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Plants growing on differ in the proportion of reduced in their roots and leaves. Using a version of a two-compartment, two-resource plant growth simulation model, the possible effects of changing the N assimilation site on plant growth were explored. Subsequently, using an experimental system designed to vary the site of reduction; chimeras (reciprocal grafts) of Pisum sativum var. Juneau and its mutant, A317, deficient in nitrate reductase. Plant growth and N metabolism at varying levels were examined in a hydroponic system. In what is thought to be the first direct plant manipulation of this kind, total dry weight, root:shoot ratio, total N and concentrations, in vitro nitrate reductase activity, in xylem exudates, and leaf chlorophyll content were compared. It was found that changing the location of nitrate reduction did not change growth significantly and caused only subtle changes in N concentration. These indifferences occurred in spite of marked changes in growth associated with availability and useability. To achieve this kind of plasticity the plants must have dramatically altered the flow and storage of and reduced N. The results are compared with existing views of biomass partitioning and regulation of N assimilation.
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