Oxygen Transport and Plant Ventilation

Abstract

Internal transport of gases is critical for plants inhabiting flood-prone areas that experience soil oxygen deficiency. Plant adaptation to hypoxia/anoxia is not based on more efficient use of molecular oxygen but on a sustained supply of oxygen to the cells. The formation of gas-filled spaces in tissues (i.e., aerenchyma) is typical of wetland species and provides a path of low resistance for the gas transport along plant organs, especially between emergent shoots and submerged roots. Mechanisms facilitating gas movement to submerged tissues include diffusion and pressurized flows (i.e., convection). Diffusion is the most common mechanism explaining the oxygen movement into, and along, plant roots. The maximum length of a root growing in oxygen-deficient soil is determined by the internal diffusion of oxygen reaching the apex. Pressurized flows are possible in stems and rhizomes of emergent and floating-leaves species. Three different types of pressurized flows have been identified: (1) humidity-induced pressurization, which are flows (positive pressure) generated in living aerial tissues, resulting from a gradient in water vapor concentration across microporous partition separating the leaf gas-spaces and the environment; (2) thermal osmosis that involves the gas flow driven by temperature differences across a microporous partition, where the movement is against the heat flow and from the cold toward the warm side; and (3) venturi-induced suction (negative pressure), which occurs when the wind blows over broken culms creating a suction that moves gases to the rhizome system, while other culms (protected from wind) act as exit points. Finally, in the opposite direction to oxygen movement toward roots, other gases accumulating in submerged tissues are transported toward shoots and vented to the atmosphere, including ethylene and potent greenhouse gases like carbon dioxide, methane, and nitrous oxide.