Nitrogen fixation associated with grasses and cereals: Recent results and perspectives for future research

Abstract

Over the last few years research in the area of biological nitrogen fixation (BNF) associated with cereals and grasses has become divided into two areas. On the one hand there have been a large number of reports of responses of field-grown plants to inoculation with N 2-fixing bacteria, principally Azospirillum spp. On the other hand there have been several reports of significant contributions of associated BNF to the nutrition of several crops, including wetland rice, sugar cane and some forage grasses. However, where BNF contributions have definitely been established no certain information is available as to the diazotrophic organisms responsible. Furthermore, certain recent reports indicate that, at least in some cases, responses of plants to inoculation with Azospirillum spp. have been shown not to be due to BNF contributions.In this paper we review some recent progress in this field, particularly at our institute in Rio de Janeiro, concerning specificity of selected Azospirillum strains in the infection of cereal roots and the promotion of responses in the host plants. The possible mechanisms of plant response are discussed including the possibility that plant growth substances or bacterial nitrate reductase are involved. The application of ~SN and N balance techniques to the quantification of plant associated BNF are considered and the possible strategies that may be adopted to further the understanding of true N 2-fixing plant/diazotroph associations.The recent discovery of many more plant-associated N2-fixing bacteria suggests that further research in this area may eventually lead to the development of such associations with applications for agricultural productivity.

Azospirillum sp . inoculation in wheat, barley and oats seeds greenhouse experiments

“…The greater accumulation of nitrogen in the grains, caused by the inoculation of bacterias of the genera Azospirillum, is due to a larger nitrogen offer for the plant. This larger offer can be consequence of nitrogen biological fixation process accomplished by the bacterias, or the capacity of certain strains in reducing the nitrate, or that the inoculated plants present a more developed root system, allowing the exploration of a larger soil area (Baldani et al, 1983;Boddey and Döbereiner, 1988;Boddey et al, 1986;Christiansen-Weniger and Van Veen, 1991;Didonet et al, 1996;Döbereiner, 1992;Fages, 1994;Ferreira et al, 1987;Kapulnik et al, 1981;Raí and Guar, 1988). Few data of inoculant application with Azospirillum in barley is found.…”

“…The promotion of plants growth inoculated with Azospirillum has been obtained in field conditions and greenhouse experiments, resulting in significant changes in several characteristics of the plants. The inoculation can cause increase in the dry weight and in the accumulation of total nitrogen of the plant, in the grain yields and in the weight of the grains, in the germination rate of the seeds and in changes in the duration of the plants growth stages (Boddey and Döbereiner, 1988;Fages, 1994;Fallik and Okon, 1996;Nur et al, 1980;Pandey et al 1998;Sumner, 1990). The beneficial effects caused by the inoculation with Azospirillum are not only due to FBN in the rhizosphere, but mainly, for the best efficiency in the absorption of water and nutrients, which happens due to a more developed root system, increasing the soil area explored by the roots.…”

Effects of integrated management of bio and chemical fertilizers on yield of maize hybrids (Zea mays L.)

“…Other studies determined that plant growth was improved even when the nitrogen fertilizer applied was reduced by 30-35% as long as the seeds had been inoculated with growth promoting bacteria. An increase in the biomass of the microbial community was related to the soil health as this had an effect on the balance and availability of nutrients in the rhizosphere of the roots that lead to a higher yield (Boddey & Dobereiner, 1988;El-kholy et al, 2005;Biari et al, 2008) …”

Bactérias endofíticas como agentes promotores do crescimento de plantas de tomateiro e de inibição in vitro de Ralstonia solanacearum

“…Os principais mecanismos pelos quais as bactérias endofíticas promovem o crescimento de plantas são a fixação de nitrogênio (BODDEY & DOBEREINER, 1995) e o controle biológico de fitopatógenos, seja este pelo antagonismo direto da microflora deletéria ou pela indução de resistência sistêmica (HALLMANN et al, 1997). No entanto, as bactérias endofíticas podem também promover o crescimento de plantas pela produção de hormônios vegetais ou de substâncias análogas (ARSHAD & FRANKENBERGER, 1991).…”