BOT 307

PLANT PHYSIOLOGY 1
Mineral Nutrients and Transport
Mineral nutrition in plants is a series of biochemical, biophysical and physiological processes through
which mineral ions are absorbed from the soil, transported and included in the metabolism. Chemical
analyses showed that the vegetal matter contains more than 60 chemical elements, but only 15 are necessary
to provide the normal growth and development of plants. During the mineral nutrition process, they absorb
mineral salts from the soil and integrate them into complex organic substances or cell structures.
Mineral nutrients are elements acquired primarily in the form of inorganic ions from the soil. The study of
how plants obtain and use mineral nutrients is called MINERAL NUTRITION. Water and dissolved
mineral salts from the soil are absorbed by the root system of the plant. The large surface area of roots and
their ability to absorb inorganic ions at low concentrations from the soil solution make mineral absorption
by plants a very effective process. After being absorbed by the roots, the mineral elements are translocated
to the various parts of the plant, where they are utilized in numerous biological functions.
ESSENTIAL NUTRIENTS
Only certain elements have been determined to be essential for plant growth. An essential element is defined
as one whose absence prevents a plant from completing its lifecycle or one that has a clear physiological
role. If plants are given these essential elements as well as energy from sunlight, they can synthesize all the
compounds they need for normal growth.
Hydrogen (H), Carbon (C), Oxygen (O), Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg),
Phosphorus (Ph), Sulfur (S), Silicon (Si), Chlorine (Cl), Iron (Fe), Boron (B), Manganese (Mn), Sodium
(Na), Zinc (Zn), Copper (Cu), Nickel (Ni), Molybdenum (Mo), are essential for most/ all higher plants.
Mineral nutrients being elements cannot be synthesized biochemically by plants. C, H, and O are obtained
from the air and water. Severally other elements are obtained from the soil.
Criteria for essentiality of elements
1. The element must be essential for the normal growth and reproduction of plants.
2. It should be indispensable i.e. no other element could replace it
3. The requirement of this element must be direct and not due to an indirect effect.
Classification of essential nutrients
Essential nutrients can be classified into two
1. Macronutrient
2. Micro/trace nutrient
Macronutrients
They are required by plants in relatively large quantities. They are called major elements of plant nutrition.
They include nitrogen, potassium, calcium, magnesium, phosphorous, sulphur, and silicon.

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Micronutrients
They are required by plants in lower concentrations. They are called trace elements of plant nutrition. They
include chlorine, iron, boron, manganese, sodium, zinc, copper, nickel, and molybdenum.

Roles of the mineral elements in plants

1. Carbon, hydrogen and oxygen: these are secured from the air and soil in the form of
carbondioxide and water respectively. The three gases are components of the protoplasm, cell wall
and most of the organic constituents of the plants.
2. Nitrogen: plants require it in the greatest amount. Plants obtain their nitrogen supply mainly in the
form of nitrate (NO3) from the soil but also as ammonium ions (NH4). Legumes, which possess
rhizobium bacteria in their nodules can utilize molecular nitrogen. Nitrate ions (NO3-) are reduced
to the NH2 group through the series of enzymatic reactions in the plant cells. Nitrogen is an
essential component of proteins, protoplasm, enzymes and chlorophyll. It is a constituent of purines
and pyrimidines which are components of nucleic acid (RNA & DNA). It also occurs in Coenzymes
which are essential for the functioning of enzymes. Plants supplied with excessive nitrogen are
usually dark green in colour, abound in foliage but with a weak developed root system. Reduction
in flowering and seed formation also occurs in several crops. Nitrogen deficiency results in
yellowing of leaves due to loss of chlorophyll (Chlorosis). Leaf development is poor and plants
become stunted due to low protein synthesis.
3. Phosphorus (H2PO4- Phosphate ions): It is an integral component of important compounds of
plant cells including phospholipids, nucleic acids, nucleo-proteins, Coenzymes and ATP. They are
involved in processes of photosynthesis, glycolysis, respiration etc. Phosphorus occurs in
abundance in the meristematic tissues and storage organs e.g. seeds & fruits.
Symptoms of P deficiency include stunted growth in young plants and a dark green coloration of
the leaves which may be malformed and contain small spots of dead tissues called Necrotic spots.
Premature leaf fall and distortion of leaf shape may also occur.
4. Potassium (K): K+
It is highly mobile in plants, with young & actively growing regions such as buds, leaves and root
tips having an abundance of it. It is important in chlorophyll development and catalyses
carbohydrate, breakdown during respiration. It activates many enzymes involved in respiration and
photosynthesis. Potassium deficiency results in weakening of leaves, chlorosis, rolling of leaves,
stunted growth & shortening of internodes.
5. Calcium (Calcium nitrate or Calcium sulphate):- it is a constituent of the middle lamella and
helps cement the wall of the cells together. Calcium is essential for the formation of cell membranes

