Date of Award
2003
Degree Type
Dissertation
First Advisor
Richard J. Hull
Abstract
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.
Recommended Citation
Bushoven, John T., "The role of nitrate and carbohydrates in modulating the partitioning of nitrate assimilation between leaves and roots in perennial ryegrass (Lolium perenne L.)" (2003). Open Access Dissertations. Paper 1897.
https://digitalcommons.uri.edu/oa_diss/1897
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