Nitrogen sources and culture pH in plant tissue culture media How does utilization of ammonium versus nitrate affect the pH of plant cell culture media over time?

Introduction / Context:
In plant tissue culture, media pH drifts as nitrogen sources are assimilated. Managing pH is vital for nutrient availability, enzyme activity, and growth of explants and callus.

Given Data / Assumptions:

• Media typically contain both NH4+ (ammonium) and NO3− (nitrate).
• Cells assimilate nitrogen via distinct pathways that consume or release protons.
• Buffers are limited; thus, assimilation impacts bulk pH.

Concept / Approach:
Assimilation of NH4+ tends to acidify the medium because uptake and incorporation release H+ equivalents to maintain charge balance. In contrast, NO3− reduction and assimilation generally consume H+ (or are coupled to OH− production), leading to a gradual rise in pH. Therefore, ammonium drives pH downward; nitrate drives pH upward.

Step-by-Step Solution:

Verification / Alternative check:
Empirical observations in MS-based media show pH decrease with predominant ammonium uptake and pH increase with nitrate uptake, confirming the biochemical rationale.

Why Other Options Are Wrong:

• Reversing the effects (option b) contradicts proton balance.
• Both rising (c) or both dropping (d) ignore the opposite ionic effects.
• Option e ignores widely observed pH drift.

Common Pitfalls:
Overlooking that relative uptake rates and initial buffering capacity modulate the magnitude of pH change, though the direction remains as stated.

Final Answer:
Ammonium utilization causes pH to drop, while nitrate utilization causes pH to rise