Table III.

Carbohydrate required for ¹⁵NH₄⁺ assimilation by maize roots

Day	Photoperiod	15NH4+ Assimilation	Carbohydrate
			Requirementa	Change in shoot	Change in root	Transport to rootb
		μmol N plant−1		μmol C plant−1
7–8	Dark	39  ± 4	117  ± 13	−319  ± 7	−57  ± 10	60  ± 15
8	Light	102  ± 18	310  ± 53	+556  ± 35	+61  ± 7	371  ± 55
8–9	Dark	91  ± 11	275  ± 34	−550  ± 12	−103  ± 6	172  ± 37
9	Light	110  ± 24	333  ± 74	+590  ± 67	+42  ± 11	375  ± 64
9–10	Dark	120  ± 28	365  ± 84	−501  ± 24	−38  ± 13	327  ± 97
10	Light	288  ± 30	872  ± 91	+1193  ± 147	+118  ± 21	990  ± 108

The diurnal assimilation of ¹⁵NH₄⁺ was measured during a 3-d exposure of maize seedlings to highly labeled ¹⁵NH₄⁺. The minimal transport of C from shoot to root required to support ¹⁵NH₄⁺ assimilation was calculated from the theoretical C requirement for ¹⁵NH₄⁺ assimilation and the changes in tissue carbohydrate. Each value is the mean of four replicates ± se.

^aCalculation of the total C requirement for ¹⁵NH₄⁺ assimilation is based on the assumption that assimilation of entering ¹⁵NH₄⁺ occurs in the root, and that the initial product of NH₄⁺ assimilation is Gln, which requires 0.505 mol of Glc equivalents mol⁻¹ NH₄⁺ to provide the necessary reductant, ATP, and C skeletons (Schubert, 1982). 

^bMinimal C transport to the root required to support ¹⁵NH₄⁺ assimilation is equal to the total C required for ¹⁵NH₄⁺ assimilation plus the change in root C content.