Soybean seedlings were grownwith N-free (- N ), NH4and NO3nutrient solutions for 20 days, NHkions were less accumulated in leaf androot tissues and a higher organic Ncontent was found in NH4 grownseedlings (Table 1 ). NO3 ions werelargely accumulated in soybean tissuesgrown with NO3 nutrient solution, accompagnied by a higher dry weight(Table l, Fig. 1 ). The oxidative andphosphorylative properties of the mitochondria isolated from roots and leaveswere measured with malate as substrate. NH4 and NO3 supplies stimulated oxidative activities of both cytochrome pathway and cyanideresistant pathway (Fig. 2). Compared withNOs nutrition, NH4 nutrition resultedin a higher cytochrome pathway activity in root mitochondria and a highercyanide-resistant pathway activity inleaf mitochondria (Fig. 2). The assimilation of NH4 into amino acids wascarried out exclusively in roots and thehigher rate of organic N synthesismight need a higher cytochrome P8thway activity to produce more ATP tosupply the energy needed. The involvement of photosynthetic energy innitrogen metabolism in leaves reducedthe utilization of respiratory energy.That resulted in an increased activityof cyanide-resistant pathway in leafmitochondria. In this case, leaf mitochondria appeared to produce more carbon skeletons than ATP for amonoacid synthesis. NO3 reduction was occurred mainly in leaves that needed alarge amount of mitochondrial NADHas reducing power and less NADH wasoxidized by cyanide-resistant pathway.The higher growth of soybean seedlingswith NOs nutrition was probablylinked to a lower cyanide-resistant respiration in leaves to avoid consumingmore carbon sources.
