Factors Affecting Absorption and Translocation of Foliar Applied Phosphorus.

Abstract

Phosphorus is known to be exported from leaves of apple (8, 9, 10), bean (4, 6, 11, 12, 13), chrysanthemum (1, 2), corn (11, 12), tomato (12) and squash (71). The present work with bean plants compares export following application by spray, leaf vein injection (4) and droplet (13) methods. Other factors investigated which affect absorption and subsequent translocation of the spray-applied phosphorus include: 1) wetting agents, 2) phosphorus concentration of the spray, 3) leaf surface (upper vs lower surface), 4) different phosphorus compounds (pH and cation), 5) time, 6) hygroscopic agent, 7) size of area sprayed, 8) age and position of sprayed leaf, and 9) phosphorus level of the plant. METHODS Red Kidney bean plants grown in one-half strength Hoagland solution under fluorescent lights on a 6:00 A.2I. to 6:00 P.NI. day were used throughout. The light intensity, temperature and relative humidity were 1000 ft-c (as measured halfway up the stem), 240 ± 1° C and 60 % + 4 %, respectively. At least two plants were used for each treatment. To obtain more uniform plants, the axillary buds were removed before vascularization was pronounced, 2 and 3 days before treatment. The phosphorus solutions containing 0.2 ml of P32-labeled phosphorus of known specific activity were sprayed onto the upper surface of the terminal leaflet of the first trifoliate leaf, unless stated otherwise , between 9:00 and 11:00 A.NM. when the plants were 18 days old (12 days after the hypocotyls had straightened). To prevent contamination of the remainder of the plant the leaflet receiving the spray was enelosed in a clear plastic box which was removed several minutes after treatment. Except in the time eourse of uptake experiments, the plants were harvested and sectioned 24 hours after the application of the tracer. Usually each plant was sectioned into three parts: 1) the sprayed leaflet, 2) the remaining two leaflets and petiole of the first trifoliate leaf, 3) and the remainder of the plant. In some cases the remainder of the plant was divided at the node of the first trifoliate leaf to obtain the upward and (lownward movement. The amount of phosphorus applied was determined by summation. The percentage of applied phosphorus which moved into the nutrient solution was negligible (about 0.03 %) and was not determined in most experiments. 1 Received revised manuscript May 14, 1957. 2 This investigation was carried out under U. S. Atomic Energy Commission, Division of Biology and Medicine, Contract No. AT (45-1)-213. Presented at the Seattle meeting of the A.A.A.S., June 12, 1956. 3 To be submitted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. When autoradiograms were made from quickly dried, pressed whole plants using no-screen x-ray film, the exposure, development and reproduction were uniform so that direct comparisons could be made. For radioactive assay the various plant sections were wet-ashed in nitric acid, made to volume and the activity determined on aliquots dried on porcelain counting discs using an end-window GM2I tube. The results were expressed in micrograms of phosphorus or in percentage of the applied phosphorus which was translocated from the sprayed leaflet. The activity applied per plant was between 5 and 20 ,uc and was constant within each experimental run. p32 labeled phosphorus will be referred to as P*. RESULTS AND DISCUSSION COMPARISON OF THREE APPLICATION-METHODS: Leaf vein injection, droplet, and spray application methods were compared (fig 1) by using a recording rate meter in conjunction with a GM tube (5 mm2 opening) positioned on the stem 2.5 cm below the node of the treated leaf. The volume of solution used, the number of microcuries of p32 employed, and the pH of the solution applied are given in the legend. Figure 2 shows an autoradiograph of treated leaflets (droplet application) from plants represented in tracings 14, 15, and 16. It is obvious that after entry the tracer was carried toward the periphery of the leaflet before entering the phloem for export. The droplet method, to be successful at all, required a pH of approximately 2.0, which produced necrosis at the place of application. For routine use the spray application proved superior. It gave only a slightly lower concentration of tracer in the stem within a minimum migration period than the injection method, produced no visible injury (except at pH extremes), and it is the method which is best adapted for field use. EFFECT OF WETTING AGENTS: The following wetting agents were tested at a concentration of 0.05 % of the active ingredient, except where stated otherwise , with spray solutions of 10 mM NaH2P*04: 1) anionic type, Igepon T-77 (sodium n-methyl-n-oleoyl taurate), Igepon T-78 (ester of sodium isethionate), Tergitol 7 (sodium heptadecyl sulfate), Vatsol OTB (dioctyl ester of sodium sulfosuccinic acid), Vatsol OS (isopropyl naphthalene sodium sulfonate) and 0.5% Dreft (sodium dodecyl sulfate); 2) cationic types; Vatsol C-61 (ethanolated alkyl guanidine am-ino complex); 3) nonionic types; Vatsol NI (nonyl phenol + 9 moles ethylene oxide) and 0.5 % Carbowax 4000 (polyethylene glycol). None increased and only two, Tergitol 7 and Vatsol OTB, significantly decreased the percentage of phosphorus translocated from the sprayed leaflet in the 24-hour interval.