Huss-Danell K, Sellstedt A
Nitrogenase activity in response to darkening and defoliation in Alnus incana

Journal of Experimental Botany: 1985 36:1352-1358

Abstract
In the Alnus-Frankia symbiosis the nitrogen-fixing root nodules are one of the sinks for carbon compounds newly formed in photosynthesis and exported from the leaves (source). The source-sink ratio of cloned plants of Alnus incana was reduced by darkening or by total or partial defoliation and the resulting nitrogenase activity (C2H2-reduction) was measured. Nitrogenase activity had nearly ceased 5 h after total defoliation but not until ca. 5 d after total darkening. Most of the activity was lost during the initial hours and days, respectively. When leaf area was reduced approximately by half nitrogenase activity decreased slightly less than by half. Removal of upper leaves seemed less harmful than removal of lower leaves one day after defoliation. On the following 2 d the treatments appeared to be similar. Thus, nitrogenase activity was largely dependent on newly formed assimilates but could also depend on stored reserves that were mobilized. Measurements of in vitro nitrogenase activity in root nodule homogenates from darkened plants indicated that nitrogenase gradually became inactivated and/or depleted after 1 and 2 d in darkness
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