SINK CONTROL OF NITROGEN UPTAKE AND ASSIMILATION DURING REGROWTH AFTER-CUTTING OF RYEGRASS (LOLIUM-PERENNE L)

Abstract. The 15N isotope was used to compare the uptake and the assimilation of NH4+ and NO3 nitrogen in ryegrass (Lolium perenne L.) during regrowth after cutting. Uptake of nitrate‐N, expressed per plant, was at all times greater than ammonium‐N uptake and assimilation decreased in roots and stubble while its assimilation was maintained at a high level in leaves. It has been suggested that ammonium assimilation is directly related to the availability of carbohydrates in the sink organ (leaves) resulting from their remobilization from the source organs (roots and stubble). Nitrate reduction decreased in all organs, while the uptake of NO3 was still high. After this first period of regrowth, nitrogen assimilation both from nitrate and ammonium increased in all the plants. Nitrate reduction capacity (expressed in μg NO3‐N reduced per g D.W. per d) is 7.5 and 22.5 times greater in leaves than in stubble and roots, respectively. Therefore, nitrogen assimilation in stubble and particularly in roots was mainly dependent on ammonium nitrogen. Copyright © 1990, Wiley Blackwell. All rights reserved