Correct Option: C. Nitrogen Detailed Explanation: When crop plants exhibit brownish coloration starting from the tips of the leaves and moving backward towards the mid-ribs, this is a classic symptom of nitrogen deficiency. Here’s a step-by-step breakdown of why nitrogen deficiency leads to this specific symptom:

1. Role of Nitrogen in Plants:
2. Nitrogen is a vital nutrient for plants, primarily because it is a key component of amino acids, which are the building blocks of proteins. It is also a crucial part of nucleic acids (DNA and RNA) and chlorophyll, the pigment responsible for photosynthesis.
3. Adequate nitrogen is essential for healthy leaf development and overall plant growth.
4. Symptoms of Nitrogen Deficiency:
5. When nitrogen is deficient, plants cannot produce enough chlorophyll, leading to reduced photosynthesis. This results in the yellowing of leaves, a condition known as chlorosis.
6. The symptoms often start with the older leaves because nitrogen is mobile within the plant. The plant reallocates nitrogen from older leaves to newer growth, leading to the characteristic yellowing and browning of older leaves, particularly from the tips backward.
7. Brownish Coloration:
8. As the deficiency progresses, the chlorotic leaves may develop necrotic (dead) tissue, which appears brown. This browning typically starts at the leaf tips and edges, moving inward, which is what is observed in the symptom described in the question.

Why Other Options Are Incorrect:

• A. Magnesium:
• Magnesium deficiency also leads to chlorosis, but it typically causes yellowing between the veins of older leaves, while the veins themselves remain green. The browning from the tips backward is not a typical symptom of magnesium deficiency.
• B. Zinc:
• Zinc deficiency can cause stunted growth and leaf distortion, but it does not typically cause the brownish coloration from the tips backward. Instead, it often leads to interveinal chlorosis and smaller leaves, particularly in younger plants.
• D. Copper:
• Copper deficiency can lead to various symptoms, including wilting and dieback of leaf tips, but it does not specifically cause the brownish coloration from the tips backward. Copper is less mobile in plants compared to nitrogen, so symptoms are often localized rather than affecting older leaves first.

Summary of Key Points:

• Nitrogen is essential for protein synthesis and chlorophyll production in plants.
• Deficiency symptoms include yellowing (chlorosis) of older leaves, progressing to browning (necrosis) from the tips backward.
• Other nutrient deficiencies (magnesium, zinc, copper) present different symptoms and do not match the described browning pattern.
• Understanding nutrient roles and deficiency symptoms is crucial for effective crop management and ensuring plant health.

This thorough understanding of nitrogen's role and the symptoms of its deficiency will help in diagnosing and addressing nutrient-related issues in crop plants effectively.