explain how rice is adapted to grow with its roots submerged in water, limited to the development of aerenchyma in roots, ethanol fermentation in roots and faster growth of stems

Respiration in Rice (Oryza sativa) under Submerged Conditions

Rice is normally cultivated in flooded paddies where the whole root system is submerged. Water‑filled soil rapidly becomes anaerobic, so the roots cannot obtain the oxygen (O₂) required for aerobic respiration. To survive, rice combines three inter‑related adaptations that are all covered in the Cambridge AS & A‑Level Biology syllabus (Topic 12 Energy & Respiration, Topic 7 Transport in Plants, Topic 4 Cell Membranes & Transport):

• Development of aerenchyma in roots (internal air‑space tissue)
• Ethanol fermentation in root cells (anaerobic ATP production)
• Accelerated elongation of the shoot (stem) to reach the air–water interface

1. Aerobic vs. Anaerobic Respiration in Submerged Roots

Why aerobic respiration stops in submerged roots: O₂ diffuses through water ~10 000 times more slowly than through air. The root tip quickly becomes hypoxic, preventing the mitochondrial electron‑transport chain from operating.

1.1. Anaerobic Pathways Required by the Syllabus

• Ethanol fermentation (plants and yeast) – the pathway used by rice roots (see Section 2).
• Lactate fermentation (animals) – included in the syllabus for comparison; the reaction is
  

  Glucose → 2 lactate + 2 ATP + 2 NAD⁺ (net ATP = 2 per glucose). Rice does not rely on this route.

2. Development of Aerenchyma in Roots

Aerenchyma are specialised tissues containing large intercellular air spaces that allow gases to move internally from the shoot to the submerged root tip. In the syllabus they are described as a bulk‑flow transport tissue analogous to xylem transport of water.

1. Formation mechanisms

   • Lysigenous aerenchyma – programmed cell death (PCD) of cortical cells creates voids.
   • Schizogenous aerenchyma – separation of cell walls without cell death.
   • Ethylene production rises under hypoxia; ethylene triggers PCD in the outer cortex, initiating lysigenous aerenchyma.

2. Functions (AO 1)

   • Provides a low‑resistance internal pathway for O₂ to diffuse from the shoot to the root tip.
   • Allows CO₂ generated by root metabolism to escape upward, preventing toxic accumulation.
   • Improves root buoyancy, keeping the root in contact with water‑logged soil.
   • Reduces the overall resistance to gas diffusion, which indirectly helps maintain favourable water‑potential gradients for water uptake (link to Topic 4.2).

3. Ethanol Fermentation in Submerged Roots

When O₂ is unavailable, rice roots switch from aerobic respiration to anaerobic ethanol fermentation to maintain a minimal ATP supply and to regenerate NAD⁺.

3.1. Overall Reaction

\[

\text{C}6\text{H}{12}\text{O}6 \;\xrightarrow{\text{glycolysis}}\; 2\;\text{CH}3\text{CH}2\text{OH} + 2\;\text{CO}2 + 2\;\text{ATP}

\]

3.2. Step‑by‑step pathway (cytosol)

1. Glycolysis – glucose → 2 pyruvate + 2 ATP + 2 NADH.
2. Pyruvate decarboxylation – pyruvate → acetaldehyde + CO₂ (enzyme: pyruvate decarboxylase).
3. Alcohol dehydrogenation – acetaldehyde + NADH → ethanol + NAD⁺ (enzyme: alcohol dehydrogenase). NAD⁺ regeneration allows glycolysis to continue.

Energy yield (AO 2): Only the 2 ATP from glycolysis are retained; oxidative phosphorylation is absent.

Why fermentation is still useful: It provides a rapid, albeit low‑yield, source of ATP and prevents the build‑up of NADH, which would otherwise halt glycolysis.

4. Accelerated Stem (Shoot) Growth – Re‑aeration Strategy

Rice seedlings respond to root hypoxia by elongating their shoots so that leaves can reach the air–water interface, restoring aerobic respiration in the photosynthetic tissues.

