Biochemistry Sem -V Paper-C-2, Unit-1 - PowerPoint Presentation

Slide1

Biochemistry Sem-VPaper-C-2, Unit-1

Dr. Rita Mahapatra

Assistant Professor

Neotech

College of applied Science and Research,

Virod

, Vadodara

Slide2

Plant cell structure

The term

c

ell

Is derived

from

the Latin

cella

, meaning storeroom

or

chamber

was given by Robert Hooke in 1665.

Plants are

multicellular

organisms composed of millions of cells with specialized functions. At maturity, such specialized cells may differ greatly from one another in their structures.

However, all plant cells have the same basic eukaryotic organization (Fig 1): They contain

1: Cytoplasm (Chloroplast, mitochondria etc.)

2:

Nucleus

3: Cell membrane

4: Cell wall

5: Chromosomes

6: Plasmodesmata

7: Filamentous cytoskeleton

Slide3

Fig 1. Plant cell structure

Slide4

Cell membraneThe cytoplasm and the nucleus and other parts of the cell are enclosed within the cell

membrane or plasma membrane

which separates cell from one another and also the cell from the surrounding medium.

The membrane is porous and allows the movement of substances or materials both inward and outward. The yellow outline in this diagram is the cell membrane.

Slide5

Cell Wall

The cell wall is the

most characteristic feature of a plant cell

The

cell wall is always

non-living but is formed and maintained

by

the

living organism

Its

primary function is

to provide protection to the contents of cell

Due

to

semi-rigid nature, the cell walls are responsible for giving

shapes to different kinds of cells during cell differentiation of

Tissues

In

multicellular

and woody plants of cell wall is differentiated into

three parts

i.e.,

the middle lamella, the primary wall and

secondary wall .

The cell wall is a very tough, flexible and sometimes fairly rigid layer that surrounds plant cells. It surrounds the cell membrane and provides these cells with structural support and protection.

In addition the cell wall is acting as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. The green outline in this diagram is the cell wall:-

Slide6

The middle lamella

It is a

common structure between

adjacent cells and therefore,

binds them

with each other.

It is an

amorphous layer and is

composed of

calcium and magnesium

pectate

.The middle lamella remains unlignifiedin case of softer living tissues namelyParenchyma, collenchyma and arenchyma, but in woody tissues Sclerenchyma it becomes highly lignified

Slide7

The primary cell wallConsists of cellulose (

45%),

hemicellulose

(25%),

pectins

(

35%) and structural proteins (

upto

8%) on the basis of dry

weight

The

primary wall is thin and elasticIt is capable of growth and expansionThe backbone of the primary wall is formed by the cellulosefibrils.The matrix is composed of hemicellulose, pectin, gums,tannins, resins, silica, waxes etc. and small structured proteins

Slide8

Secondary cell wall

The

20 wall is

very thick (lignin), rigid and inelastic and consists of three

layers known as

S1 (outer), S2 (middle) and S3 (inner)

The

microfibrils

in these layers run parallel to each other but the

directions are different in three layers.

The

microfibrils are transversely arranged in the S3 and are at an angle of10 -200 to the longitudinal axis in S2 and are at the angle of 500 in S1The lignin is formed from three different phenyl propanoid alcohols: coniferyl,

coumaryl

and

sinapyl

alcohols.

Lignin is covalently bonded to cellulose and other polysaccharides of cell wall.

Slide9

Difference between the primary andsecondary cell wall

Slide10

Functions of cell wallThey determine the morphology, growth, and development of plant

cells.

They

protect the protoplasm from invasion by viral, bacterial and

fungal pathogens.

They are rigid structures and thus help the plant

in withstanding the

gravitational forces.

They are involved in the

transport of materials and metabolites

into and out of cell.

They withstand the turgor pressure which develops within the cells due to high osmotic pressure.

Slide11

Nucleus

The nucleus is

double

membrane-enclosed organelle found in eukaryotic cells. The nucleus is generally spherical and located in the centre of the cell. The nucleus contains thread like structures called chromosomes which carry genes. Gene is a unit of inheritance in living organisms.

The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell

.

Nuclei contain a densely granular region, called

the

nucleolus

(plural

nucleoli), that is the site of ribosome

synthesis. A specific amino acid sequence called the nuclear localization signal is required for a protein to

gain

entry

into the nucleus.

The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, the nucleolus and nuclear pores.

Nuclear pores regulate the transport of molecules across the envelope.

The nucleolus is a smaller spherical body in the nucleus.

