Maarg Manthan · Topic 4.2

Plant Water Regulation (Transpiration and Osmosis)

Science & Technology › Plant Biology and Nutrition · Topic 4.2

Plant water regulation is the way plants take up, move and lose water while keeping their cells firm and their tissues working. Water moves into root cells by osmosis, from a region of higher water potential in the soil to a region of lower water potential in the cell. Pure water has the highest water potential, which is zero, and dissolved solutes lower it. Cells that take in water become turgid, which supports non-woody plants, and cells that lose water become flaccid, and in extreme loss the membrane pulls away from the wall in plasmolysis. Most of the water taken up is lost by transpiration through stomata, and the guard cells open or close the stomata to balance water loss against carbon dioxide intake.

5 min read · UPSC Prelims · Science & Technology · Plant Biology and Nutrition

At a Glance

Water enters roots by Osmosis
Pure water potential Zero (highest)
Turgid cell Full of water; firm
Stomata controlled by Guard cells
Water lost mostly by Transpiration
Exam link UPSC Prelims, Science & Technology: Plant Biology and Nutrition

Where Does It Fit?

This is the second topic of the macro on plant biology and nutrition. It applies the ideas of osmosis and transport to plant water balance and leads to the topic on reproduction. Previous topic: Photosynthesis: Light Reaction and Dark Reaction. Next topic: Reproduction in Plants: Sexual and Asexual Methods.

How Does Osmosis Control Water in Plant Cells?

Term	Meaning
Water potential	The tendency of water to move. It is the sum of solute potential, which is negative, and pressure potential, which is positive in turgid cells
Osmosis	Movement of water through a selectively permeable membrane from higher to lower water potential, as in Cell Membranes and Cell-Cell Communication/Interactions
Turgor pressure	Pressure of the cell contents against the wall, which keeps leaves and herbs firm
Plasmolysis	Shrinking of the protoplast away from the wall in a hypertonic solution. It is reversible at first
Imbibition	Absorption of water by dry materials such as seeds and wood, which causes swelling and helps seeds to germinate
Wilting	Drooping of leaves when water loss exceeds uptake and cells lose turgor

How Do Stomata Regulate Water Loss?

Guard cells: bean-shaped cells around each stoma. When they take up water and become turgid, the stoma opens, and when they lose water, it closes.
Potassium ions: light and low carbon dioxide cause potassium ions to enter guard cells, which draws in water and opens stomata.
Abscisic acid: a plant hormone released in drought that closes stomata, which saves water.
Types of transpiration: most is stomatal, with a smaller part cuticular through the leaf surface and lenticular through stems.
Daily pattern: stomata open in the day for photosynthesis and close at night in most plants, and CAM plants do the reverse, as in Photosynthesis: Light Reaction and Dark Reaction.
Trade-off: open stomata let in carbon dioxide but also let out water, so plants balance the two.

How Are Plants Adapted to Different Water Conditions?

Group	Adaptations and examples
Xerophytes	Plants of dry habitats. Thick cuticle, sunken stomata, leaves reduced to spines and water stored in stems. Cactus, Opuntia, Acacia and Euphorbia
Hydrophytes	Aquatic plants with thin or no cuticle, air spaces in tissues, and stomata on the upper surface of floating leaves. Lotus, Hydrilla and water hyacinth
Mesophytes	Plants of moderate conditions with broad leaves and stomata on both surfaces. Most crops and garden plants
Halophytes	Plants of salty soil, with salt glands and pneumatophores. Mangroves such as Rhizophora

What Else Should You Remember and Why Does It Matter?

