiments on the absorption of 2,4-D and have added experiments on the absorption of p32 by leaves