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 and lipid structures. Calcium is essential for mitosis and it is concerned with chromatin or Mitotic
spindle organization. Calcium deficiency leads to the death of meristematic regions. Chlorosis
occurs along the margin of young leaves becoming necrotic. The tips of young leaves become
hooked, cell walls become rigid and brittle. Young leaves may also appear deformed. The root
system of a calcium-deficient plant may appear brownish, short and highly branched.
6. Magnesium Mn2+:- Magnesium plays a significant role in photosynthesis and carbohydrate
metabolism. It is a constituent of chlorophyll molecules & prevents the interveinal chlorosis of the
old leaves. It helps in the maintenance of the structure of ribosomes. It helps in the activation of
enzymes involved in respiration, photosynthesis & the synthesis of DNA & RNA. Deficiency in
magnesium causes interveinal chlorosis first in the old leaves and then in the young leaves. If the
deficiency is extensive, the leaves may become yellow or white, leaf abscission may also occur.
7. Sulphur (Sulphate ions): It is a constituent of several coenzymes and vitamins essential for
metabolism. Symptoms of Sulphur deficiency are chlorosis, stunting of growth, and delayed
flowering.
8. Silicon: - Amorphous silica (SiO2.nH2O): They are deposited as amorphous silica in call walls,
intracellular spaces, and endoplasmic reticulum. It contributes to cell wall mechanical properties,
including rigidity and elasticity. Plants show enhanced growth when supplied with an adequate
amount of silicon. Silicon can ameliorate the toxicity of many heavy metals. Plants deficient in
silicon are more susceptible to lodging (falling over) and fungal infection.
9. Chlorine (Chloride ion Cl-): It is required for the water-splitting reactions of photosynthesis. It is
required for cell division in both leaves and roots. Chlorine deficiency is unknown in plants grown
in native or agricultural habitats because chloride ions are very soluble. Chloride deficiency causes
stunted root formation, reduced fruiting and inhibition of photosynthesis. It is universally required
by the plants. It also stimulates the activity of several enzymes.
10. Iron (Ferric state Fe3+): Active as ferrous (Fe2+). It is not a constituent of chlorophyll but it is
important for its synthesis. It is a component of enzymes involved in the transfer of electrons such
as cytochromes. Iron-deficient plants exhibit interveinal chlorosis in leaves and inhibit protein
synthesis.
11. Boron (Soluble borate ions (BO3-): It plays a role in cell elongation, nucleic acid synthesis,
hormone responses and membrane function. Deficiency of Boron cause Black necrosis of the young
leaves and terminal buds. Stems may be unusually stiff and brittle. Loss of Apical dominance
causes plants to become highly branched. Structures such as fruit, fleshy roots and tubers may
exhibit necrosis or abnormalities related to the breakdown of internal tissues.

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 12. Manganese (Manganese ions (Mn2+): It is required in very low concentrations as its high
concentrations become toxic to plants. It activates several enzymes in plant cells especially
decarboxylases and dehydrogenases in the Krebs cycle. Symptoms of manganese deficiency are
interveinous chlorosis associated with the development of small necrotic spots, disorganization of
the lamellar chlorophyll becoming yellow-green and finally disintegrating.
13. Sodium: - Most species utilizing the C4 and CAM pathways of carbon fixation require sodium
ions (Na+). Under sodium deficiency, these plants exhibit chlorosis and necrosis or even fail to
form flowers.
14. Zinc: - Zinc ions (Zn2+): Zinc is required for chlorophyll biosynthesis in plants. It is essential for
normal metabolism of plants but it is highly toxic in high concentrations. It is involved in the
synthesis of auxin and protein, it is involved in protein synthesis. Zinc deficiency is characterized
by a reduction in intermodal growth, older leaves may become interveinously chlorotic and then
develop white necrotic spots.
Roles of macronutrients in plants
S/N Elements Functions in cell metabolism Functions at whole-plant level
1. Nitrogen Constituent of amino acids, nitrogenous Enhanced seed and fruit production;
bases, cofactors, alkaloids, coenzymes, improved leaf and forage crop
and chlorophyll, including some production
hormones (IAA)
2. Phosphorus Required as phosphate in sugar; as ester in Rapid growth; encourages blooming
DNA, and RNA; as phospholipids in and root growth
membrane and is a constituent of ATP
3. Potassium As an activator of enzymes; essential ion Regulates opening and closing of
for protein synthesis; manufacture of stomata
sugar and starches
4. Sulphur Constituent of amino acids (cysteine and Improves root growth and seed
methionine); vitamins (thiamine and production; helps with vigorous plant
biotin); coenzyme A (required in growth and resistance to cold
respiration); formation of sulpholipids
5. Calcium Cell wall formation; maintenance of Regulates fruit quality, protects
membrane structure and permeability; cell against heat stress and disease
signalling
6. Magnesium Constituent of chlorophyll molecules and Nutrient uptake control, root
required as an enzyme activator; essential formation
for binding of ribosome subunit

Role of micronutrients
S/N Elements Functions in cell metabolism Functions at whole-plant level
1. Iron Chlorophyll formation and synthesis of Provides resistance against plant
ferredoxin and cytochromes, activates Pathogens
catalase and peroxidase and many
enzymes with iron-based cofactor
2. Molybdenum Nitrogen metabolism and nitrogen Optimizes plant growth; aids in
fixation nodule formation in leguminous
crops
3. Boron Cell wall formation along with Promotes maturity; essential for

