Question

Phosphorus (P) is an important essential nutrient.
(A) Plant roots absorb P in the \(H_2PO_4^-\) form, but under neutral to alkaline environments, \(HPO_4^{2-}\) and or \(PO_4^{3-}\) ions could also be taken up.
(B) In normal P-sufficient plants, P-content varies from 0.1% to 0.4% by weight.
(C) It is an essential ingredient for Rhizobium bacteria to convert atmospheric N (\(N_2\)) into the ammonium (\(NH_4\)) form usable by plant.
(D) Because of being immobile in plants, first signs of its deficiency appear on the older leaves.
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (B), (C) and (D) only.
• D. (A), (C) and (D) only.

Hint:

Look closely at how statement (D) describes phosphorus mobility.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question lists four statements about phosphorus nutrition in plants, and we need to work out which three are factually correct.

Step 2: Key Formula or Approach:
Check each statement one at a time against the known chemistry and biology of phosphorus, then rule out any statement that contains a wrong fact.

Step 3: Detailed Explanation:
Statement (A) is correct. In acidic to neutral soils the dominant form taken up by roots is \(H_2PO_4^-\), but as soil pH rises towards neutral and alkaline, more \(HPO_4^{2-}\) and some \(PO_4^{3-}\) become available and can also be absorbed.
Statement (B) is correct. Healthy, P-sufficient plant tissue typically contains phosphorus in the range of about 0.1% to 0.4% of dry weight, a range widely quoted in plant nutrition.
Statement (C) is correct. Biological nitrogen fixation carried out by Rhizobium inside root nodules is very energy hungry, since the nitrogenase enzyme complex needs a steady supply of ATP to break the strong triple bond in \(N_2\) and reduce it to the ammonium form the plant can use. Phosphorus is a building block of ATP, so an adequate phosphorus supply is essential for this process to run efficiently.
Statement (D) is incorrect. Phosphorus is actually a mobile nutrient in the plant, not an immobile one. Because it is mobile, the plant withdraws phosphorus from older leaves and sends it to actively growing young tissue when supply is short, so deficiency symptoms do appear on older leaves first, but the reason given in the statement, that this happens because phosphorus is immobile, is the wrong explanation and contradicts how phosphorus actually behaves.
Since (A), (B) and (C) hold up and (D) contains a factual error about mobility, the correct set is (A), (B) and (C) only.

Step 4: Final Answer:
(A), (B) and (C) only are correct, so option 1 is right.