Table II.

Predicted and observed ¹³C enrichments of malate, reflecting dual pathways of synthesis in oxygenated [1-¹³C]Glc-labeled maize root tips

Ca	Pathway
	TCA only (Glu → malate)b	PEPC only (PEP → malate)b	PEPC + TCAc
	%
C1	9.97  ± 0.99	5.8	7.42  ± 1.6
C2	22.3  ± 1.82	12.8  ± 1.1	17.47  ± 1.2
C3	22.3  ± 1.82	12.8  ± 1.1	17.54  ± 1.9
C4	9.97  ± 0.99	5.8	6.31  ± 0.62

Predicted enrichments of malate synthesized exclusively via the TCA cycle or PEPC were deduced from ¹³C enrichments of Glu and PEP, respectively. ¹³C of PEP was determined as described by Dieuaide-Noubhani et al. (1995). The enrichment of carboxyl groups of malate synthesized exclusively from PEP via PEPC was deduced from enrichment of ¹³CO₂ that would be generated from respired Glc-6-P. Values are means ± sd (n = 4).

^aAll four C's were used to measure in vivo PEPC activity (see text). 

^bPredicted. 

^cObserved in vivo.