The Effects of Exogenous Amino Acid on Acetylene Reduction Activity of Vicia faba L. cv. Fiord

Abstract

The feed back control mechanism proposed to explain the inhibition of N2 fixation by N was investigated using Vicia faba cv. Fiord. Plants were grown under controlled conditions without mineral N in coarse river sand. Asparagine was supplied to plants actively fixing N2 by absorption through cut roots and via a wick or direct injection into the stem just above the bottom leaf. Responses in N2 fixation were measured by acetylene reduction (AR). Feeding plants with [14C]-labelled asparagine showed that the amide was taken up when exogenously applied. Asparagine (10 mM) supplied by the above procedures resulted in a 50-70% inhibition of AR by 48 h. Glutamine produced a similar effect. The cut root method allowed higher levels of these amides to be supplied but the inhibition observed with 10 mM asparagine was only increased slightly with higher levels of the amide. The antibiotic Securopen prevented bacterial contamination of root solutions of asparagine and glutamine and had no effect on nodule activity. It is concluded that accumulation of asparagine of glutamine or the resultant increase in the pool of soluble N in the plant cause a feedback effect on the activity of nitrogenase.Copyright 1993, 1999 Academic Press