Evaluation of plant growth promoting and colonization ability of endophytic diazotrophs from deep water rice
Introduction
Rice is the most important staple crop in the developing world and nitrogen is the most important input required for rice production. In order to make the rice cultivation sustainable and less dependent on fertilizer nitrogen there is a need to use diazotrophic bacteria that can make biologically fixed nitrogen available for the growth of rice plants (Ladha et al., 1997). Although free living diazotrophic bacteria form natural associations with the roots and submerged portions of rice plants, their contribution of fixed nitrogen to rice plants is very low in comparison to that provided by rhizobia to the legumes (de Bruijn et al., 1995, Ladha and Reddy, 2000). The endophytic diazotrophs, however, occupy microniches in the plant tissues where they fix nitrogen without competition with other microorganisms and without the soil constraints present in the rhizosphere.
Some Brazilian sugarcane varieties, which are grown without supplementation of high concentrations of nitrogen fertilizers have been shown to derive a substantial percentage of their nitrogen requirements from N2 fixation (Boddey et al., 1995). The predominant endophytic nitrogen fixers in sugarcane belong to Acetobacter diazotrophicus and Herbaspirillum seropedicae (Cavalcante and Doebereiner, 1988, Baldani et al., 1986, James and Olivares, 1998) which are able to penetrate deeply inside the plant and spread systemically to reach aerial tissues. These endophytes have been investigated intensively for their potential in contributing biologically fixed nitrogen to their host plants.
The search for natural association and endophytic interaction of diazotrophs with rice is considered very promising especially in primitive rice varieties not bred to efficiently respond to N fertilizer (Barraquio et al., 1997, Stoltzfus et al., 1997). These rice varieties are considered excellent hosts to identify superior endophytes that may potentially impact agricultural rice production. Wild rice varieties, which are cultivated in wetlands, have been shown to harbor diazotrophic endophytic bacteria more frequently (Barraquio et al., 1997). The search for rice genotypes showing high biological nitrogen fixation, characterization of their endophytic nitrogen fixing bacteria and colonization of rice with rhizobia has attracted a great deal of attention in recent years. Presently, no information is available on the types of endophytic diazotrophs and their ability to colonize deep-water rice.
Deep-water rice is a valuable crop for the small, subsistence farmer, as it requires only small cash input. It is the staple food for over 100 million rural people, living in deep water rice areas, who are among the poorest in Asia (Catling, 1992). It is virtually the only crop that can be grown during the wet season in the deeply flooded areas in Asia and West Africa. Poor understanding of deep-water rice and its agro system had a negative effect on agricultural planning, which resulted in underestimating its importance as a staple food crop. The irreversible degradation of watersheds and the predicted rise in the world's sea level may lead to more extensive flooding and thus greater scope for deep water rice in future.
Out of nearly 9 million hectares of deep-water rice, which yields approximately 2 tons/hectare, 60% occurs in the Indian continent. Deep water rice is grown in areas usually flooding deeper than 50 cm for 1 month or longer during the growing season (Catling, 1992). The plant length usually reaches more than 140 cm. The floating rice usually start at 1 m water depth and survive chiefly by their extraordinary elongation ability, i.e. up to 5–8 cm per day for 7–10 days. The study of deep-water rice, especially the floating rice, represents the most extreme example of flood adaptability. The ratooning ability of deep-water rice represents a potential cropping pattern for deeply flooded areas.
The total nitrogen requirement for a good deep-water rice crop was estimated at 147 kg N/hectare (Rother et al., 1988). Although the aquatic ecosystems are able to meet part of their nitrogen demand, the rest needs to be supplemented. Poor farmers depending on the cultivation of deep-water rice cannot afford chemical fertilizers. The supplemented chemical fertilizer also has adverse effects, such as nitrate contamination of underground water and growth of non-target aquatic weeds. The possibility of meeting nitrogen requirements through biologically fixed nitrogen provides an ecofriendly and sustainable alternative. In view of this, diversity of endophytic diazotrophs was investigated in a traditional deepwater rice variety that grows up to a height of 20 feet in a lake not amended with N fertilizers. The aim of the study was to identify plant growth promoting endophytic bacteria, which can systemically spread, stably persist and fix nitrogen inside the rice tissues.
Section snippets
Rice genotype
Jaisurya or Desaria variety of rice is a floating type of rice, which represents one of the most extreme examples of flood adaptability. It shows a remarkable elongation ability which is needed in areas flooding regularly to depths of 3.0–3.5 m (Catling, 1992). It has been grown for the last 40 years in a large lake named Suraha Tal located in the Ballia district of Uttar Pradesh, India. The lake is not amended with chemical fertilizers, except with the surface run off from the adjacent
Diazotrophic bacteria in the seeds of deep water rice
Rice grown under water logged conditions has been shown to fix significant levels of nitrogen (Boddey et al., 1995, Wu et al., 1995). It has also been observed that cultivated wetland and wild rice genotypes harbor diazotrophic putative endophytic bacterial populations (Barraquio et al., 1997). In view of this we selected a deepwater traditional rice variety to study the biodiversity of culturable endophytic diazotrophs. A preliminary acetylene reduction assay was performed with the surface
Acknowledgements
This work was part of a frontier project on ‘Assessing opportunities of nitrogen fixation in rice: development of novel endophytic and symbiotic associations’ which was supported by Deutsche Gesellschaft fuer Technische Zusammenarbeit (GTZ). We thank S. Nordlund, K. Wilson, B. Reinhold-Hurek, A. Kumar and B. Dhar for their valuable cooperation and support. Technical assistance from Santosh Kumar Maurya is appreciated.
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