Visual Indications of S³⁵ and P³² Translocation in the Phloem

EVIDENCE THAT the phloem is the path of movement of minerals, food substances, and other materials such as vitamins, auxins and viruses in plants has been obtained bv a number of methods (Crafts, 1951). Among the visual methods has been the use of dyes, lithium salts and other substances. Mechanical methods include the separation of the wood and the bark or killing of certain stem areas by steam. Among the metabolic methods are the tracing of sugar from leaves to other areas as reflected by growth and the use of 2,4-D or other growth substances as evidenced by stem curvature. None of these methods, except possibly the first, isolates the exact path of movement. The use of radioactive tracers has introduced new refinements of techniaue which have greatly facilitated translocation studies. Although there has been no visual proof of the exact path of movement the evidence has indicated that the phloem tissue is involved. The present work is an attempt to refine this technique to include a visual method for locating the exact path of movement. MATERIALS AND METHODS.-Red Kidney bean plants were grown in a one-half strength Hoagland solution under controlled conditions of temperature. humidity, and light; namely 23? ? P?C., 60 ? 2.5 per cent relative humidity and fluorescent lighting yielding 1000 to 1200 f.-c. The plants were used twelve days after the hypocotyl straightened, when the oldest trifoliate leaf was fully matured, the second was well expanded and the third just un1folding. Two methods were used for introducing the P32 and the S35 into the plant: first, by introducing the tracer into a vein of the leaf which was severed and a flap cut so that the initial movement was toward the periphery of the leaf and from there through intact veins into the petiole and the stem (Biddulph, 1941); and second, by spraving the tracer onto the underside of the terminal leaflet of the first trifoliate leaf. Twentv-five M1. of solution were used for iniection and fifty PI. for spraying. The pH of the radioactive phosphate and sulfate solutions was approximately 6.0 and the amount of radioactivitv varied between 100 and 720 ec per treatment. The migration period of the radioactive solution veried from one hr. to one hr. and 20 min. depending upon the amount of mobile p32 which was detected in the stem of the plant by the use of a G-M tube with a window thickness of 1.4 mg./cm.2 which was attached to a portable scaler and recording circuit.