Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency

Abstract

Nitrate is reported to improve tolerance of plants towards oxygen deficiency imposed by waterlogging of the root system, but little is known of the mechanism underlying the phenomenon. We studied the metabolism of nitrate in roots under hypoxia, using soybean plants growing in a hydroponic system after suspending aeration and covering the surface of the nutrient solution with mineral oil. Nitrate depletion from the medium was greater under hypoxia than normoxia, but in the presence of chloramphenicol, consumption under hypoxia was significantly reduced. Nitrite accumulated in the medium under hypoxia and this was partially eliminated by chloramphenicol. Nitrate consumption sensitive to chloramphenicol was attributed to bacterial activity. Endogenous root nitrate was strongly reduced under hypoxia indicating mobilization. Although the transport of nitrate to the shoot via the xylem was also reduced under hypoxia, the severity of this reduction was dependent on the concentration of nitrate in the medium, suggesting that at least some of the nitrate in the xylem came from the medium. Root nitrate reductase was also strongly reduced under hypoxia, but recovered rapidly on return to normoxia. Overall, the data are consistent with two main metabolic fates for chloramphenicol-insensitive nitrate depletion under hypoxia: the reduction of some nitrate to nitrite (despite the reduced nitrate reductase activity) followed by its release to the medium (at least one-third of the nitrate consumed followed this route), and the transport of nitrate to the shoot. Nevertheless, it is highly unlikely that these metabolic routes account for all the nitrate consumed.