Guttation and transpiration: guttation is loss of liquid water through hydathodes, while transpiration is loss of vapour, as in Transport System in Plants and in a Single Cell.
Anti-transpirants: chemicals or films sprayed on leaves to reduce water loss in dry conditions.
Water use efficiency: the amount of dry matter made per unit of water, which is higher in C4 and CAM plants.
Irrigation methods: drip and sprinkler irrigation, mulching and laser levelling save water. The Pradhan Mantri Krishi Sinchayee Yojana, launched in 2015, promotes per drop more crop.
Drought-tolerant crops: breeding and genetic engineering aim to produce crops that keep growth with less water, and millets use less water than rice.
Salinity and waterlogging: salty soils lower water potential and make it harder for roots to absorb water, and waterlogged soils lack oxygen for roots.
Exam pattern: classification of plants by habitat, the meaning of osmosis terms and stomata control.
Root pressure and uptake: roots absorb water mostly through root hairs, and the large surface of root hairs makes uptake efficient, as in the study of Transport System in Plants and in a Single Cell.
Wilting point: the permanent wilting point is the soil moisture level at which plants cannot recover even if the air is humid, and it differs by soil type.
Cavitation: in drought, air bubbles can break the water column in the xylem, which blocks transport and can kill the plant.

As an analytical point, plants cannot move to find water, so they regulate its loss and uptake. Water use in agriculture is a central issue in India, where irrigation uses most of the freshwater withdrawn.

Why Is This Topic Important?

Facts Terms and plant types asked
Concept Stomatal control
Link Irrigation and drought

What Came Next?

The next topic covers reproduction in plants through sexual and asexual methods. Next topic: Reproduction in Plants: Sexual and Asexual Methods.

Key People and Terms

Water potential Tendency of water to move
Turgid Swollen with water
Plasmolysis Shrinking of protoplast
Xerophyte Dry habitat plant
Hydrophyte Water plant
Halophyte Salt-tolerant plant

Exam Corner

Points to Remember

Pure water: water potential zero.
Abscisic acid closes stomata.
Cactus: xerophyte; mangrove: halophyte.
Turgor keeps herbs firm.
Per drop more crop: PMKSY.

Do Not Confuse With

Turgid and flaccid Full of water versus losing water
Hydrophyte and halophyte Water plant versus salt plant
Osmosis and imbibition Across a membrane versus uptake by dry material

Memory Hook

Water flows to lower potential; turgid firm, plasmolysed shrunk; guard cells with potassium open, abscisic acid shuts.

Mains Angle

Questions link plant water balance with agriculture. These points add depth.

Cell level Osmosis and turgor
Leaf level Stomata
Field level Irrigation efficiency

A Question You May Face

An original practice question, not a past paper question.

“Explain how plants regulate water uptake and loss, and discuss measures to improve water use efficiency in agriculture.”

How to Answer

Osmosis Potential.
Stomata Guard cells.
Adaptation Habitats.
Farming Drip, millets.

GS Relevance

Prelims: osmosis terms, stomatal control, plant types by habitat, water-saving methods. GS Paper 3: science and technology, agriculture.

Frequently Asked Questions

What is water potential in plants?

Water potential is the tendency of water to move, measured as the sum of the solute potential and the pressure potential. Pure water has the highest value, which is zero, and water moves by osmosis from a region of higher water potential, such as moist soil, to a region of lower potential, such as the root cell.

How do stomata open and close?

Stomata open when the two guard cells around them take up water and become turgid, which is triggered by light and low carbon dioxide and involves potassium ions entering the guard cells. They close when the guard cells lose water, which happens in darkness or drought and is promoted by the hormone abscisic acid.

What is plasmolysis?

Plasmolysis is the shrinking of the protoplast of a plant cell away from the cell wall when the cell is placed in a hypertonic solution and loses water by osmosis. It can be reversed at first by putting the cell in a dilute solution, but prolonged plasmolysis kills the cell.

What are xerophytes and hydrophytes?

Xerophytes are plants adapted to dry habitats, with features such as a thick cuticle, sunken stomata, spines in place of leaves and water storage, as in cactus. Hydrophytes are aquatic plants with thin cuticles, large air spaces and floating leaves, as in lotus and Hydrilla.

How can agriculture save water?

Farmers can save water by using drip and sprinkler irrigation, mulching, laser land levelling, growing less thirsty crops such as millets and pulses, and using crop varieties that tolerate drought. The Pradhan Mantri Krishi Sinchayee Yojana, launched in 2015, promotes more crop per drop of water.

PYQ Practice — Statement Analysis

1 A turgid cell is full of water and firm.

True

It has high pressure potential.

2 Abscisic acid opens stomata.

False

It closes them.

3 Cactus is an example of a xerophyte.