rate which decreased with time. Conceivably, sodium efflux may have 

 been passive since the roots were exposed to KNO3 solutions devoid of 

 sodium during the uptake period. Furthermore, nitrate uptake was low 

 particularly in the early phases and most likely did not influence 

 sodium efflux. A slight potassium efflux occurred in the first 4 hours 

 (Fig. 27) ; however, a net influx of potassium was apparent by the end 

 of the uptake study. 



During ammonium uptake from N-^^Ht^Cl solutions, both sodium and 

 potassium efflux were apparent (Fig. 28). Sodium efflux was greater 

 than potassium efflux, and aeration decreased efflux of both ions. 

 Electrical neutrality seems to have been maintained through hydrogen 

 ion excretion (pH of external solution decreased) and by the efflux 

 of both sodium and potassium. 



Total nitrogen and ^^N in the tissue at the end of the 21-hour up- 

 take period are shown in Table 14. Although all plants were pre- 

 treated in nonaerated ammonium solutions (unlabeled ^'*NHt^) , some vari- 

 ability in the nitrogen status can be noted. The uptake of labeled 

 ^^N comprised approximately 10 percent or less of the total nitrogen 

 in the plants. Of this total nitrogen (^"^N + ^ ^N) , approximately 65 

 percent was in the shoots. Also, a higher proportion of the total 

 nitrogen was present in the insoluble fraction in the shoots. 



Of the total ^ ^N taken up by the plants, the greatest proportion 

 was transported to the shoots (39 percent) when the nitrogen form in 

 the uptake solution was ammonium (Table 14) . Nonaerated nitrate treat- 

 ments resulted in only 26 percent of the total -^^N being transported 

 to the shoots. Aerated nitrate treatments resulted in even less trans- 

 port to the shoots (10 percent). Thus, aeration seemed to affect 

 translocation when the nitrogen uptake form was nitrate but did not 

 affect it when the form was ammonium. The proportion of ^^N incor- 

 porated into the insoluble fraction was greatest in the shoots with 

 ammonium treatments but there was little difference between shoots and 

 roots with nitrate treatments. Aeration appeared to decrease ^^N in- 

 corporation into the soluble fraction of the shoots with the nitrate 

 treatment. 



The rapid uptake of ammonium substantially enhanced root GDH under 

 nonaerated conditions but, with aeration, activity was depressed (Table 

 15). The depression occurred in spite of a higher ammonium accumulation 

 in the roots of the aerated treatment. There was a slight increase in 

 root GDH activity of both nitrate treatments but shoot GDH was not 

 affected by any of the four treatments . Protein content was not 

 changed, probably because the uptake period (21 hours) was not long 

 enough for a significant effect to be noted, Soluble carbohydrate 

 content was unaffected. 



Nitrate reductase activity was essentially nil at the start of the 

 experiment and in those tissues exposed to ammonium uptake solutions 



53 



