The Web of Life (53-2) Water in Plants


Water constitutes nearly 90% of the growing tissues of plants, and 5–15% of seeds. Managing water is a critical challenge.

Water is the medium for plant biochemical reactions and transport. Water’s peculiar properties, including polarity, solvency, and viscosity, largely determine how plants are structured, as well as their metabolic dynamics.

Most biochemically important compounds are charged. Water polarity facilitates essential biochemical reactions, while its solvency tempers some reactions, such as the attraction of sodium (Na+) and chloride (Cl). While it may seem obvious that all cells take advantage of these water properties, the subtle attunement of life to the physics of chemistry remains a marvel.

The nature of the hydrogen–oxygen bond accounts for the high tensile strength of water in plant capillaries and enables water to exist as a continuum in plants: through roots, stems, and leaves. Properly managed, water easily flows.

Water’s resistance to pressure makes it a useful hydroskeleton. Land plant leaves owe their rigidity to the water pressure inside them, which is commonly 3–10 atmospheres (0.3–1 MPa). This pressure is vital during cell expansion. Plant tissues move by carefully altering relative pressures within.

Some water properties seem less useful. The rates at which oxygen and carbon dioxide diffuse in water are very low: 10,000 times slower than in air. This may have imposed a limit on the maximum thickness of leaves. To compensate, vascular plants evolved intercellular spaces in leaf tissue so that these gases need not diffuse in water by more than the length of a single cell.

Water’s relatively high viscosity creates considerable tension during transport, especially transpiration, when water is drawn through the fine capillaries of cellulose cells walls and xylem. But then, plant tissues are designed precisely to account for this level of viscosity and employ those specific properties to advantage.