Slide12

The jelly- like substance present between the cell membrane and nucleus is known as the cytoplasm. The cytoplasm is about 80% water and usually colorless . Various other components, or organelles, of cells are present in the cytoplasm. These are :Mitochondria

Chloroplast

Golgi bodies

Ribosomes

Endoplasmic reticulum

Lysosomes

Vacuole

Peroxisomes

etc.

Cytoplasm

Slide13

Chloroplast

Chloroplast belong to another group of double membrane–enclosed organelles called

plastids.

Chloroplast membranes are rich in glycosylglycerides.

Chloroplast membranes contain chlorophyll

and it’s associated proteins and are the sites of photosynthesis.

In addition to their inner and outer envelope membranes, chloroplasts possess a third system of membranes called

thylakoids

. A stack of

hylakoids

forms a

granum (plural grana) (Fig. 2). Proteins and pigments (chlorophylls and carotenoids

) that function in the photochemical events of photosynthesis are embedded in the

thylakoid

membrane.

The fluid compartment surrounding the

thylakoids

, called the

stroma, is analogous to the matrix of the

mitochondrion.

Adjacent

grana

are connected by

unstacked

membranes called

stroma lamellae

(singular

lamella).

Plastids that contain high concentrations of carotenoid pigments rather than chlorophyll are called

chromoplasts

.

They are one of the causes of the yellow, orange, or red colors of many fruits and flowers, as well as of autumn leaves

Slide14

Fig. 2. Chloroplast

Slide15

Cont.

Non pigmented

plastids are called

leucoplasts. The most

important type of leucoplast is the

amyloplast

, a

starchstoring

plastid.

Amyloplasts

are abundant in storage tissues of the shoot and root, and in seeds. Specialized amyloplasts in the root cap also serve as gravity sensors that direct root growth downward into the soil.Chlorophylls are pigments in chloroplast absorb red and blue light and reflect green and yellow light to excite electron (e-) – plant appears green.Plant absorbs CO2, H2O and O2 as raw material of photosynthesis.Carbon dioxide + water → Sugar + Oxygen +

Water + Sunlight

6 CO2 + 6H2O

→

C6H12O6 + 6O2

Slide16

Mitochondria

Mitochondria (singular

mitochondrion) are the cellular sites of respiration, a process

in which the energy released from sugar metabolism is used for the synthesis of ATP (adenosine

triphosphate

) from ADP (adenosine

diphosphate

) and inorganic phosphate (Pi) .

Mitochondria can vary in shape from spherical to tubular, but they all have a

mooth

outer membrane and a highly convoluted inner membrane (Fig. 3).

The infoldings of the inner membrane are called cristae (singular crista).The compartment enclosed by the inner membrane, the mitochondrial matrix, contains the enzymes of the pathway of intermediary metabolism called the Krebs cycle.

In contrast to the mitochondrial outer membrane and all other membranes in the cell, the inner membrane of a mitochondrion is almost 70% protein and contains some phospholipids that are unique to the organelle (e.g.,

cardiolipin

).

The proteins in and on the inner membrane have special enzymatic and transport capacities

.

The

transmembrane

enzyme

ATP synthase is coupled to the

phosphorylation

of

ADP to produce ATP.

Slide17

Fig.3 Mitochondria

Slide18

Mitochondria and Chloroplasts AreSemiautonomous Organelles

Both mitochondria and chloroplasts contain their own DNA and protein-synthesizing machinery (70S

ribosomes

, transfer RNAs, and other components) and are believed to have evolved from

endosymbiotic

bacteria.

Both plastids and mitochondria divide by fission, and mitochondria can also undergo extensive fusion to form elongated structures or networks.

The DNA of Mitochondria 200 Kb and chloroplast 145Kb, these organelles is in the form of circular chromosomes, similar to those of bacteria and very different from the linear chromosomes in the nucleus.

These DNA circles are localized in specific regions of the mitochondrial matrix or plastid stroma called

nucleoids

.

DNA replication in both mitochondria and chloroplasts is independent of DNA replication in the nucleus.

Slide19

PeroxisomesPeroxisomes are found in all eukaryotic organisms, and

in plants they are present in photosynthetic cells

Peroxisomes

function both in the removal of

hydrogens

from organic substrates, consuming O2 in the process, according to the following reaction:

RH2 + O2 → R + H2O2

(where

R is the organic

substrate)

The

potentially harmful peroxide produced in these reactions is broken down in peroxisomes by the enzyme catalase, according to the following reaction:H2O2 → H2O + 1⁄2O2Although some oxygen is regenerated during the catalasereaction, there is a net consumption of oxygen overall.

Slide20

GlyoxysomeAnother type of microbody

, the

glyoxysome

, is present

in oil-storing seeds.

