Question

A peanut farmer finds that the older leaves of his plants are turning yellow following a long period of wet weather. Suggest a reason why.

Short answer

The turning of yellow in a peanut plant indicates a deficiency of nitrogen. The plants close the aqua pores of the leaves when nitrogen is not sufficiently obtained.

Step-by-step solution

Leguminous plants

The plants which produce beans or lentils are known as leguminous plants. These plants show a symbiotic relationship withsoil bacteria or Rhizobium bacteria.

Peanut is one such leguminous plant that gives shelter for rhizobium bacteria in its roots. These bacteria provide nitrogen to the plants through nitrogen fixation.

Nitrogen-fixing bacteria

The atmospheric nitrogen is fixed in the roots of the plants with the help of nitrogen-fixing bacteria. Species of Azotobacter, Bacillus and many other soil bacteria are examples of nitrogen-fixing bacteria. These bacteria utilize oxidizing organic molecules from the plants as their nutrition.

Deficiency of nitrogen in plants

The deficiency of nitrogen causes yellowish leaves and retarded growth. The plants close the aqua pores, and therefore the leaves turn yellow and show retarded growth due to water stress.The nitrogen deficiency occurs when rhizobacteria use nitrogen to break down carbon.

Thus, yellowing of leaves or chlorosis of old leaves is due to nutrient deficiency such as nitrogen. It can also be caused by poor drainage, compacted roots, and excess alkaline compounds in the soil.