1. Akkermans A D L et al. 1989 Ann. Sci. For. 46 Suppl. Forest Tree Physiology. Eds. E Dreyer et al. pp 765s?771s. Elsevier/INRA.
2. Boddey R M and D�bereiner J 1984 In Current Developments in Biological Nitrogen Fixation. Ed. N SSubbaRao. pp 279?313. Oxford and IBH Publishing Co., New Delhi, India.
3. Bond G 1974 In The Biology of Nitrogen Fixation. Ed. AQuispel, 342?370. North Holland Publishing Co., Amsterdam.
4. Burris R H 1977 In Recent Development in Nitrogen Fixation. Eds. W ENewton et al. pp 487?511. Academic Press, London.
5. Dewan G I and SubbaRao N S 1979 Plant and Soil 53, 295?302.