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 calcium, necessary for sugar pollen grain formation and pollen tube
translocation elongation; fruit yield and quality of
temperate fruits
4. Copper Activates enzymes necessary for Provides resistance against plant
photosynthesis and respiration, Pathogens
constituent of cytochrome oxidase
and polyphenol oxidase, present in
the receptor of ethylene signal
5. Manganese Acts as a catalyst in growth process; Accelerates seed germination and
constituent of oxygen evolving complex maturity; increases the availability of
phosphate and calcium
6. Zinc Chlorophyll formation; involved in Provides resistance against plant
respiration and nitrogen metabolism; Pathogens
regulates functioning of DNA/RNA
polymerase
7. Chlorine Formation of cytochromes; light Regulation of stomatal movement;
reaction of photosynthesis delayed senescence
8. Nickel Constituent of urease Increases crop yield

Deficiency symptoms of different nutrient elements in plants

S\N Elements Deficiency symptoms Symptoms due to excess supply
1. Nitrogen Lateral bud dormancy, wrinkled Dark bluish-green leaves; high shoot/root
cereal grains ratio; new growth will be succulent and
susceptible to disease; insect infestation
and drought stress; flower abort and lack
of fruit set
2. Phosphorus Premature leaf fall and flower bud; No direct effect on the plant, but will
delay in seed germination; chlorosis; show deficiency symptoms of Zn, Fe, Mn,
and necrosis first in older leaves or Ca; maturity often delayed; poor
vascular tissues; shoot growth is less and
root growth is more
3. Potassium Loss of apical dominance; interveinal Excess of potassium causes Mg and Ca
chlorosis first in older leaves; scorched deficiency
leaf tips; short internodes; and dieback
4. Calcium Stunted growth; degeneration of Deficiency symptoms of magnesium can
meristems, especially root meristem; be seen, and if concentration further
growing tips of roots and leaves will increases, potassium deficiency may also
turn brown and die; fruit quality will be occur
affected; decay of the conductive
tissue in lower region of stem wilt
easily
5. Magnesium Interveinal chlorosis and anthocyanin Occurs rarely; results in cation
pigmentation appear; older leaves imbalance; plant shows calcium
affected first; premature leaf and/or potassium deficiency
abscission
6. Sulphur Chlorosis first in young leaves; Premature senescence of leaves
reduced nodulation in legumes,
stunted and delayed growth;
anthocyanin accumulation;
defoliation in tea
7. Iron Interveinal chlorosis appearing first in Occurs rarely; results in bronze colored
young leaves, slow growth of the plant leaves with brown spots; symptoms
frequently seen in rice leaves
8. Manganese Interveinal chlorosis with gray spots Older leaves show brown spots
on young leaves; malformed leaves,

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 white streaks on the leaves of some surrounded by a chlorotic zone; tree fruits,
plants; plant growth is slow referred to as measles
9. Zinc Upper leaves show interveinal Fe deficiency develops; toxicity is severe;
chlorosis; stunted growth; dieback; plants severely stunted and eventually die
internodes will be short and plants will
be stunted
10. Copper Leaf tip necrosis; blackening of potato Fe deficiency may be induced with very
tubers; bark becomes rough and splits; slow growth; root tips may die
loss of apical dominance
11. Boron Death of root and shoot tips; Leaf tips and margins turn brown and die;
abscission of flowers; reduced stunted growth toxic to many plants
nodulation in legumes; interveinal
chlorosis with marginal necrosis and in
folding; pollination is reduced; no
internode elongation giving a
compressed appearance
12. Molybdenum Similar to nitrogen; slight retardation Not of common occurrence
of growth; chlorosis and necrosis of
old and middle leaves; sometimes leaf
margins get rolled, new growth is
malformed, and flower formation is
restricted
13. Chlorine Bronze colour in leaves; wilting of Premature yellowing of the lower leaves
leaves; swollen root tips; stunted root with burning of leaf margins and tips;
growth wilting and leaf abscission in woody
plants

FACTORS AFFECTING MINERAL UPTAKE

They include:-
1. Light: - Plants growing in bright light show rapid solute uptake than those grown under weak light.
2. Temperature:-High temp up to 40OC increases solute uptake. The slow growth of plants growing
in cold climates is due to the slow uptake of solutes.
3. pH:-Affects growth and uptake of solutes when the pH of soil is between 5-7 the growth is maximal.
4. Aeration: - Roots must have a sufficient quantity of oxygen for optimal absorbance of solutes.
5. Nutrient level of plant: - If plant roots have enough amount of specific solutes then absorption of
that solute is comparatively slow as compared with the deficient roots.
6. Growth:-Rapidly dividing and growing cells accumulate more of ions. Mature cells accumulate
fewer amounts of ions.

