Biology PLANT WATER RELATIONS

### KEY TOPICS

star Comparison of Different Transport Processes
star Plant water Relations
star Water Potential

### COMPARISON OF DIFFERENT TRANSPORT PROCESSES

● The Table gives a comparison of the different transport mechanisms.

● color{violet}("Proteins in the membrane") are responsible for color{brown}("facilitated diffusion") and color{brown}("active transport") and hence show common characterstics of

star Being highly color{brown}("selective; ")
star They are liable to color{brown}("saturate,")
star Respond to color{brown}("inhibitors") and
star Are under color{brown}("hormonal") regulation.

● But color{violet}("diffusion whether facilitated") or not – take place only color{brown}("along a gradient") and do color{brown}("not use energy")

### PLANT-WATER RELATIONS

● color{brown}("Water") is essential for all color{violet}("physiological activities") of the plant and plays a very important role in color{violet}("all living organisms.")

● It provides the medium in which most substances are color{violet}("dissolved.")

● The color{brown}("protoplasm") of the cells is nothing but water in which color{violet}("different molecules") are dissolved and (several particles) suspended.

● A color{brown}("watermelon") has over 92 per cent water; most color{brown}("herbaceous plants") have only about 10 to 15 per cent of its fresh weight as color{violet}("dry matter.")

● Of course, distribution of water within a plant varies – color{brown}("woody parts") have relatively very little water, while soft parts mostly contain water.

● A color{brown}("seed") may appear dry but it still has water – otherwise it would not be alive and respiring!

● color{violet}("Terrestrial plants") take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., color{brown}("transpiration.")

● A mature corn plant absorbs almost color{brown}("three litres") of water in a day, while a mustard plant absorbs water equal to its color{brown}("own weight") in about 5 hours.

● Because of this high demand for water, it is not surprising that water is often the color{brown}("limiting factor") for plant growth and productivity in both agricultural and color{violet}("natural environments.")

### WATER POTENTIAL

● To color{violet}("comprehend plant-water relations"), an understanding of certain standard terms is necessary.

● color{brown}("Water potential" (ψ_w)) is a concept fundamental to understanding color{violet}("water movement.")

● color{brown}("Solute potential" (ψ_s)) and color{brown}("pressure potential" (ψ_p)) are the two main components that determine color{violet}("water potential.")

● Water molecules possess color{brown}("kinetic energy.")

● In color{violet}("liquid and gaseous") form they are in random motion that is both rapid and constant.

● The greater the color{brown}("concentration") of water in a system, the greater is its color{violet}("kinetic energy or ‘water potential’.")

● Hence, it is obvious that color{brown}("pure water") will have the greatest color{violet}("water potential.")

● If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.

● Thus water will move from the system containing water at higher water potential to the one having low water potential.

● This process of movement of substances down a gradient of free energy is called color{brown}("diffusion.")

● color{violet}("Water potential") is denoted by the Greek symbol color{brown}("Psi" "or" ψ )and is expressed in pressure units such as color{brown}("pascals (Pa).")

● By convention, the water potential of pure water at standard temperatures, which is not under any pressure,
is taken to be color{brown}("zero.")

● If some solute is dissolved in color{violet}("pure water,") the solution has fewer free water and the concentration of water decreases, reducing its color{violet}("water potential.")

● Hence, all solutions have a lower water potential than color{violet}("pure water;") the magnitude of this lowering due to dissolution of a solute is called color{brown}("solute potential") or color{brown}(ψ_s. ψ_s) is always negative.

● The more thecolor{brown}(" solute molecules"), the lower (more negative) is the s .

● For a solution at atmospheric pressure (water potential) ψ_w = (solute potential) ψ_s.

● If a pressure greater than atmospheric pressure is applied to color{violet}(" pure water") or a solution, its color{violet}("water potential") increases.

● It is equivalent to color{violet}("pumping water ") from one place to another.

● Pressure can build up in a color{violet}("plant system") when water enters a plant cell due to color{violet}("diffusion causing ") a pressure built up against the cell wall, it makes the cell color{brown}("turgid ") this increases the color{brown}("pressure potential.")

● Pressure potential is usually positive, though in plants color{brown}("negative potential or tension") in the water column
in the color{violet}("xylem plays") a major role in water transport up a stem.

● color{brown}("Pressure potential") is denoted as Psi_p.

● color{violet}("Water potential of a cell") is affected by both solute and pressure potential. The relationship between them is as follows:
Psi_w = Psi_s + Psi_p