12.2.12—Rice adaptations to flooding

Floodwater slows gas diffusion and can restrict oxygen reaching submerged roots. Rice survives and actively grows in waterlogged conditions by improving gas access and tolerating the low-yield, ethanol-producing conditions of anaerobic respiration.

• Upward growth: in some rice types, faster growth away from the waterline keeps leaves above the water. Stomata can then access oxygen and carbon dioxide from air.
• Aerenchyma: air spaces in specialised root and stem tissue provide a pathway for gases entering through above-water stomata. Oxygen and carbon dioxide can diffuse through the plant to tissues above and below the water, including submerged roots.
• Anaerobic ATP supply: when aerobic respiration is limited, ethanol fermentation regenerates oxidised NAD so glycolysis can continue. This preserves a small ATP supply, but it is less efficient than aerobic respiration.
• Ethanol tolerance: ethanol produced by fermentation can accumulate and damage plant tissue. Rice tolerates higher ethanol levels and produces more ethanol dehydrogenase, which breaks down ethanol; this permits anaerobic respiration to continue for longer and supports survival and growth.

The features solve different parts of the flooding problem: aerenchyma and upward growth improve gas access, whereas ethanol tolerance reduces the cost of relying on fermentation. They do not restore the full ATP yield of aerobic respiration, and not every rice type must show every feature. Temperature and substrate-concentration effects belong to the later investigation objectives.