Question

Which of the following association is favourable for P nutrition in higher plants ?

• A. Rhizobium–Legume symbiosis
• B. Azolla–Anabaena symbiosis
• C. Actinorrhizal symbiosis
• D. Mycorrhizal association

Hint:

Arbuscular Mycorrhizal (AM) fungi produce highly branched structures called arbuscules within root cortical cells, which serve as the primary site of nutrient and carbon exchange between the plant and the fungus.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Phosphorus (P) is an essential macronutrient required by plants, but it is highly immobile in soil, often precipitating as insoluble complexes.
Plants develop symbiotic relationships with soil microorganisms to improve nutrient absorption.

Step 2: Detailed Explanation:
Let us evaluate each of the symbiotic associations listed:
1. Mycorrhizal Association (D): This is a mutualistic relationship between plant roots and soil fungi.
The fungal hyphae extend far into the soil, creating an extensive network that reaches beyond the nutrient depletion zone surrounding root hairs.
This network dramatically increases the absorption area, allowing the fungi to capture and deliver highly immobile phosphorus ions directly to the host plant.
Thus, this association is highly favorable for plant phosphorus nutrition.
2. Rhizobium–Legume symbiosis (A), Azolla–Anabaena symbiosis (B), and Actinorrhizal symbiosis (C): These are specialized relationships with nitrogen-fixing bacteria (*Rhizobium*, cyanobacteria, or *Frankia*).
They are responsible for converting atmospheric nitrogen ($\text{N}_2$) into ammonia ($\text{NH}_3$) to support plant nitrogen nutrition, not phosphorus.

Step 3: Final Answer:
The association favorable for phosphorus nutrition is the Mycorrhizal association, corresponding to option (D).