The Discovery of Symbiotic Nitrogen Fixation

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

The Discovery of Symbiotic Nitrogen Fixation

The Discovery of Symbiotic Nitrogen Fixation
In the 19th century, even though scientists had understood the value of mineral nutrition of plants some suspected that plants could obtain nitrogen from the atmosphere.

However, it was fortuitous that two German chemists, Herman Hel1riegel and Herman Wilfarth, presented very convincing experimental reports in 1886 and 1888 to distinguish the innate ability of legumes to fix elemental nitrogen in the atmosphere from the inability of cereal plants to perform the same function.

Earlier, Boussingault in 1883 had done somewhat similar experiments but was not able to clearly point out the special symbiotic ability of legumes.

Results of pot experiment conducted by Boussingault (from Fred et al.,1932)

Plant Cultivated	Duration of culture (months)	Weight (g)		Nitrogen (g)		Gain or loss in nitrogen
Plant Cultivated	Duration of culture (months)	Seed	Crop	Seed	Crop	Gain or loss in nitrogen
Clover	2	1.576	3.220	0.110	0.120	10.010
	3	1.632	6.288	0.114	0.156	0.042
Wheat	2	1.526	2.300	0.043	0.040	-0.003
	3	2.018	4.260	0.057	0.060	0.003
Pea	3	1.211	4.990	0.047	0.100	0.053

An important result obtained by Hellriegel and Wilfarth (from Fred et al., 1932)

No.	Nitrogen in calcium nitrate per pot (g)	Oats	Peas
		Average weight of grains and straw (g)	Average weight of seed and vines (g)
1	none	0.390	4.380
2	0.056	5.876	4.128
3	0.112	10.961	9.132
4	0.168	15.997	-
5	0.224	21.357	9.725
6	0.336	30.175	11.352

In 1679, Malpighi provided the first diagrams and descriptions of root nodules of plants but considered them as insect galls. In 1866 Woronin saw fungal hyphae in root nodules which looked like threads, probably the 'infection threads' as we now know them. Therefore, subsequent ob- servations led to the belief that root nodules were caused by fungi. Ward in 1889 studied the development of root nodules from the root hair to the origin of nodule protrusions.

More than one hundred years have elapsed since Hellriegel and Wilfarth presented their findings in several lectures delivered in 1886 to learned societies. Their conclusions were: (1) The Leguminoseae behave with regard to their nitrogen nutrition principally different from the Gramineae, (2) The Gramineae can rely for their nitrogen nutrition solely on the nitrogenous substances which are present for assimilation in the soil and their development is directly related to the available nitrogen supplies, (3) Besides the soil nitrogen, the Leguminoseae can use nitrogen from a second source available in the form of free elementary nitrogen of the atmosphere,

(4) The Leguminoseae by themselves cannot assimilate atmospheric nitrogen but need the cooperation of vital microorganisms, (5) It is essential that certain species of microorganisms enter into symbiosis with the legume, (6) The root nodule is not merely a storehouse of protein but has a causal relation with the assimilation of free nitrogen. Such startling observations made after careful experimentation, have no doubt stood the test of time as we now look back more than one hundred years later.

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