Maintaining pH in plant tissue culture — which approach is standard practice?
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AUse organic acids such as citric, fumaric, malic, and succinic acids to buffer the medium
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BUse synthetic buffers (e.g., MES, MOPS, HEPES, or Tris) to stabilize pH
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CBoth (a) and (b) are commonly used together to maintain pH
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DAdjust with ammonium salts that both feed nitrogen and stabilize pH long-term
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EDo nothing; pH remains constant in sealed vessels
Answer
Correct Answer: Use synthetic buffers (e.g., MES, MOPS, HEPES, or Tris) to stabilize pH
Explanation
Introduction / Context:pH stability in culture media affects nutrient availability, enzyme activities, and morphogenesis. Media are typically adjusted (e.g., to pH ~5.6–5.8) before autoclaving, but metabolic processes can shift pH over time, so buffering is often beneficial.
Given Data / Assumptions:
- Common buffers used in plant culture include MES and other Good's buffers.
- Organic acids listed are metabolic intermediates, not standard buffers for media.
- Ammonium salts alter pH during uptake and are not stable buffers.
Concept / Approach:
Synthetic zwitterionic buffers (e.g., MES, MOPS, HEPES) provide reliable buffering capacity in the slightly acidic range appropriate for plant culture. They minimize pH drift caused by ammonium assimilation or exudates. Organic acids are not used as primary buffers; Tris is less ideal at acidic pH but appears in some specialized formulations. Ammonium salts can acidify media as NH4+ is taken up, so they do not “maintain” pH.
Step-by-Step Solution:
Identify target pH (around 5.6–5.8).Select suitable buffering agents: MES or similar Good’s buffers.Exclude organic acids and ammonium salts as stable buffers.Choose the option naming synthetic buffers.Verification / Alternative check:
Media supplier guides and tissue culture manuals recommend MES for pH stabilization in many protocols.
Why Other Options Are Wrong:
Organic acids are not standard buffers for media; ammonium salts alter pH during assimilation; pH does not remain constant without buffering.
Common Pitfalls:
Over-buffering can chelate micronutrients or interfere with growth; use appropriate concentrations.
Final Answer:
Use synthetic buffers (e.g., MES, MOPS, HEPES, or Tris) to stabilize pH