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TRANSPORT IN PLANTS
Transport of water and various ions in plants occurs over very long distances, from roots to the tips of
growing stem (upward direction), with or without metabolic energy through complex plant tissue called
xylem. The photosynthetic plants synthesise their food mainly in the green leaves. This food or
photosynthates move to all parts of the plant through phloem tissue in upward, downward, and lateral
direction (multidirectional transport). Transport of substances like mineral nutrients, organic nutrients,
water, plant growth regulators, etc., also occurs within a plant cell and from one cell to the other by either
simple diffusion or cytoplasmic streaming or through intercellular channels. Angiosperms (flowering
plants), therefore, consist of a well organised, complex, continuous traffic of various substances moving in
different directions in order to maintain their integrity.
TYPES OF TRANSPORT IN PLANTS
1. Diffusion
Diffusion is a physical process (independent of a living system) where solids, liquids or gases move
passively (without energy expenditure) from a region of higher concentration to a region of lower
concentration through the space available. In a plant, many substances move from one part of the cell to the
other, or from cell to cell, or over short distances by the process of diffusion. Since diffusion is a slow
process, the diffusing particles move slowly across a concentration gradient in a random fashion without
any energy expenditure. Different types of particles can diffuse simultaneously through the same space.
Carbon dioxide from the surrounding environment (produced as a by-product of respiration) is used for
photosynthesis in the leaves, during which oxygen is also released. This short-distance gaseous movement
of carbon dioxide and oxygen within the plant parts and between the plant and its environment occurs solely
by diffusion.
Rate of diffusion of different substances is affected by:
• The concentration gradient of the diffusing particles
• The membrane permeability through which the diffusing particles move
• Size of the diffusing particles (smaller substances diffuse faster)
• Temperature
• Pressure

2. Facilitated Diffusion
Facilitated diffusion of specific substances occurs through special proteins present in the plant cell
membrane without expenditure of ATP energy from a high concentration area of the substances in
consideration to a low concentration area.

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A gradient must already be present for diffusion to occur. The diffusion rate depends on the size of the
substances; smaller substances diffuse faster. The diffusion of any substance across a membrane also
depends on its solubility in liquids. Substances soluble in lipids diffuse through the membrane faster.
Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their movement
has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane.
Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration this would
require input of energy.
Membrane proteins are highly specific for a specific molecule to be diffused. This property of membrane
proteins allows the cell to selectively uptake and release substances across its membrane according to its
requirements.
Rate of transportation of a specific diffusing particle will reach a maximum when all the transport proteins
are used up (saturated), thus showing saturation effect. Facilitated diffusion is sensitive to inhibitors of the
membrane proteins.
The membrane proteins form channels for molecules to pass through. Some of these channels are always
open; others can be controlled. For example, porins are large membrane proteins present in the outer
membranes of the plastids, mitochondria, and some bacteria through which many molecules equivalent to
the size of small proteins can pass. Aquaporins or water channels are membrane proteins for passive
transport of water and water-soluble substances. There are eight different types of aquaporins recorded.
There are two major types of membrane proteins:
1. Carrier proteins (carriers or transporters): They bind to a specific diffusing particle and deliver it
to the other side of the membrane. Some carrier proteins allow movement of a molecule across a
membrane independent of other molecules, the process is called uniport whereas some allow
movement only if two types of molecules move together through them simultaneously. This is
called cotransport.
Cotransport is of two types:
• In a symport, both molecules cross the membrane in the same direction at the same time.
• In an antiport, both molecules cross the membrane in opposite directions simultaneously.
2. Channel proteins: They allow passive diffusion of specific particles of an appropriate size through
them.
Active Transport
This is the movement of substances across a biological membrane through carrier membrane proteins by
utilising energy from ATP against a concentration gradient. This is the primary mechanism for most mineral
uptake, especially for macronutrients and micronutrients present in low concentrations. Carrier proteins
used in active transport are called pumps. They are highly specific and sensitive to inhibitors that react with

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protein side chains. These pumps use energy to carry substances across the cell membrane from a lower
concentration to a higher concentration (uphill transport). They also exhibit saturation effect.
Active transport is faster than the passive transport (diffusion or facilitated diffusion).

Translocation of Mineral Ions

Once the mineral ions reach the xylem, their further transport is accomplished through the transpiration
stream. The plant utilises these minerals at the growing regions (e.g., apical and lateral meristems, young
leaves, developing flowers, fruits and seeds) and the storage areas. The mineral ions are unloaded at the
fine vein endings through diffusion and active uptake by the cells of the growing and storing regions. There
is constant movement of minerals in a plant, especially from older, senescing parts to younger leaves. Some
elements like phosphorus, sulphur, nitrogen and potassium are most readily mobilised, whereas calcium
and other such elements that are structural components are not remobilised. Nitrogen is carried through
xylem as inorganic ions and transported through phloem in organic form as amino acids and related
compounds. Also, some phosphorus and sulphur are carried as organic compounds.

Role of the Root System in Mineral Acquisition

The root system plays a crucial role in a plant's ability to acquire minerals from the soil. The root system
acts as a vital interface between the plant and the soil environment. Through its various functions, it plays
a critical role in ensuring the plant has access to the essential mineral nutrients it needs for healthy growth
and development. Some of the role of the root system in mineral acquisition are:
1. Increased Surface Area for Absorption: Roots, especially fine root hairs, significantly increase the
surface area available for contact with soil particles. This larger surface area allows for more efficient
uptake of dissolved minerals by the plant.
2. Selective Absorption: Root cells actively control the uptake of specific mineral ions from the soil
solution. This is achieved through specialized protein channels in the cell membrane. These channels allow
the passage of desired minerals while filtering out unwanted or potentially harmful substances.
3. Chemical Modifications: Plants can release various chemicals through their roots, known as root
exudates. These exudates can:
o Lower the pH of the surrounding soil, making some minerals more soluble and easier to absorb.
o Chelate certain micronutrients (like iron) that are often bound tightly to soil particles, making them
more available for uptake.
o Attract beneficial microbes like mycorrhizae, which form symbiotic relationships with plant roots
and assist in mineral acquisition.

