Question

The youngest leaf is brownish or dead due to deficiency of _______ element

• A. Boron
• B. Manganese
• C. Phosphorus
• D. Potassium

Hint:

Immobile nutrients show deficiency symptoms at the plant's growing tips:
Boron deficiency $\rightarrow$ Distorted, brown, or dead youngest leaves (apical bud necrosis).
Calcium deficiency $\rightarrow$ Hooked leaf tips and necrosis of the apical meristem.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nutrient deficiency symptoms in plants are determined by the physiological role of the nutrient and its mobility within the plant vascular system.
Deficiencies of highly immobile nutrients appear first on the youngest leaves and growing points (apical meristems).

Step 2: Detailed Explanation:
Boron ($\text{B}$) is a highly immobile micronutrient in plants, as it cannot be easily re-translocated from older leaves to developing tissues.
Boron is essential for cell wall synthesis, cell division, membrane integrity, sugar transport, and pollen tube growth.
Because boron is immobile, growing meristems rely on a continuous supply from the transpiration stream.
Under boron deficiency, cell division and structural integrity in the growing tip fail, causing the youngest leaves to become distorted, turn brown, and eventually die.
This symptom is commonly referred to as the "death of the apical growing point."
Let us review the deficiency symptoms of the other options:
1. Manganese: Deficiency causes interveinal chlorosis on young to middle leaves, but does not typically cause immediate necrosis of the growing tip.
2. Phosphorus: Deficiency causes dark green foliage with purple or reddish-purple pigmentation on older leaves due to anthocyanin accumulation.
3. Potassium: Deficiency causes marginal chlorosis and scorching (necrosis) on the older leaves.

Step 3: Final Answer:
The youngest leaf becomes brownish or dead due to a deficiency of Boron, which corresponds to Option (A).