Plant nitrate assimilation: After roots absorb nitrate (NO3−) from soil, into which inorganic form is it primarily reduced within the plant before amino acid synthesis?

Introduction / Context:
Plants assimilate nitrate through a reductive pathway prior to incorporating nitrogen into amino acids. Knowing the intermediate inorganic nitrogen form is foundational to plant biochemistry and agronomy.

Given Data / Assumptions:

• Nitrate is taken up by roots from soil solution.
• Nitrate must be reduced prior to incorporation into organic molecules.
• The reduction steps occur via specific enzymes in roots and/or shoots.

Concept / Approach:
Nitrate is reduced to nitrite (via nitrate reductase) and then to ammonium (via nitrite reductase). The ammonium is incorporated into amino acids through the GS-GOGAT pathway, yielding glutamine/glutamate as primary amino donors.

Step-by-Step Solution:

Verification / Alternative check:
Enzyme assays and isotopic labeling confirm ammonium as the inorganic node before organic incorporation.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing uptake form (NO3−) with assimilation product (NH4+); overlooking the two-step reduction.

Final Answer:
Ammonia/ammonium (NH3/NH4+).