Abstract:
Soybean (Glycine ~ (L.) Merr. cv. Harosoy 63) plants inoculated with
Rhizobium japonicum were grown in vermiculite in the presence or absence of
nitrate fertilization for up to 6 weeks after planting. Overall growth of
nodulated plants was enhanced in the presence of nitrate fertilization, while
the extent of nodule development was reduced. Although the number of nodules
was not affected by nitrate fertilization when plants were grown at a light
intensity limiting for photosynthesis, at light intensities approaching or
exceeding the light saturation point for photosynthesis, nitrate fertilization
resulted in at least a 30% reduction in nodule numbers.
The mature, first trifoliate leaf of 21 day old plants was allowed to
photoassimi1ate 14C02. One hour after·· the initial exposure to 14C02, the
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plants were harvested and the 14C radioactivity was determined in the 80%
ethanol-soluble fraction: in. o:rider to assess· "the extent of photoassimilate
export and the pattern of distribution of exported 14C. The magnitude of
14C export was not affected by the presence of nitrate fertilization.
However, there was a significant effect on the distribution pattern,
particularly with regard to the partitioning of 14C-photosynthate between
the nodules and the root tissue. In the presence of nitrate fertilization,
less than 6% of the exported 14C photosynthate was recovered from the nodules,
with much larger amounts (approximately 37%) being recovered from the root
tissue. In the absence of nitrate fertilization, recovery of exported
14C-photosynthate from the nodules (19 to 27%) was approximately equal to
that from the root tissue (24 to 33%).
By initiating- or terminating the applications of nitrate at 14 days of
age, it was determined that the period from day 14 to day 21 after planting
was particularly significant for the development of nodules initiated
earlier. Addition of nitrate fertilization at this time inhibited further
nodule development while stimulating plant growth, whereas removal of
nitrate fertilization stimulated nodule development. The results obtained
are consistent with the hypothesis that nodule development is inhibited by
nitrate fertilization through a reduction in the availability of photosynthate
to the nodules.
