Stem Nodulating Legumes

Rhizobium and Legume Root Nodulation
Leguminous Plants
The Discovery of Symbiotic Nitrogen Fixation
Rhizobium Classification
Cultural Characteristics
Genetics
Flavonoids Induce Nodualation
Novel Approaches to Extend Root Nodulation
Ecology
Infection
Lectins
Structure of the Nodule
Function of the Nodule
Nitrogenase Hydrogenase Interrelationship
Stem Nodulating Legumes
Factors Affecting Nodulation
Temperature and Light
Hydrogen Ion Concentration
Mineral Nutrition

Stem Nodulating Legumes

	Stem Nodulating Legumes There are two genera of legumes which are known to have nodules on stem-Aeschynomene and Sesbania. Of the 150-250 species of Aeschynomene, nodules on stem have been reported in A. indica, A. aspera, A. elaphroxylon, A. villosa, A. evenia, A. paniculata and A. afaspera. These teports have come from India, Mali, Ghana, Venezuela, Brazil, U.S.A., Japan, Zimbabwe, Java, South Africa, Zambia, South America, Puerto Rico and Argen ina. In India, A. indica and A. aspera occur widely in waterlogged situations.
The genus Sesbania has 170 species which are annual as well as perennial and grow in warmer regions of both the hemispheres. S. rostrata is an annual plant which bears nodules on stems as well as on roots. It was first described a few years ago by a group of scientists in Senegal as a plant equivalent (or higher) to soybean in nitrogen fixation to the extent of 200 kg N₂/ha in 50 days by virtue of the stem nodules. Subsequently, the plant has been tested at the International Rice Research Institute, Philippines and other places such as India and Thailand and found to establish well.
What merits the attention of rice agronomists is the green manure potential of this plant. From published reports, the increase in the yield of rice by the application S. rostrata green manure could be as high as 3.7 t/ha. This increase is more than that obtainable by 60 kg N /ha of chemical fertilizer application which can only result in an increase in rice yield to the extent of 1.7 t/ha. In Coimbatore, India, S. rostrata application alone yielded 2350 kg/ha rice in comparison with the application of Azolla filiculoides which resulted in the production of 2030 kg/ha rice.
There are other green manure and green leaf manure crops which have been used in rice cultivation by several generation of rice farmers in India and other countries. In India, species of Crotalaria, Indigofera, Lathyrus, Pongamia, Tephrosia, Trifolium and Sesbania have been used for.", long time. The high N content of these crops and their ability to get easily degraded in wet rice soils of Asia are the main criteria for adopting a given plant species as a suitable green manure candidate of choice. Astragalus sinicus, a species widely used in China, has 108 kg N/ha whereasS. rostrata contains 267 kg/ha. The commonly used Indian green manure species, Sesbania aculeata has around 96-122 kg N/ha. In Senegal, using ¹⁵N labelled fertilizers, a comparison was made between stem and root nodulated S. rostrata and root nodulated S. sesban to see if the additional facility provided by stem nodulating habit conferred greater nitro en-fixing capacity. It was calculated that S. rostrata fixed 83-109 kg N₂ ha^-1whereas S. sesban fixed only 7-18 kg N₂ ha^-l under similar conditions of growth reflecting the superiority of the stem nodulating legume.
	The nitrogen content of rice grain and the organic matter content of the soil have been found to increase by green manuring of fallow soil prior to transplanting rice. Other attributes such as the availability of zinc, the hydraulic conductivity, water holding capacity and aggregate stability of soil also appear to be improved by green manuring. In addition, the practice also prevents loss of soil-nitrogen by decreasing denitrification processes. These benefits are offset by certain limiations: green manuring is labour intensive and sometimes necessitates the cultivator to set apart land for growing green manure species which otherwise could have produced a cash crop. In other words, the economical considerations of the green manure practice are not clear especially in situations where fertilizer nitrogen can be afforded.

Fundamental research on stem nodulating green manure species has been directed mainly to an understanding of the rhizobiology and genetics of the microsymbiont producing stem nodules. The species of Rhizobium inducing both stem and root nodules is designated as Azorhizobium caulinodans.

The bacterium is capable of high levels of nitrogen fixation in the free living state up to 1500 nM C₂H₄/mg protein/hour at an unusually high oxygen tension of 3 per cent. Stem nodulation is extensive (50 g/plant) and acetylene reduction activity corresponds to 550-600 mm C₂H₂ reduced per hour. Besides these attributes nodulation on stem in S. rostrata and nitrogen fixation in situ are relatively insensitive to levels of combined nitrogen in the soil. The regulation of nif gene expression in A. caulinodans is complex and shares regulatory features common with those understood for Rhizobium meliloti and Bradyrhizobium japonicum.

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