BIOLOGICAL NITROGEN-FIXATION ASSOCIATED WITH SUGAR-CANE AND RICE - CONTRIBUTIONS AND PROSPECTS FOR IMPROVEMENT

N-15 isotope and N balance studies performed over the last few years h ave shown that several Brazilian varieties of sugarcane are capable of obtaining over 60% of their nitrogen (>150 kg N ha(-1) year(-1)) from biological nitrogen fixation (BNF). This may be due to the fact that this crop in Brazil has been systematically bred for high yields with low fertilizer N inputs. In the case of wetland rice, N balance experi ments performed both in the field and in pots suggest that 30 to 60 N ha(-1) crop(-1) may be obtained from plant-associated BNF and that dif ferent varieties have different capacities to obtain N from this sourc e. N-15(2) incorporation studies have proved that wetland rice can obt ain at least some N from BNF and acetylene reduction (AR) assays also indicate differences in N-2-fixing ability between different rice vari eties. However in situ AR field estimates suggest plant-associated BNF inputs to be less than 8 kg N ha(-1) crop(-1). The problems associate d with the use of the N-15 dilution technique for BNF quantification a re discussed and illustrated with data from a recent study performed a t EMBRAPA-CNPAB. Although many species of diazotrophs have been isolat ed from the rhizosphere of both sugarcane and wetland rice, the recent discovery of endophytic N-2-fixing bacteria within roots, shoots and leaves of both crops suggests, at least in the case of sugarcane, that these bacteria may be the most important contributors to the observed BNF contributions. In sugarcane both Acetobacter diazotrophicus and H erbaspirillum spp. have been found within roots and aerial tissues and these microorganisms, unlike Azospirillum spp, and other rhizospheric diazotrophs, have been shown to survive poorly in soil. Herbaspirillu m spp. are found in many graminaceous crops, including rice (in roots and aerial tissue), and are able to survive and pass from crop to crop in the seeds. The physiology, ecology and infection of plants by thes e endophytes are fully discussed in this paper, The sugarcane/endophyt ic diazotroph association is the first efficient N-2-fixing system to be discovered associated with any member of the gramineae. As yet the individual roles of the different diazotrophs in this system have not been elucidated and far more work on the physiology and anatomy of thi s system is required. However, the understanding gained in these studi es should serve as a foundation for the improvement/development of sim ilar N-2-fixing systems in wetland rice and other cereal crops.