Nitrates Essential for Protein Synthesis in Plants

Steps:

• Examine the graph: wheat yield increases with nitrate fertilizer up to an optimal level, indicating nitrates boost growth.
• Recall plant biology: nitrates (NO3-) are absorbed and reduced to ammonia for amino acid production.
• Link to yield: proteins form enzymes and structures vital for photosynthesis and biomass, explaining higher crop output.
• Conclude: the graph supports nitrates' role in protein formation as key to improved yield.

Why D is correct:

• Nitrates provide nitrogen, the essential building block for amino acids, which polymerize into proteins (biochemical definition of protein synthesis).

Why the others are wrong:

• A: Chlorophyll formation requires magnesium, not nitrates; nitrates aid proteins in leaves indirectly.
• B: Yield is not strictly proportional; graphs typically show a plateau or decline beyond optimal nitrate levels.
• C: Graph may show benefits but lacks evidence of inhibition from excess nitrates.

Final answer: D