Root Nodule Enzymes of Ammonia Assimilation in Alfalfa (Medicago sativa L.) : DEVELOPMENTAL PATTERNS AND RESPONSE TO APPLIED NITROGEN

Nitrogenase-dependent acetylene reduction activity of glasshouse-grown alfalfa (Medicago sativa L.) decreased rapidly in response both to harvesting (80% shoot removal) and applied NO₃⁻ at 40 and 80 kilograms N per hectare. Acetylene reduction activity of harvested plants grown on 0 kilogram N per hectare began to recover by day 15 as shoot regrowth became significant. In contrast, acetylene reduction activity of all plants treated with 80 kilograms NO₃⁻-N per hectare and harvested plants treated with 40 kilograms NO₃⁻-N per hectare remained low for the duration of the experiment. Acetylene reduction of unharvested alfalfa treated with 40 kilograms N per hectare declined to an intermediate level and appeared to recover slightly by day 15. Changes in N₂-fixing capacity were accompanied by similar changes in levels of nodule soluble protein.