Water Absorption in Plants
Unlike aquatic plants, terrestrial plants need to absorb water from the soil all the time to maintain turgidity, metabolic activities, and the development and growth of the plant. It is vital to comprehend the structure of the soil, its water content and factors; and also, the plant structures associated with the absorption of water.
Soil is comprised of fine rock particulate of different sizes derived from the weathered igneous and sedimentary rocks. Environmental elements like, heat, wind, rain, cold, river streams, and oceanic waves act on the rock particles, which break them down to smaller-sized particles which on the accumulation in a shallow or flat surface, constitute the soil.
The process of soil formation on this Earth is a constant procedure and it is happening for the past 4.0 billion years and will continue as long as this earth exists.
Based on the size, structures of soil particles, and the structure of natural and inorganic components, soils have actually been categorized into rocky, coarse, sandy clay, and loamy soils. Rock particles of big sizes do not hold any water in between them and any such soil including rock particles that do not hold water in between them is not good for the development of the root system. Even the sandy soil of such small-sized rocks benefits aeration however not for water retention.
On the other hand, clay soils have colloidal particles that can hold water however extremely bad in aeration. However, the loam soil is excellent, due to the fact that it has a mixture of clay, sand, and decayed organic material called humus. This soil supplies great aeration and appropriate capillary areas to hold water. Hence this soil is thought to be the very best soil for the opulent development of the root system.
Structures Associated with Absorption of Water
In higher plants water is soaked up through root hairs that are in contact with soil water and form a root hair zone a little behind the root tips. Root hairs are tubular hair-like prolongations of the cells of the skin layer (when epidermis bears root hairs it is likewise called piliferous layer) of the roots. The walls of root hairs are permeable and include pectic compounds and cellulose which are highly hydrophilic (water caring) in nature. Root hairs consist of vacuoles filled with cell sap.
When roots lengthen, the older hairs die and new root hairs are developed so that they are in contact with fresh supplies of water in the soil.
Types of Water Absorption
Water absorption is of two types, passive and active.
1.Passive Water Absorption
The force for this kind of water absorption comes from the aerial parts of the plant due to loss of water in transpiration. This produces tension or low water potential of numerous environments in the xylem channels. The creation of stress in the xylem channels of the plant is evident from:
- (i) A negative pressure is commonly found in the xylem sap. It is because of it that water does not spill out if a cut is offered to a shoot.
- (ii) Water can be soaked up by a shoot even in the absence of the root system.
- (iii) The rate of water absorption is roughly equal to the rate of transpiration.
Root hairs operate as small osmotic systems. Each root hair has a thin permeable cell wall, a semipermeable cytoplasm, and an osmotically active cell sap present in the primary vacuole. Because of the latter, a root hair cell has a water potential of -3 to -8 bars.
2.Active Water Absorption
It is the absorption of water due to forces present in the root. Living cells in active metabolic conditions are necessary for this. Auxins are known to improve water absorption (even from hypertonic solution) while breathing inhibitors reduce the same.
Therefore, energy (from respiration) is involved in active water absorption. Water absorption from soil and its inward motion may happen due to osmosis. Passage of water from living cells to the xylem channels can happen by:
- (i) Build-up of sugars or salts in the tracheary aspects of xylem due to either secretion by the nearby living cells or left there during the decay of their protoplasts.
- (ii) Development of bioelectric potential favorable for movement of water into xylary channels.
- (iii) Active pumping of water by the surrounding living cells into tracheary components.
Field Capacity of Soil
The amount of water present within the soil as capillary water that is offered for the root system is typically referred to as the beneficial water content of the soil. Hence the capacity of soil to hold the optimum amount of utilizable or useful water is known as ‘Field capacity of the soil’, this again relies on the nature of the soil.
Sandy and rocky soils are poor soils in terms of water holding potential. While clay soils have excellent water retention capability but they have the worst aeration. Nevertheless, loam soil is the best for it keeps an excellent amount of water and likewise, it has excellent aeration. However, the field capacity of the soil can be figured out by finding out the difference between the weight of completely wet soil and that of the dry soil of a known amount.
As roots with their numerous branches and millions of root hairs diminish water from capillary spaces, water from the other regions move into the depleted spaces; in some cases, the water moves upwards from the water level and fills up the capillary areas, but the refilling or replenishment process takes its own time.
Thus, plants experience a deficiency of water for a short period of time, which is referred to as a short-lived or temporary wilting point. In some cases, the diminished water is not replaced for a long period of time, under such conditions, plants die, and such a state is called long-term or permanentwilting point.