Transport of Water in Plants

Introduction

Water is transported from soil to leaves through a tissue known as Xylem. Root hairs present on the root tend to absorb water from the soil and the osmosis process transports water to the leaves. The structure of roots, stems and leaves in a plant serve as tools for the transportation of water throughout the plant. Phloem and xylem are the main tissues that hold the responsibility of organic compounds transportation and water movement respectively in the plant. Regulation of stomata, water potential, and evapotranspiration are the process that influences the way in which water is transported throughout plants.

Water Potential

Plants are considered phenomenal hydraulic engineers as they use basic physics law along with simple manipulation of potential energy. Plants tend to move water up to the 116 meters tall trees. In addition to this, plants work as hydraulics for generating enough force which can buckle sidewalks and split rocks. Water potential can be described as the potential energy of water when water moves between two systems.

Solute Potential (ΨS)

Solute potential is also called osmotic potential. When some solute is dissolved in pure water, the concentration of water decreases. The presence of solute particles reduces the free energy of water and thus decreases the potential. Hence, all solutions have lower water. The magnitude (= amount) by which the water potential is lowered due to the presence of solute is called solute potential. T is denoted by the symbol ψs, the solute potential of solution is always less than zero or has negative value. The more solute molecules in a solution, the lower (more negative) the solute potential (ψs). For a solution at atmospheric pressure the water potential is equal to solute potential i.e., ψ = ψs.

Pressure Potential (ΨP)

When a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is just like pumping water from one place to another. Our heart builds up pressure for the circulation of blood in the body. Pressure can build up in a plant system, when water enters a plant cell due to diffusion, and makes the cell turgid. This increases the pressure potential. Thus, the pressure which develops in a plant cell or system due to entry or exit of water from it is called pressure potential. Pressure potential is denoted as ψp. A positive hydrostatic pressure in a plant cell is also called turgor pressure. Loss of water produces a negative hydrostatic pressure or tension. It develops in xylem due to loss of water in transpiration. This is very important in transport of sap over long distances in plants.

Water potential is affected by both solute and pressure potential. The relationship between them is as follows.

                 ψw = ψ s + ψp.

Phloem

 It is the vascular tissue that transports photosynthates from the leaves to the basal parts. Vascular cambium is an undifferentiated tissue that holds the responsibility of secondary growth and repair of phloem which is damaged. In other words, phloem is the living tissue that holds the responsibility of transporting organic material in varied parts of a plant.

Xylem

As stated above, the xylem is the tissue that holds the responsibility of transporting water to the leaves of the plant from the soil. The cells of the xylem are present in the stem, root and leaves are interconnected which form a conducting channel that reaches every part of a plant. The cells present in the roots sources ions from the soil that leads to the creation of differences in the concentration level of ions between the soil and roots. Due to this reason, continuous water movement is maintained in the xylem. Xylem plays a significant role in the transportation of water absorbed from the soil to different parts of the plant. Overall, xylem can be defined as the complex tissue present in plants, and it holds the responsibility of transporting water in the plants.

Osmosis

 Osmosis present in the plants causes osmotic pressure which leads to the transportation of one cell to another. Transpiration also causes continuous water loss in plants along with suction pressure due to the continuous forcing of water into the xylem cells located in the roots. Osmosis can be defined as the spontaneous passage of water through semipermeable membranes. With regard to osmosis in the plant, basic diffusion of molecules is carried out through semipermeable cells of plants by forcing higher solute concentration from a lower solute concentration. Plant cells at the root serve as guard cells that fill up water, swell water up for stomata and let the excess water out.

Transportation from Roots

• Water is transported to all the parts of the plants through xylem cells of the root. The manner in which water is transported is as follows
• Water goes through the roots of the plants as the xylem cells of roots continuously absorb water. After absorption, water is sent up to all the parts of the plant through cells present in the stem
•  A little amount of water sent to varied parts of plants is retained or used for photosynthesis. The remaining water is evaporated into the atmosphere through the Stomata. Stomata is present in the epidermis of the leaves and other external parts of the plant
• The xylem tissues, which are narrow in diameter, create a greater force of water transport in capillary tubes, fibers, and tracheid
• Situations in which xylem vessels get empty due to loss of water because of  transpiration, the water rises into leaves by the capillary force

Movement of Water in Roots

Negative water potential continues to drive momentary water inside the roots, as the potential of soil is much higher than that of the root. Additionally, the potential of the ground tissue is much higher than that of root vascular tissues. Once water is absorbed by the root hair, water moves through the ground tissue through one of three following routes before it enters the xylem cells of the plant:

•  Symplast: The meaning of sym is the same or share; thus, symplast is a shared cytoplasm. In this pathway, water moves from one cell of the cytoplasm to the next through the process of plasmodesmata. These plasmodesmata tend to physically join different plant cells before reaching the xylem cells
• Transmembrane pathway: In this pathway, water tends to move through varied water channels which are present in the plant cells known as plasma membranes, from one cell to the next one until it reaches the xylem eventually
• Apoplast: This term means outside of and with regards to water transportation it means outside of the cell. Through this pathway, water does not move through the plasma membrane of cells, rather it travels through cell walls that are surrounded by plant cells 

Conclusion

We read about the transportation of water in plants in this article. Also, we gained knowledge of modes of transport and water potential.In the case of transportation in plants, the biggest constraint is water as it ends up being a limiting factor in growth. To overcome this problem, trees and other plants have the perfect system for the absorption and translocation of water.