Figure 3.

Whole-plant contents of NH₄⁺ (A), soluble N (B), insoluble N (C), and total N (D) derived from endogenous (¹⁴N) sources and from exogenously supplied NH₄⁺ (¹⁵N) during a 3-d continuous exposure of maize seedlings to highly labeled ¹⁵NH₄⁺. Each value is the mean of four replicates ± se. The diurnal rates of ¹⁵NH₄⁺ uptake (in micromoles per gram fresh weight per hour) are noted in D.