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4. Exploration and Anchoring: The root system constantly grows and explores the soil, allowing the plant
to access a larger volume of soil and potentially reach deeper layers where mineral concentrations might be
higher. Roots also anchor the plant in the ground, providing stability and preventing it from being uprooted
by wind or water.

Chelation and Mineral Nutrition

Chelation is a natural process that prevents absorbed nutrients from precipitation. It allows the nutrients to
move freely in soil and increases its availability to plants. In plants, proteins, peptides, porphyrins,
carboxylic acids, and amino acids act as natural chelating agents. Other naturally occurring organic acids,
such as malonic acid and gluconic acid, also play an important role in plant mineral nutrition. Organic acids
and amino acids, such as citric acid and glycine, are also naturally occurring chelating agents. A strong
chelating agent may bind the mineral too strongly and make it unavailable to plants. On the other hand, a
weak chelating agent may not be able to protect the chelated minerals from chemical reactions with other
compounds and thereby reduce their availability to plants.

Hyperaccumulators
Some plants take up high concentrations of metal elements from the soil and store them in their aerial
tissues. These plants are known as hyperaccumulators or metallophytes. The elemental concentration in the
above-ground part ranges from 100- to 1000-fold higher than the observed concentration in non-
hyperaccumulator species. They are unique as they can be utilized in biogeochemical and phytoremediation
studies. There are around 450 hyperaccumulators plants and nickel is the most accumulated metal. In
addition to nickel, arsenic, cobalt, manganese, lead, cadmium, zinc, selenium, and copper are also being
accumulated by the plants. For example, Brassica can accumulate up to 30,000 μg.g-1 zinc and 1300 μg.g-1
cadmium.
Why should some plant accumulate metals at such high concentration?
Most probably hyperaccumulated metals provide defense against herbivores and pathogens. In Nicotiana
caerulescens, there is significant inhibition of the bacterial pathogen P. syringae by zinc accumulation.
They also have significant roles in phytoremediation, an eco-friendly method of removal of heavy metals
from polluted soils. Hyperaccumulators also have potential significance in phytomining, recovering, or
phytoextraction of metals from plants.

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Fertilization with Mineral Nutrients
High agricultural yields depend strongly on fertilization with mineral nutrients. The yield of most crop
plants increases linearly with the amount of fertilizer that they absorb. Crop plants, however, typically use
less than half of the fertilizer applied. The remaining minerals may:
- Leach into surface waters or ground water
- Become attached to soil particles
- Contribute to air pollution and water pollution.
Plants are proving useful for removing deleterious minerals from toxic waste dumps.
In addition to nutrients being added to the soil as fertilizers, some mineral nutrients can be applied to the
leaves as sprays, in a process known as Foliar Application and the leaves can absorb the applied nutrients.
Foliar application can reduce the lag time between application and uptake by the plant. For foliar application
to be successful, damage to the leaves must be minimized. If foliar sprays are applied on a hot day, when
evaporation is high, salts may accumulate on the leaf surface and cause burning or scorching. Spraying on
cool days or in the evening helps to alleviate this problem.

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 Transpiration and Stomatal Physiology
Transpiration: It is the loss of water from the aerial parts of the plant in the form of water vapours. Leaves
are the principal organs of transpiration, and most of the transpiration takes place through their stomata.
Types of Transpiration
1. Stomatal Transpiration: This is when water vapour is lost through the leaves, stems and other
organs of the plant. The amount of water lost through stomatal transpiration is very high, usually
about 80% of water lost by plants are from the stomata.
2. Cuticular Transpiration: here, a small amount of water vapour is lost from the leaves and
herbaceous stems by direct evaporation from the epidermal cells through the very thin cuticle.
Cuticles are waxy layers covering and protecting the epidermis of leaves, young shoots and aerial
plant organs. Although the cuticle is composed of waxes and other hydrophobic substances and is
generally impermeable to water, small quantities of water vapor can pass through. The contribution
of cuticular transpiration to leaf water loss varies considerably between species. It is to some extent
dependent on the thickness of the cuticle. Thicker cuticles are characteristic of plants growing in
full sun or dry habitats, while cuticles are generally thinner on the leaves of plants growing in
shaded or moist habitats. Cuticular transpiration may become more significant, particularly for
leaves with thin cuticles, under dry conditions when stomatal transpiration is prevented by closure
of the stomata.
3. Lenticular Transpiration: this is the loss of water vapours through the lenticels of fruits and
woody stems.
Significance of transpiration:
1. As transpiration helps in the movement of xylem sap, it increases the absorption of mineral
nutrients by the roots from the soil.
2. It causes cooling effect on leaf and plant surface.
3. It produces suction pressure for absorption, ascent of sap, mineral translocation and distribution if
minerals.
4. Transpiration decreases heating of leaves by solar radiations.
5. It maintains turgidity as well as aids in hydrological cycle.
Apart from transpiration, plants lose water through:
1. Guttation: it is the exudation of droplets of water from the margin and tips of leaves. The exudation
takes place through groups of leaf cells called hydathodes. A Hydathode is an opening or pore in the
leaf epidermis. Guttation water is rich in organic and in organic solutes, loss of water by guttation are
slight, though some tropical plants may lose as much as 100ml of water per leaf in a night.

