Studies on Foliar Penetration

Abstract

A further study has been made of the factors determining the level of penetra tion of 2,4-dichlorophenoxyacetic acid (2,4-D) into leaves. The technique involves the use of leaf disks and 2,4-D containing carbon-14 in the carboxyl group. For Phaseolus vulgaris the influence of the pH of the applied solution is greater in the light than in the dark. Between o and 1,000 f.c. at 27o C there is a small increase in the rate of penetration into the abaxial surface. Under these conditions the rates remain constant up to 56 hours. This linear relationship holds for con centrations ranging from 100 to 1,000 mg/1, and the rate of penetration is directly proportional to the concentration. For intensities in excess of 1,000 f.c. the light response is markedly different: over the first few hours there is a steady and relatively slow rate of penetration which is followed by a second phase when the rate is greatly accelerated. This accelerated rate can be reversed by transferring the disks to darkness, and does not take place at Io C. Likewise, if excised disks are left for more than one hour in either the light or the dark before applying 2,4-D, then there is subsequently no phase of accelerated penetration. The course of penetration into the adaxial surface exhibits no accelerated rate, and compared to the abaxial surface the rates are lower. For leaves of Ligustrum ovalifolium, which lack stomata on the adaxial surface, the rates of penetration at 27e C into both surfaces remain constant in either light or darkness. For both the adaxial and abaxial surfaces, the rate progressively increases from o to 2,000 f.c. and there is no phase of accelerated penetration. Penetration into the adaxial surface is less. At Io C the rates for both surfaces in either light or darkness are depressed. If disks of Phaseolus are irradiated with ultraviolet light, subsequent penetra tion is markedly depressed in the light at 27e C, but in the dark or at 1° C it is enhanced. On the basis of these findings it is concluded that both physical and metabolic factors control the rate of penetration of 2,4-D. Transport through the cuticle will be dependent on adsorption and the length of the diffusion paths. Once diffusion gradients have been established between the surface of the cuticle and the outer surface of the cytoplasm in the epidermal cells, the steepness will be dependent on the rates at which 2,4-D is either converted into some metabolite or moved away from the surface or into other cells. The relative importance Journal of Experimental Botany, Vol. 16, No. 46, pp. 24-47, January 1965. This content downloaded from 207.46.13.114 on Thu, 26 May 2016 06:36:40 UTC All use subject to http://about.jstor.org/terms Sargent and Blackman—Studies on Foliar Penetration. 2 25 of physical and metabolic processes will be dependent on the permeability and thickness of the cuticle and the level of metabolic activity. The possible role of ectodesmata in determining both the length and steepness of the diffusion paths is discussed.