The role of nitrate and carbohydrates in modulating the partitioning of nitrate assimilation between leaves and roots in perennial ryegrass (Lolium perenne L.)

Nitrate (NO3−) assimilation in perennial grasses occurs predominantly in leaves. A growing body of evidence indicates that NO3− transport to and assimilation in leaves reduces the partitioning of photosynthetic products to roots. Since roots are dependent on leaf derived carbohydrates to support metabolism and growth, concentrating NO3− assimilation in roots may increase their sink activity and growth. The rate-limiting step in the assimilation of NO3− is its reduction to nitrite (NO2−) catalyzed by nitrate reductase (NR). The first objective of the present investigation was to compare the results obtained from the in vivo/in vitro NR activity (NRA) assays and to assess their ability to estimate in situ rate of NO3− assimilation in perennial ryegrass (Lolium perenne L.). The second objective of the present investigation was to determine if NR capacity and/or NR access to NO 3− limits root NO3− assimilation in perennial grasses. The third objective of the present investigation was to confirm that a reduction in photosynthate supply to roots is correlated with increased transport of NO3− to leaves in a perennial grass. We also determined if the capacity of roots to assimilate NO3− is in turn, limited by photosynthate supply from leaves. In grasses grown in 0.01, 0.09, 0.1 or 0.5 mM NO3− the leaves remained the predominate site of NO3 − reduction regardless of assay method used. As the NO 3− supply increased, more NO3 − was transported to leaves before its reduction and assimilation. At external NO3− concentrations above 0.09 mM, it appears that the root capacity for NO3− assimilation becomes saturated in this grass. Reducing NO3 − transport from roots indicated that one of the factors limiting root NRA may be the rate at which NO3− translocation occurs from roots to leaves. Increased leaf NO3− concentrations were inversely proportional to the amount of photosynthate partitioned to roots on a short-term basis as indicated by partitioning of photoassimilated 14CO2, as well as on a long-term basis as indicated by biomass partitioning. Increased carbohydrate supply to roots did enhance the root capacity for NO3− assimilation. These data support the hypothesis that root capacity for NO3− assimilation in perennial ryegrass is carbohydrate limited