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Differences between Transpiration and Guttation
Transpiration Guttation
a. This takes place during the day. This occurs during the night.
b. Water is lost as vapour. Water is lost as liquid.
c. Water transpired is pure. Water lost is rich in minerals, e.t.c
d. Stomata bring about this process. This takes place through hydathodes.
e. It is a regulated and controlled phenomenon. It is an uncontrolled phenomenon.
f. In this process, temperature of plant and leaf No such effect takes place.
is decreased.

2. Bleeding: It is the process of slow exudation of a watery solution from an incision made in a plant.

Rate of water loss in plant: The amount of water lost from a plant varies in different species. The quantities
of water lost from a plant may be much larger than the amount actually utilized by the plant in
photosynthesis and general other metabolic processes.
Water Requirement: water requirement of a plant is indicative of the number of pounds of water absorbed
by the plant in producing one (1) pound of solid matter.
Water Balance: This represents the difference between water gain and water expenditure by the plant.
Water Deficit: This occurs when water losses exceed water uptake/intake, therefore, causing a negative
water balance.
Indices of Transpiration
Transpiration intensity represents the index reflecting the amount of water evaporated from a surface unit
per one unit of time. This depends on the species, light intensity, temperature, wind and speed. During the
24 hour cycle the intensity of the transpiration forms a characteristic curve in which the minimum values
are registered in the morning, then they gradually increase at 1–2 PM (when they reach the maximum
values), and later decrease until 6 PM (when the intensity is very weak).
Transpiration productivity is the index reflecting the amount of organic mass accumulated in plants
during the elimination of a kilogram of water through transpiration (1–8 g of dry matter per 1 kg of water).
The transpiration coefficient is the amount of water needed for the synthesis of a gram of dry mass, it
varies from 250–300 to 700–800 l/kg, depending on plant species.

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 Stomata as Transpiring Organs
A stoma (plural- stomata) is a tiny pore found in the epidermis of leaves, stems and other organs that
controls the rate of gas exchange. Stomata are surrounded by two guard cells which control their opening
& closing.
Stomata consist of:
• Two guard cells
• Subsidiary or Accessory cells
1. The Guard Cells:
It is a kidney/bean shaped cell. They are specialized plant cells in the epidermis of leaves, stems and other
organs that are used to control gas exchange. They are cells surrounding each stoma. The guard cells possess
chloroplast making them involved in photosynthesis, though at reduced rate. Guard cells are pairs with a
gap between them that forms a stomatal pore.
Stomatal pores are largest when water is freely available, and the guard cells are turgid. Stomatal pores are
closed when water availability is critically low and the guard cells become flaccid. Guard cells help to
regulate the rate of transpiration by opening and closing the stomata. The cell wall of the guard cells on the
pore-side is thicker and more inelastic than the wall adjacent to the surrounding epidermal cells. As water
enters the cell, the thin side bulges outward like a balloon and draws the thick side along with it, forming a
crescent; the combined crescents form the opening of the pore. When water leaves the cells or is lost, the
guard cells decrease in volume; their walls straighten and the aperture is closed. Opening and Closure of
the stomatal pore is mediated by changes in the turgor pressure of the two guard cells.
Tugor Pressure is caused by osmotic flow of water in and out of the plant cell. It is the pressure exerted
by fluid in a cell that presses the cell membrane against the cell wall. Turgor is the pressure exerted by
water against the cell wall. The turgor pressure of guard cells is controlled by movements of larger quantities
of ions and sugars into and out of the guard cells.
Plants can also regulate transpiration intensity and the volume of eliminated water. This regulation is carried
out by the alteration of guard cell shape that contributes to opening and closing of the stomatal pore (the
osteole). The mechanism of such changes in shape is based on cell turgidity variation.
Passive regulation, conditioned by changes in the turgidity state occurring in neighboring cells surrounding
the guard cells. Passive movements are hydropassive (those of opening or closing stomatal pores), they are
determined by changes in water content. Hydropassive closure of stomata is related to the mechanical
pressure exerted by the neighboring cells of the epidermis under full turgidity. Hydropassive opening occurs
when this pressure is released during weak water deficit conditions.
Active regulation, caused by turgidity state changes taking place directly in the guard cells. Active
movements can be both hydroactive (those of opening and closing), dependent on water content and

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photoactive (also of opening and closing) caused by light. Hydroactive opening and closure of the stomata
occur when transpiration levels become greater than water absorption by the roots and when the decrease
in guard cell turgidity reaches critical levels. The photoactive opening and closure is caused mainly by the
photosynthesis process occurring in guard cells.

2. Subsidiary or Accessory Cells:

They are specialized epidermal cells which surround the guard cells. They support guard cells function by
offering a mechanical advantage that facilitates guard cell movements, and/or by acting as a reservoir for
water and ions.

Distribution of Stomata
In vascular plants, the number, size and distribution of stomata varies widely. Mature leaves can contain
between 50 and 500 stomata. Dicotyledons usually have more stomata on the lower surface of the leaves
than the upper surface. Monocotyledons such as Onion, Oat and Maize may have about the same number
of stomata on both leaf surfaces. In plants with floating leaves, stomata may be found only on the upper
epidermis, and submerged leaves may lack stomata entirely.
Amphistomatous leaves are leaves with stomata on both the upper and lower surfaces.
Hypostomatous leaves are leaves with stomata only on the lower surface.
Epistomatous or Hyperstomatous leaves are leaves with stomata only on the upper surface.
When fully opened, the stomata pore has a width of 3 to 12μ and a length of 10 to 40μ. The number of
stomata range from 100 to 60,000 per square centimeter.
The number of stomata per unit of a leaf is called Stomata Frequency.

