FACTORS AFFECTING WATER UPTAKE BY PLANTS

Описание: This lesson is on:
Factors that affect water uptake by plants
a) Available soil water
Plants can only absorb soil moisture that is free. If there is hygroscopic moisture absorption is very low. Hygroscopic moisture is found when the moisture in the soil is held at below permanent wilting point. Plant roots can easily absorb the soil moisture in between field capacity and permanent wilting point. When the soil moisture decrease below the wilting point, plant roots have to exert more pressure and thus rate of absorption decreases. On the other hand, when the soil is completely saturated with water or in excess of field capacity, then soil temperature and aeration are poor and this condition also affects the absorption of water.

b) Aeration of the soil
Absorption is mainly faster in well aerated soils than badly aerated one. Aeration affects the process of aerobic metabolism of the roots. Poor aeration is mainly due to excess soil moisture or compaction. However roots of species such as the hydrophytes absorb water even in an aerobic condition.

c) Soil temperature
Soil temperature is known to influence water absorption and ultimately transpiration to a considerable extent. In many plants, normal absorption of water take place at temperature of soil between 20 – 35°C. Increasing or decreasing soil temperature of soil between 20 – 35°C inhibits absorption. Cold soil is as physiologically dry. In most instances, temperature above 40oC does not support water absorption and plant can show signs of wilting. A freezing or low temperature slows down the rate of water absorption because of following causes.
a) Decreased root elongation
b) Increased viscosity of water
c) decreased permeability of cell membranes
d) Increased resistance to movement of water in to roots, this is caused by decreased permeability of cell membrane and the increased viscosity.

d) Concentration of soil solution
The rate of water absorption is inversely proportional to the concentration of minerals present in soil. When concentration of minerals is high rate of water absorption is low and vice versa. Water absorption only takes place in appropriate soil solution. Soil should be hypotonic & Plant must be hypertonic to carry out the process of endosmosis. If the concentration of soil minerals is high, it decreases the rate of absorption & plasmolysis & wilting takes place.

e) Root system
Root system is the plant factor which is directly related to the absorption of water from soil. Under favorable soil water, potential soil temperature, aeration, and roots system of the plants strongly influence the uptake of water. When growth of roots (root system) is more, uptake of water is also more under favorable soil conditions. Good root system that has developed during favorable growing periods make a plant can draw its moisture supply from deeper soil layers. Plants vary genetically in their rooting characteristics. Vegetable crops such as onions have a adventitious rooting system and are unable to use all the soil water within the root zone. Forage grasses, sorghum, maize and such other crops have very fibrous, dense roots.



f) Rate of transpiration
The rate of water absorption is directly proportional to the rate of transpiration. The rate of water absorption increases due to increase in the transpiration because passive water absorption increases due to transpiration. Transpiration produces transpiration pull in the xylem vessels. It pulls water upward. Thus negative pressure is produced in the xylem of root. It draws water from the soil.