THE LEVELS OF NITRATE REDUCTASE AND MOCO IN ANNUAL RYEGRASS AS AFFECTED BY NITRATE AND AMMONIUM NUTRITION

The influence of NH4+ And NO3- on the activity of nitrate reductase(NR ; EC 1. 6.6.1) and the level of the molybdenum cofactor (MoCo) was stu died in annual ryegrass (Lolium multiflorum cv Westerwoldicum). Additi on of 4.5 mM NH4+ to plants grown with NO3- Increased NR activity in t he shoots, but decreased it in the root. Total MoCo increased in both roots and shoots. Plants grown with only NH4+ increased their MoCo con tent with N concentration in the medium, while NR activity remained at constitutive levels. NR activity, total MoCo content, readily availab le MoCo (determined without a heat pretreatment) and NO3- accumulation increased in plant root and shoot within 1 day in NO3-. NH4+ triggere d the increase of total and readily available MoCo content in shoot an d root above the constitutive level after 1 day of induction. MoCo bio synthesis may be regulated by NH4+ end NO3- or one of their common ass imilation products. N-regulation of MoCo in the roots differs from the shoots in that: (a) total root MoCo increased proportionally more tha n NR in plants grown with NO3-, while total shoot MoCo and NR activity levels were quite similar; and (b) The NH4+ induced increase in MoCo was proportionally higher in roots than in shoots, suggesting a higher demand of MoCo in the root for the biosynthesis of molybdo-enzymes ot her than NR. (C) 1998 Elsevier Science Ireland Ltd. All rights reserve d.