In this article we will discuss about:- 1. Introduction to Plant – Water Relations 2. Water Potential of Plants 3. Plasmolysis.
Introduction to Plant – Water Relations:
Actually, life itself originated in the hot primordial soup of the ancient oceans few billions of years ago. This primordial soup had water in abundance as a solvent. Water is indispensable for life. The maintenance of life on earth is impossible without water. The protoplasm has to have a high degree of hydration to carry out physiological processes. Almost all plant processes require enzymes for the various biochemical reactions.
These reactions require more than 90% of water to function. In many biochemical reactions water is either a reactant or a product or both. The oxygen so essential for the life of aerobic organisms is obtained from water during photosynthesis by green plants. Water provides the hydrogen necessary for sugar formation during photosynthesis.
Water is essential for all physiological activities of the plant and plays a very important role in all living organisms. It provides the medium in which most substances are dissolved. The protoplasm of the cells is nothing but water in which different molecules are dissolved and (several particles) suspended.
A watermelon has over 92 per cent water; most herbaceous plants have only about 10 to 15 per cent of its fresh weight as dry matter. Of course, distribution of water within a plant varies – woody parts have relatively very little water, while soft parts mostly contain water. A seed may appear dry but it still has water – otherwise it would not be alive and respiring!
Terrestrial plants take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration. A mature corn plant absorbs almost three litres of water in a day, while a mustard plant absorbs water equal to its own weight in about 5 hours. Because of this high demand for water, it is not surprising that water is often the limiting factor for plant growth and productivity in both agricultural and natural environments.
Properties of Water:
Water has many unique properties which enabled nature to make it a medium of life and for life:
(i) Many chemical substances easily dissolve in water; therefore water is called a universal solvent.
(ii) Cohesive and Adhesive Forces:
Water shows cohesive forces (attraction between similar molecules is called cohesion).
Water molecules also stick together with other substances which have charged groups of atoms or molecules and this force of attraction between dissimilar molecules is called adhesion.
Surface tension is due to the cohesive property of water molecules,
Adhesive and cohesive forces account for the process of capillary action or capillarity.
(iii) Temperature Stabilisation:
The amount of heat energy required to raise the temperature of water by 1 °C is very large. Water has a high specific heat. It has high heat of vaporization.
Water Potential of Plants:
Potential is a way of representing free energy. Every component of a system possesses free energy, which is available for doing work. Water potential is the chemical potential of water. Water potential of a system is defined as the difference between the free energy of water in the system and the free energy of pure water.
The term water potential is represented by the Greek alphabet psi (Ψ) as measured in Bars, Pascals Megapascals, (MPa)/ Kilopascals, (KPa) per mole of water (unit mass of water).
Pure water has the highest water potential of o or zero bar:
Pure water is not seen in plants and soil system since some solutes are always dissolved in it. All descriptions in plant water relations are expressed in the concept of water potential. A dilute sugar solution will have a less negative Ψ and a concentrated sugar solution will have a more negative Ψ.
In these two solutions are separated by a semipermeable membrane that will only permit movement of solvent (water) molecules then water will move from the dilute sugar solution to the concentrated sugar solution. We can also state that water moves from a region of less negative water potential to a region of more negative y. This can also be stated as water moves from a region of higher water potential to a region of lower water potential.
[In the earlier concept we would have said water moves from high DP to low DP or low DPD to high DPD since dilute solution has high DP and concentrated solution has low DP of solvent or water molecules].
Components of Water Potential (Ψ or Ψw):
Water potential has three components namely:
1. Solute potential or Ψs,
2. Pressure potential or Ψp and
3. Matric potential or Ψm. Ψ or Ψw = Ψs + Ψp + Ψm
Currently we do not use Ψw for water potential but we represent water potential by Ψ only.
1. Solute Potential or Ψs:
Solute potential is also called osmotic potential and is always of a negative value. Defined as reduction in the potential of a solvent (Ψw) due to the addition of solutes is known as potential or osmotic potential. It has always a negative value is the potential with which pure water diffuses towards a solution. It is represented as Ψs. The more the solute molecules the more the negative is Ψs. for a solution at atmospheric pressure is Ψw = Ψs – 0. The osmotic potential of pure water is zero.
