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.