(A) sucrose concentration in the culture medium
(B) CO2 concentration in the culture medium
(C) agar concentration in the culture medium
(D) NH4+ concentration in the culture medium
Photoautotrophic micropropagation enables plantlets to grow sugar-free by fixing CO2 via photosynthesis, mimicking ex vitro conditions for better acclimatization. This SEO article solves Q.55 with full option explanations for biotech students.
Correct Answer
(B) CO2 concentration in the culture medium drives photoautotrophic systems.
Elevating CO2 (e.g., 1000-3000 μmol mol⁻¹) compensates for low internal vessel levels (dropping to 100-200 μmol mol⁻¹ during light), enabling photosynthesis without sucrose.
Gas-permeable vessels plus CO2 enrichment yield robust plantlets with high survival post-transfer.
Option Analysis
(A) Sucrose Concentration
Sucrose supports photomixotrophic growth as a carbon source but inhibits photoautotrophy by repressing photosynthetic genes and causing hyperhydricity.
Increasing it defeats sugar-free goals.
(B) CO2 Concentration
Key for photoautotrophy: High CO2 maintains saturating levels inside vessels despite respiration/photosynthesis fluctuations, boosting biomass at low PPFD (50-150 μmol m⁻² s⁻¹).
24-h enrichment outperforms photoperiod-only.
(C) Agar Concentration
Agar provides physical support; higher levels reduce water availability but don’t enable autotrophy—may even limit gas exchange if too solid.
(D) NH4+ Concentration
NH4+ is a nitrogen source; excess causes toxicity, poor rooting, and ammonia buildup, unrelated to photosynthetic carbon fixation.
Factor Comparison
| Factor | Photoautotrophic Role | Effect of Increase | Sucrose-Free Viability |
|---|---|---|---|
| Sucrose | Carbon supplement | Inhibits PS genes | No |
| CO2 | PS substrate | Boosts growth | Yes |
| Agar | Gelling agent | Minor support | Neutral |
| NH4+ | N nutrition | Toxicity risk | No |
Exam tip: Photoautotrophy needs CO2 enrichment + ventilation + high PPFD; sucrose-free medium is diagnostic.