• Hormonal control – hypoxia induces synthesis of gibberellins (GA) in the stem. GA stimulates cell division and elongation in the intercalary meristem of the internodes.
• Energy use – the limited ATP from fermentation is directed toward active transport, cell‑wall synthesis and cell expansion.
• Outcome – once the shoot emerges above water, atmospheric O₂ diffuses down the aerenchyma, re‑establishing aerobic respiration in the roots and increasing carbohydrate supply to the whole plant.

5. Links to Other Syllabus Topics

• Water potential (Topic 4.2) – aerenchyma reduces the resistance to gas diffusion, which helps maintain a favourable water‑potential gradient for water uptake by keeping root metabolism active.
• Transport in plants (Topic 7) – aerenchyma is a specialised bulk‑flow transport tissue for internal O₂, analogous to xylem transport of water.
• Energy & ATP (Topic 12.1) – the ATP generated by fermentation fuels the gibberellin‑driven growth response.

6. Practical Investigation (AO 3)

Objective: Compare the respiratory behaviour and growth of rice seedlings under submerged versus non‑submerged conditions.

• Apparatus: sealed respirometer bottles, KOH pellet (CO₂ absorber), graduated syringe or gas‑collection burette, glucose solution (optional), ruler, ethylene detector (optional).
• Variables

  • Independent – water level (fully flooded vs. aerated soil).
  • Dependent – volume of CO₂ produced, O₂ consumption (if measured), shoot elongation rate.
  • Controlled – temperature, light intensity, seedling age, glucose concentration, container size.

• Procedure (outline)

  1. Place equal numbers of 7‑day‑old seedlings in two airtight chambers; add water to one chamber to submerge the roots.
  2. Insert a KOH pellet to absorb CO₂ and a graduated syringe to record the volume of gas displaced over 2 h.
  3. If an O₂ sensor is available, record O₂ consumption simultaneously.
  4. Measure shoot length before and after the experiment; calculate elongation rate (mm h⁻¹).

• Data handling (AO 2)

  • Calculate RQ = (V_CO₂ produced) / (V_O₂ consumed). In the submerged treatment V_O₂ will be very low, giving an apparent RQ close to 0.
  • Use the gas‑volume data to estimate the amount of ATP generated (2 ATP per glucose for fermentation vs. ~30 ATP for aerobic respiration).
  • Present results in a table and a bar chart comparing the two treatments.

• Evaluation points

  • Possible leakage of gases from the chamber – minimise by using airtight seals.
  • Temperature fluctuations affect gas solubility; keep chambers in a water‑bath at constant temperature.
  • KOH absorbs CO₂ but does not affect O₂; if O₂ is measured, ensure the sensor is calibrated.
  • Biological variation – use at least three replicates per treatment and calculate mean ± standard deviation.

7. Summary of Adaptations (AO 2)

Key Points for Examination (AO 1 & AO 2)

• Describe the four stages of aerobic respiration and state why the link reaction, Krebs cycle and oxidative phosphorylation cannot occur in submerged roots.
• Write the balanced equation for ethanol fermentation and give the net ATP yield (2 ATP per glucose).
• Briefly describe lactate fermentation (reaction and ATP yield) to satisfy the syllabus requirement.
• Explain how ethylene triggers aerenchyma formation (lysigenous vs. schizogenous).
• State the role of gibberellins in rapid internode elongation under hypoxia.
• Link the three adaptations in a cause‑and‑effect chain: submergence → hypoxia → ↑ ethylene → aerenchyma & fermentation → limited ATP → ↑ GA → shoot elongation → re‑aeration.
• Contrast the RQ for aerobic carbohydrate respiration (≈1.0) with the lack of a meaningful RQ for anaerobic fermentation.
• Outline a practical method to investigate the effect of submergence on respiration and growth, including data handling and sources of error.

¹ The figure “≈30 ATP” is a conventional textbook estimate; modern biochemistry gives a range of 30–32 ATP per glucose depending on the shuttle systems used. The exact number is not required for the Cambridge syllabus – the important point is that aerobic respiration yields far more ATP than anaerobic fermentation.