6 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



culation, is usually determined by measurement of the time elapsing from 

 injection of a substance into the blood at one point to its detection at 

 another point. Obviously, the in vivo type of measurement is ideal for 

 studies of this nature, many of which have been reviewed by Strajman 

 and Pace (41). Earher workers used radium C (Bi-^^). However, Na^^ 

 is now usually preferred. In general, faster circulation times were found 

 for children, and slower circulation times were found for patients suffering 

 from cardiac diseases and peripheral vascular diseases. A reduction in 

 circulation time was also found during the digestive period and during 

 exercise and low environmental temperatures. Another useful procedure 

 for the evaluation of pathologic and therapeutic variables on blood cir- 

 culation has been the measurement of the rate of accumulation of radio- 

 isotopes in the extremities, hands or feet. For measurement of peripheral 

 flow, the clearance of Na^^ or P^^ from tissues is often emplo3'^ed. Radio- 

 phosphorus has been used for the investigation of fetal circulation (42) 

 and body-fluid circulation in insects (43). 



In plant studies the rate of translocation of various elements and the 

 effect of such treatments as temperature variations, use of respiratory 

 poisons, and killing of the stems by steam have yielded pertinent informa- 

 tion as to theories and mode of nutrient transport. Chen (44), for exam- 

 ple, described the simultaneous movement in the phloem tissue in oppo- 

 site directions of C'^-labeled organic solutes and P^Mabeled minerals, 

 which is contrary to theories based on a unidirectional mass flow. Studies 

 on the influence of petiole temperature on the rate of distribution of foliar- 

 applied P^^, K^^, Ca*^, and Cs'" have been described by Swanson and 

 Whitney (36). When the isotopes were studied simultaneously in pairs, 

 the results indicated independent rates of export. Translocation from 

 the leaf was markedly inhibited by both low and high petiole tempera- 

 tures, and killing a 5-mm zone on the petiole by steaming completely 

 stopped the outward movement of P^^, K"*', and Cs^" from the leaf. 



Care must be taken in the interpretation of any study in which the 

 radioactivity is administered at some location in a system and the time 

 measured for it to appear at another site. The appearance of the first 

 trace of the injected substance cannot be observed since generally the 

 concentration will be lower than the detectable limit of measurement. 

 Thus the measured time of arrival will be a function of the amount of 

 radioactivity used, the sensitivity of the measurement, and the volume 

 of circulating fluid with which the isotope becomes diluted. 



SOME QUANTITATIVE ASPECTS 



Many of the procedures discussed so far do not require more than the 

 qualitative detection of radioactivity. Since the amount of radioactivity 



