Summary
Symbiotic N2 fixation, NO −3 assimilation and protein accumulation in the shoots were measured simultaneously in alfalfa (Medicago sativa L.) grown in the field or in pots, in order to study how the balance between the two modes of nitrogen nutrition could be influenced by agronomic factors, such as harvest, mineral nitrogen supply and drought stress.
During periods of rapid growth, fixation and assimilation may function simultaneously; they are antagonistic at the beginning and at the end of the growth cycle, when the nitrogen requirement of the plant is lower. When nitrogen nutrition does not limit growth, mineral nitrogen supply favours assimilation at the expense of fixation, but does not modify the amount of nitrogen accumulated, which is adjusted to the growth capacity of the plant.
After cutting, nitrate assimilation compensated for the decrease in fixation and supplied the plant with the nitrogen required by the regrowth, the proliferation of which determined the fixation recovery.
Drought stress decreased N2 fixation much more than NO −3 assimilation. The latter made growth recovery possible when water supply conditions became normal again.
These results suggested the existence of an optimum level of nitrate assimilation, which differed depending on the age of the plants and allowed both maximum growth and fixing activity.
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Wery, J., Turc, O. & Salsac, L. Relationship between growth, nitrogen fixation and assimilation in a legume (Medicago sativa L.). Plant Soil 96, 17–29 (1986). https://doi.org/10.1007/BF02374992
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DOI: https://doi.org/10.1007/BF02374992