Résumé
The
sensitivity of N2 fixation to drought stress in soybean
(Glycine max Merr.) has been shown to be associated with
high ureide accumulation in the shoots, which has led to the hypothesis
that N2 fixation during drought is decreased by a feedback
mechanism. The ureide feedback hypothesis was tested directly by
measuring the effect of 10 mm ureide applied by stem
infusion or in the nutrient solution of hydroponically grown plants on
acetylene reduction activity (ARA). An almost complete inhibition of
ARA was observed within 4 to 7 d after treatment, accompanied by
an increase in ureide concentration in the shoot but not in the
nodules. The inhibition of ARA resulting from ureide treatments was
dependent on the concentration of applied ureide. Urea also inhibited
ARA but asparagine resulted in the greatest inhibition of nodule
activity. Because ureides did not accumulate in the nodule upon ureide
treatment, it was concluded that they were not directly inhibitory to
the nodules but that their influence mediated through a derivative
compound, with asparagine being a potential candidate. Ureide treatment
resulted in a continual decrease in nodule permeability to
O2 simultaneous with the inhibition of nitrogenase activity
during a 5-d treatment period, although it was not clear whether the
latter phenomenon was a consequence or a cause of the decrease in the
nodule permeability to O2.