Absorption of Water by Plants

Absorption of Water by Plants

Absorption of water by cells and roots

Availability of Water in the Soil Soil is the major source of water for plants. The plants absorb water through root hairs from the soil. There are four types of water, but all of them are not available to the plants.

Gravitational Water When the water enters the soil and passes through the spaces between the soil particles and reaches the water table, the type of soil water is called gravitational water. This water lies far below and is generally not available to the plant roots.

Hygroscopic Water This is the form of water which is held by soil particles on soil surface. The water is held tightly around the soil particles due to cohesive and adhesive forces. These forces greatly reduce the water potential and thus this type of water in soil is not available to plants.

Capillary Water In smaller pores of the soil, water is held against the force of gravity by capillary forces and is called capillary water. This form of water is most important to plantsand constitutes the only available source of water to plants.

Run Away Water All the rain water falling on the soil is not retained by it. Run away water does not enter the soil and gets drained away from soil surface and reaches to the water table.

Plants absorb water mostly from soil by their roots. As a general rule, a plant's root system is about as extensive as its shoot system. Water and minerals enter the plant from the soil through the surface of the fine branches of its roots. Several centimeters of the tip of each root are covered with numerous root hairs. Each root hair is a tubular projection of an epidermal cell. Root hairs provide the plant with an extensive surface area and maximum access to the water supply.

Plants have extensive root system to absorb water and minerals

Parts of a root Maximum absorption takes place through root hairs

Corn root with prolific growth of root hairs for uptake of water and nutrients from the soil.

For example, a single rye plant (Secalecerale) was found to have a root system that occupied 0.08 m 3 of soil. The plant had an estimated 13.8 million roots covered with 14 billion root hairs, providing a combined surface area of 632 m 2 (equivalent to oneseventh the area of a football field).

As the root tip grows into the soil, new root hairs emerge behind it, and thus new reservoirs of water are continually being tapped. Meanwhile, older root hairs usually die.

Water Movement Through A Plant

Plant physiologists discuss water relations in terms of water potential (Ψw), an expression of the free energy of water in a system. In this concept, water moves from a high to a low water potential. The water potential of pure water is set arbitrarily at 0. In a solution or under tension, the water potential is lower, a negative value.

Plant water transport is readily explained in terms of water potential. Water enters a root hair cell from the soil because cell solutes lower the water potential (Ψw). In the xylem vessels, water is under tension and has a still lower Ψw, and water is pulled from the endodermis and across the root.

Up in the leaf, the bundle sheath cells contain solutes and have a low Ψw; they pull water from the xylem. Photosynthetic cells in the leaf, with their high sugar content, have a still lower Ψwand draw water from adjacent cells in a chain extending to the bundle sheath.

As water evaporates from the leaf cells and diffuses out of the leaf, the lowered Ψwof these cells pulls water from adjacent cells, generating the transpiration pull. Outside the leaf, if the relative humidity is low, Ψwis low and water diffuses out of the intercellular spaces.

Water and minerals move from the root hair cell to cells deeper in the root.

Aquaporins are specific transport proteins - aid in passive movement of water only.

There are two types of movement of water in cell 1.Apoplasticmovement 2.Symplasticmovement

Mechanism of Water Absorption Water is absorbed in plants by two methods: 1. Active absorption 2. Passive absorption

Active Absorption Water is absorbed due to activities going on in roots. Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root cells such as respiration. Absorption takes place against concentration gradient -even when the concentration of cell sap is lower than that of soil water.

Passive Absorption Passive absorption is by osmosis. Passive absorption takes place along the concentration gradient -when the concentration of cell sap is higher than that of soil water. Water is absorbed when transpiration rate is high or soil is dry.

Mycorrhiza Mycorrhizae ( Myco= fungus, and Rhiza = root) are fungi that grow in mutually beneficial (symbiotic) or sometimes harmful relationships with the roots of vascular plants. More than 90 percent of plants have a mycorrhizalsymbiontthat plays an important role in biochemical soil processes.

Some mycorrhizaepenetrate into the cells of plant roots (endomycorrhiza) while others (ectomycorrhiza) grow between the roots of woody plants. Mycorrhizaefacilitate water and mineral exchange with soil, contribute to disease and toxicity resistance and help in the colonization of barren soil or degraded landscapes.

In exchange for these benefits, the fungi gain access to carbohydrates produced by the plant and transferred from the leaves to the root system. Mycorrhizalplants are at a particular advantage in nutrient-poor soils where they enable plants to exploit the limited resources of water and nutrients available to them.

Amanita A mycorrhizal fungus

Sequoia sempervirens The tallest tree of the word

The End