increased by nitrate. Manganese concentration in the shoots was great- 

 er with the nitrate treatment. Phosphorus, zinc, copper, and iron 

 concentrations were not significantly affected by nitrogen source. 

 Significant changes in mineral composition result from the two sources 

 but whether these were instrumental in the growth effects is presently 

 unknown . 



c. Experiment 5 . After germination, plants were transferred to 

 nonaerated ammonium solutions (Table 1} for a 6-week growth period. 

 The plants were grown under growth chamber conditions. At the end of 

 6 weeks, the plants were transferred to a similar nutrient solution 

 lacking nitrogen for 5 days. 



A 21-hour uptake period with four treatments was initiated. The 

 treatments were ammonium-nonaerated, ammonium- aerated, nitrate-non- 

 aerated, and nitrate-aerated. Each treatment was replicated twice 

 with three plants per replicate. Three plants were placed in 50- or 

 100-milliliter containers in which an initial 0.5-millimole ammonium 

 or nitrate concentration was present. The solution concentration was 

 then measured and renewed at different time intervals throughout the 

 21-hour uptake period. The nitrogen sources were labeled with 98-per- 

 cent ^N in all uptake solutions. Ammonium was provided as -^^NH^Cl 

 and nitrate as K^^N03; the only other nutrient present was 1 x 10 '^ 

 M CaSOi^. The uptake solutions were continuously stirred with magnetic 

 stirring bars. Lights were maintained at 3,000-foot candles through- 

 out the uptake period. 



At the completion of the uptake period the plants were removed, 

 rinsed in distilled water for 1 minute, and the shoots were separated 

 from the roots. Analyses for carbohydrate, protein, GDH, NR, and 

 endogenous NHi^ and NO3 were determined on a part of the tissue. An- 

 other part was then separated into a soluble and an insoluble fraction 

 using a methanol : chloroform: water (13:4:3) procedure, and the incor- 

 poration of -^^N into the soluble and insoluble fractions were deter- 

 mined. 



Ammonium uptake exceeded nitrate uptake at the end of the experi- 

 ment by a factor of 4 for nonaerated solutions and 5 for aerated solu- 

 tions (Fig. 26). The rate of ammonium uptake was high initially and 

 then decreased to a linear rate after 6 hours. This high initial rate 

 was probably due to the 5-day minus-nitrogen pretreatment period which 

 decreased plant nitrogen reserves. Nitrate uptake was very low for the 

 first 6 hours (nondetectable in the first 2 hours) but the rate then 

 slowly increased throughout the uptake period. The data suggest that 

 nitrate uptake was substrate-inducible. Aeration slightly decreased 

 uptake from ammonium and nitrate solutions but did not change the 

 general uptake pattern. 



Roots exposed to K-^^NOs lost sodium to the ambient solution during 

 the uptake period; the cumulative loss (Fig. 27) showed a high initial 



50 



