BASIC DIFFICULTIES IN TRACER METHODOLOGY 69 



physiological response will be greatly dependent upon such factors as the 

 amount of dietary and carrier iodine and the functional state of the 

 thyroid. 



Peacock et al. (25) reported that human donors who had carried of the 

 order of 4800 disintegrations of Fe^^ (or 40,000 disintegrations of Fe") 

 per minute per milliliter of blood showed no clinical radiation effects 

 when observed over periods of several years. Considerations in regard 

 to the use of radioactive iron in human beings have been reviewed by 

 McFarlane (26). 



Hassett and Jenkins (27), using larvae of yellow-fever mosquitoes, 

 noted radiation effects at concentrations of 0.05 /zc/ml of P^'- in the rearing 

 medium with early instars. Resistance increased with age, but at con- 

 centrations above 5.0 /ic/ml practically no adults emerged. Retardation 

 or inhibition of growth, and death occurred at high concentrations. 

 Mating and egg laying occurred normally in adults reared from larvae in 

 P^' solutions of 1.0 ^tc/ml or less. 



Phosphorus 32 has been and is being widely used for important agro- 

 nomic and plant physiological studies. Early workers observed no differ- 

 ences in the yields of plants grown with and without radiophosphorus, 

 and little consideration was given to radiation effects. In 1949 Russell 

 and Martin (28) reported that barley plants grown in nutrient solution 

 containing 10 juc/liter of P'- showed a reduction in the weight of roots 

 produced which was accentuated at the lower total-phosphate levels. At 

 50 Mc/liter there was also a change in the phosphorus content of the roots. 

 Shoot changes were equivocal. Dion et al. (29) reported no injury to 

 wheat when applications of P2O5 were made at 12, 24, and 48 lb/acre, 

 with P^- being incorporated at 26 /xc/g phosphorus. Russell, Adams, and 

 Martin (30) grew barley in pots containing 1500 g of soil plus the equiv- 

 alent of 8 lb of total phosphorus per acre and varying amounts of P^'. 

 Although neither shoot nor root weight was affected, there was a depressed 

 phosphorus absorption. 



Blume and coworkers (31) have reported extended observations on 

 radiation effects and histologic changes induced in barley plants grown in 

 nutrient solution. It was clear that injury to the plant primarily 

 depended on the accumulation of P'^- in the tissues. Injury to the root 

 by external radiation from the solution was minor. It was shown that 

 the specific activity of the solution was the determining factor rather than 

 the level of P^^ in the solution. Obviously at high specific activities the 

 plant absorbs more P^-. The damage occurred only in zones of active 

 division, and the measurement of arc lengths of shoot-tip cells was used 

 as an indication of radiation damage. Some of these points are self- 

 explanatory from the data in Table 2-2 (32). Evidence was presented 

 for radiation effects at as low as 2 ptc/liter at a total phosphorus concen- 



