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

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Respiration: Adaptations of Rice to Submerged Roots

Respiration in Rice (Oryza sativa) under Submerged Conditions

When rice is cultivated in flooded paddies, its roots are often completely submerged in water. This creates an anaerobic environment that limits the availability of oxygen for aerobic respiration. Rice overcomes this challenge through three inter‑related adaptations:

  • Development of aerenchyma in roots.

  • Ethanol fermentation in root cells.

  • Accelerated elongation of the shoot (stem) to restore contact with air.

1. Development of Aerenchyma in Roots

Aerenchyma are specialised tissues containing large intercellular air spaces that facilitate internal gas diffusion from the shoot to the submerged root tip.

  1. Formation process:

    • Cell death (lysigenous aerenchyma) or cell separation (schizogenous aerenchyma) creates voids.

    • Ethylene produced under hypoxia triggers programmed cell death in cortical cells.

  2. Function:

    • Reduces the diffusion distance for O₂.

    • Allows CO₂ produced in roots to escape upward, preventing toxic accumulation.

Suggested diagram: Cross‑section of a rice root showing aerenchyma air spaces extending from the stele to the outer cortex.

2. Ethanol Fermentation in Roots

When oxygen is scarce, rice roots switch from aerobic respiration to anaerobic ethanol fermentation to maintain ATP production.

The overall reaction can be written as:

\$\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}\$

Key steps:

  1. Glycolysis: Glucose is broken down to pyruvate, yielding a net 2 ATP per glucose molecule.

  2. Pyruvate decarboxylation: Pyruvate → Acetaldehyde + CO₂ (catalysed by pyruvate decarboxylase).

  3. Alcohol dehydrogenation: Acetaldehyde + NADH → Ethanol + NAD⁺ (catalysed by alcohol dehydrogenase), regenerating NAD⁺ for glycolysis.

Although ethanol fermentation is far less efficient than aerobic respiration, it enables the root cells to survive until oxygen becomes available again.

3. Faster Growth of Stems (Shoot Elongation)

Rice accelerates stem elongation to reach the water surface, where atmospheric oxygen is accessible. This rapid growth is driven by:

  • Increased synthesis of gibberellins (GA) under hypoxic conditions.

  • Enhanced cell division and elongation in the intercalary meristem of the internodes.

  • Utilisation of the limited ATP from fermentation to power growth processes.

By emerging above water, the shoot restores aerobic respiration in the leaves, which in turn supplies more carbohydrates to the root system.

Summary of Adaptations

AdaptationMechanismBenefit under Submergence
Aerenchyma developmentEthylene‑induced formation of air spaces in root cortexFacilitates internal O₂ transport and CO₂ removal
Ethanol fermentationGlycolysis → pyruvate decarboxylation → alcohol dehydrogenationProvides ATP and regenerates NAD⁺ when O₂ is absent
Accelerated stem growthGA‑mediated cell division/elongation in intercalary meristemAllows shoot to reach air, re‑establishing aerobic respiration

Key Points for Examination

  • Explain how aerenchyma reduces the diffusion path for gases.

  • Write the balanced equation for ethanol fermentation and identify the ATP yield.

  • Describe the hormonal control (gibberellins) of rapid internode elongation under hypoxia.

  • Link the three adaptations in a cause‑and‑effect chain: submergence → ethylene → aerenchyma & fermentation → limited ATP → GA increase → stem elongation → re‑aeration.