Table II.

Diurnal rates of ¹⁵NH₄⁺ uptake, ¹⁴NH₄⁺ uptake, and ¹⁴NH₄⁺ release by maize roots

Day	Diurnal Period	Length	Total (15NH4+ + 14NH4+) Uptake	True 14NH4+ Release	True Root 14N Releasea
		h	μmol plant−1	period−1	%
7–8	Dark	10	49.8	6.9	4.0
8	Light	14	103.1	11.6	6.7
8–9	Dark	10	63.9	0.2	0.1
9	Light	14	177.3	11.1	6.4
9–10	Dark	10	90.6	5.2	3.0
10	Light	14	193.3	8.3	4.8
Total			678.0	43.3	25.0

Roots of maize seedlings were exposed continuously to highly labeled ¹⁵NH₄⁺ for 3 d. NH₄⁺ flux rates were calculated from periodic analysis of the uptake solution, as illustrated in Table I.

^a14NH₄⁺ release of the total ¹⁴N present in roots harvested on the 7th d (174 μmol ¹⁴N root⁻¹).