A recently discovered nitrogen-fixing association is that between grasses, such as sugar cane, maize, rice, wheat, sorghum, and other graminaceous species and endophytic diazotrophic bacteria that can colonize the plant interior without causing symptoms of disease. These bacteria enter the plants at root tips or at the emergence points of lateral roots and penetrate the root cortex, the stelar tissues, and the xylem vessels through which they may migrate toward the shoot. Endophytic diazotrophic bacteria are generally restricted to intercellular spaces and especially to the xylem vessels where the low pO2 and the high bacterial respiration rate create the microaerobic conditions needed for nitrogenase activity. Some of these diazotrophic microorganisms, such as those belonging to the genera Acetobacter, Herbaspirillum, Azospirillum, and Azoarcus, are of extreme interest since they can significantly contribute to the nitrogen requirement of the graminaceous plants. Certain rice varieties, for instance, can obtain over 30% of their nitrogen from these endo-phytes and some Brazilian sugar cane varieties up to 80%, with a total contribution of more than 170 kgNha-1 yr-1.
Studies of these N2-fixing associations form a topical field of research whose aim is to explore the possibility of both enhancing the N2 fixation and extending this efficient system to other cereals. This would greatly reduce the use of nitrogen fertilizers with considerable economic benefits, and, above all, with enormous environmental advantages. Over two-thirds of arable lands, in fact, are dedicated to the growing of cereals, which provide 80% of the food for the world's populations.
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