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 Hypotheses of Stoma Opening and Closing
1. Photosynthetic guard cells:
According to Von Mohl and Schwendener, sugar is produced by the chloroplasts of the guard cells through
photosynthesis. The sugar is soluble and hence increases the concentration of sap of guard cells. The
increase of osmotic pressure of guard cells leads to endosmosis of water from neighbouring cells into guard
cells and they become turgid. This results in opening of stomata.
2. Starch-sugar inter conversion hypothesis:
This hypothesis states that the opening and closing of stomata is controlled by phosphorylase enzyme.
During daytime, the starch converts into glucose (sugar) by the activity of phosphorylase enzyme. The
increasing concentration of sugar in the guard cells causes endosmosis from neighboring cells. Hence, the
guard cells become turgid and stomata opens. The sugar present in guard cell converts into the starch in the
absence of light or during night. The starch is insoluble, and hence the cell sap of the guard cell remains of
much lower concentration in comparison to neighboring cells. Exosmosis from the guard cells takes place
by making them flaccid and the stomata is closed. The starch-sugar inter-conversion depends upon the
acidity (pH) and alkalinity of the cell sap of guard cells. During night, photosynthesis is absent thus the
carbon dioxide gets accumulated in the guard cells. This converts the cell sap in to weak acidic starch. The
carbon dioxide is utilized in the process of photosynthesis during daytime and the cell sap becomes alkaline
and the starch converts in to sugar.
3. Concentrations of CO2 hypothesis:
This hypothesis for opening and closing of stomata was proposed by Bonner and Galston. It relies upon the
concentration of the carbon dioxide (CO2) present in the stomatal chamber. It is independent of the presence
or absence of light. Normally 0.03% of carbon dioxide is present in the atmosphere. When the density of
the CO2 in the sub-stomatal chamber also becomes 0.03%, then the guard cells become flaccid and the
stomata become closed. As the density of CO2 decreases gradually, the stoma starts to open and it opens
gradually lengthwise until the density of CO2 becomes 0.01%. Now the stomata are completely open and
they are not open further beyond this density. Photosynthesis occurs in day time and much of the carbon
dioxide is being utilized in the process, the density becomes lesser than 0.03% and the stomata stays open
during day time. During night or in the darkness, photosynthesis is absent, the density of carbon dioxide
remains 0.03%. The guard cell remains flaccid, and the stomata remains closed.
4. Active potassium (K+) theory:
This theory is also termed as hormonal regulation theory or malate switch theory or potassium malate
theory. This theory was proposed by Levitt in 1974. The role of potassium (K+) in stomatal opening is now
most accepted world-wide. In 1967, Fujino, for the first time observed that opening of stomata takes place
due to the influx of K+ ions concentration. The osmotic concentration of guard cells is increased by the

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influx of K+ and causes stomatal opening. The uptake of potassium K+ controls the gradient in the water
potential. This in turn triggers endosmosis into the guard cells increasing the turgor pressure. ATP aids in
entry of K+ ions into the guard cells. Levitt (1974) observed that proton (H+) uptake by the guard cell’s
chloroplasts occurs with the help of ATP. This leads to rise of pH in guard cells. Increase in pH converts
starch into organic acid, such as malic acid. Malic acid again dissociates to form H+ and malate anion. The
absorption of potassium K+ ions is balanced by one of the following:
a. Uptake of Cl-
b. Transport of H+ ions from organic acids, such as malic acid
c. By negative charges of organic acids when they lose H+ ions
The accumulation of large concentration of K+ ions in guard cells is ionically balanced by the uptake of
negatively charged ions, i.e., chloride and malate. The hydrolysis of starch causes the accumulation of high
amount of malate in guard cells of open stomata. A passive or highly catalyzed excretion of K+ and Cl- from
the guard cells to the epidermal tissue results in stomatal closure in general and subsidiary cells in particular.
It is considered that subsidiary cells have an active re-absorption mechanism of K+.

Daily Periodicity of Stomatal Movements

The stomata of all plants show daily periodicities of opening and closing as their behaviour depends on
climatic factors.
Behaviour of Stomata under Standard Condition
The stomata of most mesophytic plants are open during the entire or most of the day light period and close
at night, their maximum diffusive capacity being attained during mid-day. The stomata open at dawn under
the influence of light factor, and maximum opening of stomata occurs in less than an hour. Under standard
conditions, the water contents and turgor of the leaf cells decrease progressively during the day, and as a
result, stomata begin to close during the mid-day, and completely close before sunset.
Behaviour of Stomata on a Hot Day (Afternoon)
The internal water deficit develops in the leaves at midday, causing the stomata to partially close. This
result in the decrease in transpiration, which permits an increase in water content of the leaves, and after
some time, the stomata widen again, and transpiration attains secondary maxima during the early afternoon.
Subsequently, the water deficit of the leaf increases again, and stomata closes again by sunset.
Behaviour of Stomata when Temperature is Low
When the sky is overcast and temperature is low, and soil is well watered, the stomata of most species are
not so wide open as in bright light, and do not remain open as long as under standard conditions.

