12.2.10—Lactate and ethanol fermentation

Waterlogging restricts gas diffusion to roots and can reduce oxygen available for aerobic respiration. Rice survives and grows by improving gas access through its tissues and by tolerating the limited ATP yield and toxic products of ethanol fermentation.

• Keep gas access: upward growth can keep leaves above the waterline, where stomata can exchange oxygen and carbon dioxide with air.
• Move gases through the plant: aerenchyma in roots and stems contains air spaces. These spaces allow gases entering through above-water stomata to diffuse to tissues above and below the water, including submerged roots.
• Use anaerobic ATP production when oxygen is scarce: ethanol fermentation allows glycolysis to continue, so cells still receive a small ATP supply. This is a low-yield solution because the mitochondrial aerobic stages cannot continue without sufficient oxygen.
• Limit ethanol damage: rice tolerates higher ethanol levels than some other plants and produces more ethanol dehydrogenase, which breaks down ethanol. This reduces the toxicity barrier to continuing anaerobic respiration.
• Growth boundary: these adaptations support survival and active growth in flooded conditions, but anaerobic respiration still has low energy output and can produce damaging waste products; it is not equivalent to fully aerobic respiration.

Not every rice plant or waterlogged plant must show every listed feature. Keep this card focused on gas-space transport, ethanol-fermentation tolerance and the associated energy/toxicity trade-off. Do not infer general effects of temperature or substrate concentration.