150 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



solution was introduced rapidly, the vessel sealed with a glass plate, and 

 the contents agitated gently throughout the tagging period. At termina- 

 tion, the Hghts were extinguished, and the plants were killed by 5 min 

 maceration at top speed in the Blendor and were then prepared for frac- 

 tional analysis. 



Plant Injection. A detailed review of methods of plant injection has 

 been compiled by Roach (40). The procedures, although designed for 

 general plant physiological and nutritional studies, may be useful for 

 radioisotope administration and are therefore briefly outhned. In botan- 

 ical work the term injection refers to the introduction of a liquid into a 

 plant through a cut or hole in an organ or by other methods without 

 wounding, even though no force is used. This is contrasted with the 

 medical or veterinary use of the term to indicate the forcing of a fluid into 

 the body. Consideration of the point of application in relation to plant 

 anatomy and the subsequent transport of material is treated excellently 

 by Roach (40) and is beyond the scope of this text, so that only the 

 mechanics of making the solution available to the plant will be covered. 



Interveinal Leaf Injection. A small incision is made in the leaf at a 

 chosen point. The liquid is contained in a reservoir and is drawn up a 

 filter-paper wick or a darning-cotton wick which passes through and 

 makes contact with the lips of the incision. The wick can be sealed into 

 the bottom of the reservoir with paraffin wax and can thereby support the 



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Leaf-tiv Injection. The tip of a leaf or leaflet is cut off at right angles 

 to the midrib and immediately submerged in the liquid to be injected. 

 The liquid is drawn in through the cut edge of the leaf as a result of tran- 

 spiration and travels along the veins and veinlets to replace the tissue 

 transpiration losses. 



External Application to the Leaf. It is often of interest to apply radio- 

 active materials to the leaf and measure the rate of translocation to other 

 parts of the plant. It is difficult, however, to accomplish quantitative 

 administration. Swanson and Whitney (41) overcame this difficulty by 

 application of the radioisotope to the leaf surface (usually a 0.01-ml drop) 

 and calculation of movement from a ratio of the activity at some distant 

 part to that in the petiole of the treated leaf. The results showed that 

 a reliable measure of rate of transport was obtained even though there 

 was considerable variability between plants as to the actual amount of 

 isotope absorbed into the leaf. With P^'^ it was found that a pH of about 

 3 gave good absorption and that the uptake was greatly diminished with 

 solutions of pH higher than 4. 



Leafstalk Injection. This method was used very satisfactorily with 

 apple" trees (40). The leaf is cut off so as to leave a short length of stalk 

 which is connected by rubber tubing to the drawn-out end of a test tube. 



