Absorption and Translocation of 2,4-Dichlorophenoxyacetic Acid and P 32 by Leaves

Abstract

Foliar applications of nutrient minerals and the increasing use of systemic pesticides have served to reemphasize the importance of absorption and trans-location 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 sur-factants 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 Swan-son 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. and on the translation of 2,4-D and p32. The radioactive element has been used as a check on the results with the herbicide. MIATERIALS AND METHODS Soybean seedlings were the main experimental material. Uniform plants growing in pots were used when the unifoliolate leaves were mature but before expansion of the trifoliolate leaves had begun. Treatment with 2,4-D usually consisted of wetting 2/3 of one unifoliolate leaf on each plant with a solution containing 750 ppm of Na-2,4-D and 0.1 % of Triton B-1956. Penetration periods were controlled by washing the treated leaves thoroughly after a specified penetration time, typically two hours. The method of washing with a series of water and detergent solutions has been shown (12) to reduce residual 2,4-D on the leaf below that required for a significant effect on the plant. Translocation time was controlled by removing the treated leaf and its petiole after a specified time. Since significant penetration was obtained in two hours while significant transloca-tion from the treated leaves required more than four hours (cf table II), it was possible to separate the effects of temperature, for example, on penetration and on translocation. The effect of the 2,4-D absorbed (and subsequently translocated) before the treated leaves were washed, or absorbed and translocated before the treated leaves were removed, was measured as a reduction in the growth of the plant parts above the unifoliolate leaves. An experimental unit consisted of two, uniform, treated seedling plants in one four-inch pot. Ten replications, or 20 plants, were commonly used for each treatment. The minimum number of replications was six in a few experiments when a larger number of suitable plants was not available. All data were treated by analysis of variance. Values for least significant differences at the 5 and the 1 % levels (LSDO.05 and LSDo.01) are given in the tables whenever these values aid in the interpretation of the data. Plumule growth of soybean seedlings is variable under the best conditions and this variability is shown in the relatively large values required for least significant differences. The p32 was obtained as a high specific activity solution (approximately 5 mc/ml) of the phosphate ion in a weak HCl solution at a pH of about 4.5. In general, 1 ,ac of p32 was dissolved in 10 ul of solution and applied with a micropipette to the central area of one unifoliolate leaf on each treated plants. Treated leaves were washed or removed at specified periods after applying the p32, as for experiments with 2,4-D. p32 was measured in dried plant briquets by the counting method of 'Mackenzie and Dean (20). When 225