The promising future of Sesbania rostrata Reducing the need for commercial fertilizer

A seminar in Dakar, Senegal, in January 1988 broke new ground by bringing together two types of researchers who do not often meet: those doing fundamental scientific research and those working on the practical application of such results. The occasion was an international seminar on Sesbania rostrata which was organised by the CTA in collaboration with ORSTOM (the French development research agency) ISRA (Senegal's national agricultural research institute), and the genetics laboratory of the University of Ghent in Belgium.

The participants from more than 30 countries, discussed the results of their work and identified the problems posed by the integration of S. rostrata (and other plants with nitrogen-fixing nodules on . their stems) in the cultivation practices of African and Asian farmers. S. rostrata is an adventitious nitrogenfixing plant that is frequently found in the Senegal River valley as well as throughout the Sahel.

Although well known for many years, this plant was not studied in any depth until 1979 when ORSTOM researcher, Dr B. Dreyfus, discovered nitrogen-fixing nodules on its stems. This discovery hailed the beginning of basic research which rapidly developed throughout the world in 20 or so laboratories which analysed the physiology and genetics of the micro-organism responsible for the stem nodules. It is a new type that combines the properties of free nitrogen-fixing bacteria (like Klebsiella) and symbiotic bacteria (like Rhizobium). In fact, it was during this very seminar that this new bacterium was officially baptized as Azorhizobium caulinodans. At the same time as this basic research was started, agronomic researchers from 20 or so African and Asian countries began to examine the possibility of exploiting this new system capable of fixing high levels of nitrogen whether for irrigated crops (particularly with rice) or for rain-fed crops (notably with row crops).

The seminar gave a platform for the results of the latest research on the symbiosis of S. rostrata and A. caulinodans which appears more and more often as a priority subject for experimental trials. Such discoveries dealt first of all with the genetics of Azorhizabium. The substances that initiate and regulate nodulation have now been identified as well as the new genes that fix the nitrogen. The study of the interaction of Sesbania-Azorhizabium at the cellular level has, for the first time, enabled the in vitro development of nodules based on the infection of S. rostrata protoplasm by Azorhizabium. Such nodules will no doubt enable future research in this area to advance current knowledge of the cellular interaction between these two organisms.

The recent result of a plant/host mutant that does not have nodule sites opens the way to the identification of genes coded for this remarkable characteristic of nitrogenfixing stem nodules. The seminar also provided the opportunity for an update on the most recent molecular biology techniques capable of producing transgenic plants which have resistance to viruses, insects or herbicides.

Researchers at the seminar also confirmed preliminary observations which suggested that S. rostrata has considerable nitrogen-fixing potential. This exceptional symbiosis between S. rostrata and Azorhizabium explains the success of the use of S. rostrata as a green manure which, in almost all cases, enables the yield of irrigated rice to be doubled.

As for row crops, the contribution of S. rostrata to nitrogen supplies of associated plants is less signficant but still substantial. New uses of S. rostrata were also presented, including its ability to provide forage, fuelwood and pulp. In Senegal, it has even been shown that its leaves can be used for human consumption. The comprehensive study of the integration of S. rostrata in agriculture has already identified certain limits to its use, notably its sensitivity to a short photoperiodicity and to certain nematodes in areas that were flooded.

At the end of the seminar, numerous recommendations were made primarily to co-ordinate both field and laboratory research. They will be included in the proceedings which will be jointly published by CTA and ORSTOM.