Step 1: Understanding the Question:
The question asks to identify the specific nutrient deficiency that leads to a physiological disorder in cauliflower known as "Buttoning".
Step 2: Key Formula or Approach:
First, define "Buttoning". It is the premature formation of a small, non-marketable head or "curd" while the plant is still small. The approach is to understand the role of each nutrient and how its deficiency would affect plant growth, leading to this disorder.
Step 3: Detailed Explanation:
For a cauliflower to form a large, healthy curd, it must first undergo sufficient vegetative growth (i.e., develop a large frame of leaves).
(A) Nitrogen (N): Nitrogen is the most critical nutrient for vegetative growth (leaf and stem development). A deficiency of nitrogen stunts the plant's growth. A small, starved plant will be triggered to enter its reproductive phase early, resulting in the formation of a tiny, premature "button" curd.
Deficiencies of other nutrients cause different disorders:
(B) Phosphorus (P): Deficiency can cause purplish discoloration and stunted growth, but is not the primary cause of buttoning.
(C) Potassium (K): Deficiency affects water regulation and can cause browning on leaf margins.
(D) Boron (B): Deficiency causes browning of the curd and hollow stems, not buttoning.
Step 4: Final Answer:
The primary cause of poor vegetative growth leading to buttoning in cauliflower is Nitrogen deficiency. Therefore, option (A) is correct.