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WATER ABSORPTION BY ROOTS

WATER ABSORPTION BY ROOTS

Intimate contact between the surface of the root and the soil
is essential for effective water absorption by the root. This
contact provides the surface area needed for water uptake
and is maximized by the growth of the root and of root
hairs into the soil. Root hairs are microscopic extensions of
root epidermal cells that greatly increase the surface area
of the root, thus providing greater capacity for absorption
of ions and water from the soil. When 4-month-old rye
(Secale) plants were examined, their root hairs were found
to constitute more than 60% of the surface area of the roots

Water enters the root most readily in the apical part of the
root that includes the root hair zone. More mature regions of
the root often have an outer layer of protective tissue, called
an exodermis or hypodermis, that contains hydrophobic materials in its walls and is relatively impermeable to water.
The intimate contact between the soil and the root surface is easily ruptured when the soil is disturbed. It is for
this reason that newly transplanted seedlings and plants


 Root hairs make intimate contact with soil particles and
greatly amplify the surface area that can be used for water absorption by
the plant. The soil is a mixture of particles (sand, clay, silt, and organic
material), water, dissolved solutes, and air. Water is adsorbed to the surface of the soil particles. As water is absorbed by the plant, the soil solution recedes into smaller pockets, channels, and crevices between the soil
particles. At the air–water interfaces, this recession causes the surface of
the soil solution to develop concave menisci (curved interfaces between
air and water marked in the figure by arrows), and brings the solution
into tension (negative pressure) by surface tension. As more water is
removed from the soil, more acute menisci are formed, resulting in
greater tensions (more negative pressures).

greatly amplify the surface area that can be used for water absorption bythe plant. The soil is a mixture of particles (sand, clay, silt, and organicmaterial), water, dissolved solutes, and air. Water is adsorbed to the surface of the soil particles. As water is absorbed by the plant, the soil solution recedes into smaller pockets, channels, and crevices between the soilparticles. At the air–water interfaces, this recession causes the surface ofthe soil solution to develop concave menisci (curved interfaces betweenair and water marked in the figure by arrows), and brings the solutioninto tension (negative pressure) by surface tension. As more water isremoved from the soil, more acute menisci are formed, resulting ingreater tensions (more negative pressures).

need to be protected from water loss for the first few days
after transplantation. Thereafter, new root growth into the
soil reestablishes soil–root contact, and the plant can better
withstand water stress.
Let’s consider how water moves within the root, and the
factors that determine the rate of water uptake into the root

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