Contribution of nitrogen fixed by Azospirillum to the n nutrition of spring wheat in Israel

Acetylene reduction activity (ARA) was measured in cores containing roots of various Israeli wild and cultivated wheat lines colonized by Azospirillum. The inoculated plants were grown under greenhouse or field conditions. Although, no measurable ARA was detected during earlier stages of wheat development, 50-600 nmol C₂H₄ g^-1 dry root h^-1 was measured during heading and flowering stages. By using N yield balance and ¹⁵N dilution techniques, it was found that Triticum aestivum cv. Miriam inoculated with Azospirillum accumulated 20% more N (¹⁴N and ¹⁵N) at the booting stage than did the uninoculated control. This difference in N content became less apparent in grains. No significant ¹⁵N dilution could be found and the contribution of atmospheric N₂ to the N content of grains of inoculated plants was negligible. It was concluded that the potential contribution of biological N₂ fixation to spring wheat cultivation in Israel is very low.