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Behaviour of Stomata in Dry Weather
The stomata of most xerophytic plants in very dry weather open incompletely in the morning, and soon
close again before noon as a result of loss of water by the leaf. Under the conditions of maximum heat and
dryness, they remain closed whole day long and night, and open only for a short time during the early
morning hours.

The behaviour of stomata also varies with different plant species. In potato, cabbage, beet etc. under
optimum conditions of water supply, stomata tend to remain open during the day and night. The nocturnal
opening is favoured by high temperature and reduced partial pressure of the oxygen in the intercellular
spaces of leaf. In cereals, e.g; maize and rice, stomata always shut at night. Moreover, they close very early
in the evening and at the slightest deficiency of moisture. They may close even in the morning hours. In
alfalfa, they open during the day and close at night.

Factors Controlling Transpiration

1. Internal Factors
2. External Factors
1. Internal Factors
a. Root-Shoot ratio: Parker (1949) found that the rate of transpiration increased with an increase in root-
shoot ratio, provided other conditions were favourable for transpiration. Sorghum with more extensive root
system than that of corn, transpired at a higher rate per unit of leaf surface than corn. The sorghum root
system provides more water to the shoot than the corn root system.
b. Leaf Area: The greater the leaf area, the greater the water loss. The removal of leaves, i.e. decreasing
leaf area from plant increases the rate of transpiration per unit leaf area of the plant, but the total water loss
is greater in the unpruned ones. This is because the root system of pruned plants is providing a greater
amount of water to a smaller number of leaves, thus increasing the transpiration efficiency.
c. Leaf Structure: Plants growing in dry areas show a number of structural modifications, especially in
leaves, such as thick cuticle, thick cell wall, well-developed palisade, sunken stomata, a covering of hairs
e.t.c. These features reduce water loss. Under dry conditions, the stomata are closed and cuticular
transpiration is the main source of water loss.
In addition, leaf size and shape, spacing, distribution and structure of stomata, water content of leaves
greatly affect the rate of transpiration.

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 2. Environmental or External factors
a) Light: Light affects directly by heating the leaf, and thus raising its temperature indirectly favours the
opening of the stomata. The rate of transpiration increases in light, In the dark, stomata are closed and
transpiration ceases.
b) Carbondioxide: It is mainly the internal level of CO2 which controls the opening and closing of stomata.
In most plants, low CO2 concentration in intercellular spaces causes the stomatal opening during night
time as well. High concentrations of CO2 in intercellular spaces cause partial closure of stomata during
daytime. Once stomata closes, external CO2 concentration does not affect the stomatal movement. High
level of intracellular CO2 induces closure of stomata in the same way as ABA-induced closure.
c) Temperature: A rise in temperature of the leaf or both leaf and air within a certain physiological range
will increase the rate of transpiration. Stomata close at temperature approaching 0 0C and opens with
increase in temperature up to about 360C. When the temperature is warm, water evaporates more readily
and plants close their stomata to prevent water loss.
d) Wind: High wind velocity causes stomata closure. When plants are suddenly exposed to wind, there is
sharp increase in the rate of transpiration followed by a gradual falling of this increase. This is because of
cooling effect which wind produces on the evaporating surfaces.
e) Availability of Soil Water: If the supply of water to the leaves is not adequate, the rate of transpiration
decreases. If this condition is prolonged, a water deficit will result and the plant will appear wilted.

Anti-Transpirants
These are the chemicals which decrease water loss from plants due to transpiration. They are also useful
for avoiding transplantation shock to the nursery plants and plants raised in tissue culture. An antitranspirant
should be nontoxic and affect only stomata. It should not cause permanent damage to stomatal mechanism,
and the effect should persist only for short duration. There are mainly four types of antitranspirants:
1. Stomatal-closing types – some fungicides, like phenyl mercuric acetate (PMA), and herbicides, like
atrazine, act as antitranspirants by closing the stomata.
2. Film-forming type: These chemicals form coating on the surface of leaves. Hence, stomatal pores are
blocked resulting in reduction in the rate of transpiration. The material used is either plastic or waxy in
nature.
3. Reflectance type: These are white materials which form a coating on the leaves and increase the leaf
reflectance. Materials like kaolin, hydrated lime, calcium carbonate, magnesium carbonate, and zinc
sulfate are used to reduce transpiration in this category.

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4. Growth retardant: Abscisic acid (ABA) can bring stomatal closure. Chemicals like cycocel reduce
shoot growth and increase root growth, thus enabling the plants to resist drought. It is useful for
improving water status of the plant.

Ecological Adaptations to Reduce Transpiration

Plants develop many adaptations to avoid water loss due to transpiration. The presence of thick cuticle,
sunken stomata, and stomata only on the lower side of the leaves are some of the adaptive mechanisms
developed by plants. Plants growing in desert experience more water crisis. They need to restrict the rate
of transpiration. The ecological adaptations of desert plants are reduction of leaf area, presence of thick
cuticle, sunken stomata, and stomata opening during night, special water storage capacity, modified root
structure, and C4 photosynthesis.

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