Question

Chlorosis in the older leaves is deficiency symptom of

• A. Phosphorus
• B. Nitrogen
• C. Potassium
• D. Sulfur

Hint:

A simple mnemonic for mobile elements is "N-P-K-Mg". Symptoms for these always start in the bottom (older) leaves.
For Nitrogen, think "Uniform yellowing".
For Magnesium, think "Interveinal chlorosis".
For Potassium, think "Burned edges".

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the nutrient whose deficiency causes chlorosis (yellowing of leaves) specifically in the older leaves first.
The appearance of deficiency symptoms depends on the mobility of the element within the plant.
Detailed Explanation:

• Concept of Mobility:
Nutrients that are mobile within the plant (like Nitrogen, Phosphorus, and Potassium) are exported from older, senescing leaves to younger, actively growing parts when the supply is low.
Consequently, deficiency symptoms for these mobile elements appear first in older leaves.
Nutrients that are immobile (like Calcium and Sulfur) stay in the older leaves, so deficiency symptoms appear first in the young, developing tissues.

• Nitrogen (\( N \)) Deficiency:
Nitrogen is a primary constituent of proteins, nucleic acids, and chlorophyll.
It is highly mobile. When nitrogen is deficient, the plant breaks down chlorophyll in older leaves to salvage nitrogen for the shoot apex.
This leads to uniform chlorosis (yellowing) starting from the tips and moving along the midrib in older leaves.

• Comparison with other options:
Phosphorus: Deficiency often causes dark green coloration or purple/red tints due to anthocyanin accumulation, not primary chlorosis.
Potassium: Deficiency symptoms also appear in older leaves, but they typically manifest as marginal chlorosis or "scorching" (necrosis of leaf edges).
Sulfur: Deficiency causes chlorosis similar to Nitrogen, but since Sulfur is relatively immobile, the yellowing appears first in younger leaves.

Step 3: Final Answer:
Chlorosis in older leaves is a hallmark symptom of Nitrogen deficiency.