When leaves of crop plants show brownish coloration from the tips of the mid-rib...

When the leaves of crop plants show brownish coloration from the tips of the mid-ribs backward, it indicates a deficiency symptom. The question is asking which nutrient is lacking in the plant when this symptom is observed.

The options given are magnesium, zinc, nitrogen, copper, and phosphorus. To determine the correct answer, we need to understand the role of each nutrient in plant growth and development.

Magnesium is an essential nutrient that helps in the formation of chlorophyll, the pigment responsible for the green color of leaves. A deficiency in magnesium usually leads to yellowing of leaves, not brownish coloration.

Zinc is another important nutrient involved in various plant processes, including enzyme activity and protein synthesis. Zinc deficiency can cause stunted growth and yellowing of leaves, but not brownish coloration.

Nitrogen is a major nutrient required for plant growth and is involved in various metabolic processes. A deficiency in nitrogen can lead to overall yellowing of leaves, but not brownish coloration.

Copper is a trace element that plays a role in enzyme activity and plant metabolism. Copper deficiency can result in distorted growth and wilting, but not brownish coloration.

Phosphorus is a vital nutrient involved in energy transfer and the development of roots, flowers, and fruits. A deficiency in phosphorus can cause purplish or reddish-brown coloration on leaves, starting from the tips of the mid-ribs backward. Therefore, the correct answer is Option E: phosphorus.

In summary, when the leaves of crop plants show brownish coloration from the tips of the mid-ribs backward, it indicates a deficiency of phosphorus in the plant. Phosphorus is an essential nutrient for plant growth and development, and its deficiency can result in discoloration of leaves.