Leguminous Plants

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

Leguminous Plants

Leguminous Plants
Leguminous plants are classified into three major botanical subfamilies of the family Leguminoseae-the Ceasalpinioideae, the Mimosoideae and the Papilionoideae. There are nearly 750 genera and 18,000-19,000 species of leguminous plants of which 500 genera and approximately 10,000 species belong to the subfamily Papilionoideae.

Not all legumes bear nodules on their root system and it is known that certain tree forms do not possess them at all. Hardly 16% of Leguminoseae have so far been examined for nodulation of which 95% of Mimosoideae, 26% of Ceasalpinioideae and 90% of Papilionoideae possess root nodules.

The origin of leguminous plants and the evolution of bacterial symbiosis are largely speculative. Evidence from fossil legumes does not provide much help in judging the exact time of origin of Leguminoseae. The plants are likely to have originated in sub-humid tropical, subtropical or temperate regions. The type of soil in which the first symbiotic legumes developed is also conjectural. It might have been in acid, neutral, alkaline or calcareous soil.

Nodule bacteria were probably free-living nitrogen fixers before they became symbiotic under conditions of low availability of essential soil nutrients. It is likely that the slow growing, symbiotically promiscuous cowpea-type Rhizobiumwas the ancestral type of nodule bacterium which has persisted till today in association with many modern genera of Leguminoseae.

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