Associative symbiosis of Azospirillum lipoferum with dicotyledonous succulent plants of the Indian desert

The roots of several dicotyledonous xerophytic plants exhibited nitrogenase activity. Azospirillum lipoferum was isolated from the roots of these plants including several species of Opuntia having crassulacean acid metabolism. These isolates showed high rates of acetylene reduction; maximum nitrogenase activity was observed with the isolate from Opuntia vulgaris. Organic acids rather than sugars were preferred as carbon source for two selected isolates studied. However, no activity was detected with formic, oxalic, or lactic acid. Inorganic nitrogen sources caused significant reduction in nitrogenase activity, while organic sources such as amino acids and proteins either stimulated or did not significantly inhibit the activity. However, urea completely inhibited formation of nitrogenase activity. Growth measured as optical density was enhanced with all the nitrogen sources except potassium nitrite.