Class 11 Transport in plants Absorption of Water
ABSORPTION OF WATER
l The movement of water from the soil to the xylem of the root is called absorption of water. For water absorption from soil through root, the roots must be metabolically active, respiring aerobically and continuously growing
REGIONS OF ROOTS
l Root cap region– – No absorption of water.
l Meristematic region– No absorption of water.
l Growing region– Region of elongation–Only some amount of water absorption.
l Root hair region– Maximum absorption.
l Mature region– No absorption.
STRUCTURE OF ROOT HAIR
l It is a delicate tubular elongation of the epidermal cell (trichoblast) of the root. It is 1mm to 1cm long and 10 in diameters. It is short-lived, lasting only for few days. It is more developed in herbs.
(Number = several hundreds/mm2 of root surface).
l It has a permeable cell wall, with an outer layer of pectic materials and inner of cellulose. The former possessing great water imbibing property and due to being sticky it brings closer contact between soil water and root hair. The cell membrane is selectively permeable. Protoplasm is a single nucleus at the tip.
l Root hair cells function as a tiny osmotic system as they have a D.P.D of 3–8 bars while D.P.D of soil water is 0.1 to 0.3 bars.
FUNCTIONS OF ROOT HAIR
l It increases the absorptive surface. By their growth, they move to the soil particle in search of water.
SOURCE OF SOIL WATER
l The ultimate source of all soil water is atmospheric precipitation (rainfall).
l Water reaching the deeply saturated zones of soil is called gravitational water. Gravitational water is not a permanent constituent of soil water. It occurs only after a good rain or irrigation. Excess water is drained downwardly under the influence of gravity through soil macrospores larger than. The phenomenon is called percolation.
FORMS OF SOIL WATER
l Capillary water: It is present in between capillary spaces (having a diameter of less than 20) of soil particles. Capillary water is held in the soil by capillary forces. It, therefore, does not fall down to water table by gravity.
l Hygroscopic water: It is present in a thin film around soil particles due to adhesive force This is not available to plant roots since the water is held by soil particles with very strong attractive forces.
l Combined water: It lies in chemically combined form. It is also not available to plants.
l Water vapour: It occurs in soil air spaces (macropores).
l Of these only capillary water is available to plants.
Ä The total amount of water present in the soil is called holard.
Ä Of this the available water to the plant is called chresard
Ä Water which cannot be absorbed is called enchard or non-available water.
Ä Field capacity: The total amount of water retained by a unit weight of soil under the controlled condition is called field capacity. It is 25–35% in common loam soil.
Ä Storage capacity: The amount of capillary water retained by a unit weight of soil under the controlled condition is called storage capacity.
Ä Water holding capacity: The percentage of moisture held in the form of thin film in a fully saturated soil is called water holding capacity.
Ä Permanent wilting percentage (PWP) or Wilting coefficient: The percentage of water that remains in the soil when permanent wilting is attained is called wilting coefficient.
l Major proportion of water flow in the root cortex occurs via the apoplast, as the cortical cells are loosely packed, and thus, the cortex offers no resistance. The apoplastic water movement beyond the cortex is blocked by the Casparian strip present in the endodermis. The Casparian strip is composed of wax-like substances called suberin, which blocks water and solute movement through the cell wall of the endodermis. Thus, beyond endodermis, water is forced to move through the cell membranes. This movement of water through the cell membrane is called transmembrane pathway. In this pathway, water may also cross through the tonoplast surrounding the vacuole. Once the water reaches root xylem, transpiration drives the water to move to the leaves through the stem.
l Some plants use mycorrhiza for absorption of water. A mycorrhiza is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root and they penetrate the root cells. The hyphae have a very large surface area that absorbs mineral ions and water from the soil. Some plants have an obligate association with the mycorrhizae. For example, Pinus seeds cannot germinate and establish without the presence of mycorrhizae.