2. Pressure Potential or Ψp:
It is the pressure potential. It is the pressure exerted by the cytoplasm on the cell wall. It is actually solute pressure developed in a turgid cell. It is always a positive value. When a cell has plenty of water in the cell sap or cell solution, (mostly found in large central vacuoles of living cells of the plant) the cell is said to be turgid and in a turgid cell the protoplast will exert an outward pressure on the cell wall called turgor pressure, the cell wall exerts a equal inward pressure on the protoplast called wall pressure. A turgid cell will have a positive Ψp and the Ψ will be less negative due to the presence of dilute cell sap with plenty of water. In a plasmolysed cell, Ψ can be highly negative since the cell has lost most of its water.
3. Matric Potential or Ψm:
It is the matric potential, it is generated by imbibition forces causing adhesion of water to wettable or water loving surfaces of cell walls, which has water loving or hydrophilic substances, such substances occur in cytoplasm also. The Ψm of parenchyma cells (vacuolated with large central vacuole) could be as low as -1.01 Mpa and is not taken into account for most of the calculations of Ψ.
Plasmolysis in Plants:
When we take a turgid cell (or a tissue made up of turgid cells) and immerse it in hypertonic solution water molecules will start leaving the cell by exosmosis (water will osmose out from the less -ve Ψ of turgid cell to the more -ve Ψ of the external hypertonic solution. At some stage the plasma membrane will just start withdrawing from the cell wall (at the corners). This stage is called incipient plasmolysis.
Later on the cell will undergoes total exosmosis and become a plasmolysed cell with a shrunken vacuole which has a highly concentrated cell sap due to loss of water. The plasma membrane withdraws into the interior of the cell. It is connected to the cell wall at the regions of plasmodesmata.
The plasmolysed cell is highly -ve (Ψ = -30 bars) due to the high concentration of cell sap in the shrunken vacuole. Ψ will be low.
When kept in contact, water will move from a turgid cell to a plasmolysed cell along the gradient i.e., less negative Ψ to more negative Ψ. This is very important in the process of water movement of various physiological processes in plants.
When a plasmolysed cell is placed in water or hypotonic solution, water will enter into it and it will turn turgid. This process is called deplasmolysis. Plasmolysis and deplasmolysis are reversible processes.
Significance of Plasmolysis:
(i) Fish, meat, pickles, and jams are salted or sweetened to a high degree in concentrated brine (salt) solutions or sugar solutions. All the living bacterial / fungal cells are killed by plasmolysis. This increases the shelf life of food items.
(ii) Salting kills weeds (unwanted plants) by plasmolysing their cells.
(iii) If the soil solution is made highly concentrated by putting more fertilisers then root cells will die by getting plasmolysed. This should be avoided in agriculture.
Water Relations of Turgid and Plasmolysed Cells:
A turgid cell has an abundance of water and most of the water is in the cell sap of the central vacuole. The central vacuole is large sized and the protoplast has the plasma membrane touching the cell wall. In fact, the protoplast exerts an outwardly directed High Turgor Pressure (TP) on the cell wall. The cell wall exerts an inwardly directed Wall Pressure (WP) on the protoplast.
The cell wall being very strong enables the cell to retain the high TP without damage to itself.
The cells pass through an intermediate condition between the fully turgid and fully plasmolysed states. This in between state is the flaccid state when the cell has lost sufficient water to lose turgidity, but the central vacuole is not fully turgid and not fully shrunk like that of a turgid and plasmolysed cell respectively.
Comparison of Different Transport Processes:
Table 11.1 gives a comparison of the different transport mechanisms. Proteins in the membrane are responsible for facilitated diffusion and active transport and hence show common characteristics of being highly selective; they are liable to saturate, respond to inhibitors and are under hormonal regulation. But diffusion whether facilitated or not – take place only along a gradient and does not use energy.
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