Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

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Abstract
During vegetative growth in controlled environments, the pattern of distribution of 14C-labelled assimilates to shoot and root, and to the meristems of the shoot, was measured in red and white clover plants either wholly dependent on N2 fixation in root nodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot were exposed to 14CO2, nodulated plants of both clovers generally exported more of their labelled assimilates to root (+nodules), than equivalent plants utilizing nitrate nitrogen, and this was offset by reduced export to branches (red clover) or stolons (white clover). The intensity of these effects varied with experiment. The export of labelled assimilate to growing leaves at the terminal meristem of the donor shoot was not influenced by source of nitrogen. Internode elongation in the donor shoot utilized no labelled assimilate. Whole plants of white clover exposed to 14CO2 on seven occasions over 32 days exhibited the same effect on export to root (+nodules), which increased slightly in intensity with increasing plant age. Nodulated plants had larger root: shoot ratios than their equivalents utilizing nitrate nitrogen.