6.1. Root Colonization and Infection Groundnuts are nodulated by a wide range of species of rhizobia. Proliferation of compatible rhizobia in the rhizosphere of legumes is the first step towards nodule formation. There is no clear specificity in groundnut nodulation and root colonization. In most legumes rhizobia enter through root hairs via an "infection thread", but in groundnut roots the invasion process is rather different. Normal root hairs are absent; instead tufted rosettes of hairs are found in the junctions of root axils. It is at these junctions that nodulation occurs.
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6.2. Nodule Development Once the rhizobia have entered the root and occupied the space between the root hair wall and the adjoining epidermal and cortical cells, the cells adjacent to the point of Rhizobium penetration separate at their middle lamellas and the resultant spaces become filled with bacteria, forming intercellular zones of infection. The bacteria penetrate into progressively deeper cell layers and intracellular infection then occurs. Soon after intracellular infection, the bacteria multiply rapidly.
Further development of the nodule occurs by repeated division of the infected host cells, and the bacteria become transformed, into different morphological forms known as bacteroids. As the size of bacteroids is relatively large, the number of bacteroids per unit nodule weight in groundnut is small when compared to other tropical legumes such as cowpea.The initial spherical shape of legume nodules can develop into various morphological variants, but in groundnut they remain more or less spherical throughout. 6.3. Nodule Function The bacteroids contain the nitrogenase enzyme which reduces gaseous nitrogen.
The effectiveness of the nodule to fix nitrogen depends on the presence of leghemoglobin, which gives a pink coloration to the nodule tissue. Quantitative determination of leghemoglobin during certain stages of plant growth (e.g. flowering) could be used as an indicator of dinitrogen fixation in groundnuts. The host plant provides energy in the form of carbohydrates for dinitrogen fixation and the carbon skeleton for the assimilation of reduced nitrogen (NH4).
The fixed nitrogen is transferred to shoots mainly in the form of o-methylene glutamin. Not all the rhizobia that produce nodules fix nitrogen. It is important therefore to select good inoculant strains for survival in rhizosphere, competition with indigenous strains, infectivity and fixation potential.
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