Glyoxysomes

contain the

glyoxylate

cycle enzymes, which help convert stored fatty acids into

sugars that can be translocated throughout the young plant to provide energy for growth

Because both types of

microbodies (peroxisome & glyoxysome) carry out oxidative reactions, it has been suggested they may have evolved from primitive respiratory organelles that were superseded by mitochondria.

Slide21

Vacuoles

Mature living plant cells contain large, water-filled

central vacuoles

that can occupy 80 to 90% of the total volume

of the cell.

Each

vacuole is surrounded by

a

vacuolar

membrane, or

tonoplast

. Many cells also have cytoplasmic strands that run through the vacuole, but each transvacuolar strand is surrounded by the tonoplast.The vacuole contains water and dissolved inorganic ions, organic

acids,

ugars

, enzymes, and a variety of

secondary metabolites,

which often play roles in plant defense.

Active

solute accumulation provides the

osmotic driving

force for water uptake by the vacuole, which

is required

for plant cell enlargement.

The

turgor

pressure generated

by this water uptake provides the

structural rigidity

needed to keep herbaceous plants upright,

since they

lack the lignified support tissues of woody plants

.

Slide22

Vacuoles contain many hydrolytic enzymes, such as proteases, ribonucleases, glycosidases

, and

phosphatases

, as well as

peroxidases

and a variety of secondary metabolites, which often play roles in plant defense. Active solute accumulation provides the osmotic driving force for water uptake by the vacuole, which is required for plant cell

nlargement

.

The

turgor

pressure generated by this water uptake provides the structural rigidity needed to keep herbaceous plants upright, since they lack the lignified support tissues of woody plants.Specialized protein-storing vacuoles, called protein bodies, are abundant in seeds. During germination the

storage proteins

in the protein bodies are hydrolyzed to

amino acids

and exported to the cytosol for use in protein synthesis.

The hydrolytic enzymes are stored in

specialized lytic

vacuoles, which fuse with the protein bodies to

initiate

the

breakdown process

Slide23

Plasmodesmata

Plasmodesmata are microscopic channels which allow molecules to travel between plant cells. Unlike animal cells, every plant cell is surrounded by a cell wall. Neighboring plant cells are therefore separated by a pair of cell walls, forming an extracellular domain known as the apoplast.

Although cell walls allow small soluble proteins and other solutes to pass through them, Plasmodesmata enable direct, regulated,

symplastic

intercellular transport of substances between cells

.

Plasmodesmata (singular

plasmodesma

) are tubular extensions

of the plasma membrane, 40 to 50 nm in diameter, that traverse the cell wall and connect the

cytoplasms

of adjacent cells. Because most plant cells are interconnected in this way, their

cytoplasms

form a continuum referred to as the

symplast

. Intercellular transport of solutes through

plasmodesmata

is thus called

symplastic

transport

Slide24

Filamentous cytoskeleton

The

cytoskeletal

components—microtubules,

microfilaments, and intermediate filaments—participate

in a

variety of

processes involving intracellular movements,

such as

mitosis,

cytoplasmic

streaming, secretory vesicle transport cell plate formation, and cellulose microfibril deposition.The process by which cells reproduce is called the cell cycle.The filamentous cytoskeleton is a network of fibers composed of proteins contained within a cell's cytoplasm. It is a dynamic structure, parts of which are constantly destroyed, renewed or newly constructed.

Here is a multitude of functions the cytoskeleton can perform: It gives the cell shape and mechanical resistance to deformation, it stabilizes entire tissues, it can actively contract, hereby deforming the cell and the cell's environment and allowing cells to migrate, it divides chromosomes.

It

is involved in the division of a mother cell into two daughter cells etc. The functions which this cytoskeleton can perform depend on the type of cell and the organism.

Slide25

Functions of Water in Plants

Water is important in the life of plants because it makes

up the

matrix and medium in which most

biochemical processes

essential for life take

place.

It

has been said that the study of plant physiology is, for the most part, the study of plant water relations. This is understandable, given the central role water plays in a large number of plant processes.

Among the many functions of water in plants are the following: · it serves as a medium (and sometimes substrate) for biochemical reactions in cells, since many enzymes are dissolved in the cell water · structural support – water provides the “

turgor

pressure” that gives many cells their shape; thus, many tissues will lose their structure and wilt when water availability is inadequate.cell enlargement – turgor pressure provides the physical force needed to expand cells during growth. Transport of solutes between organs, via the xylem and phloem vessels evaporative cooling of leaves during transpiration.Next to light, water availability is probably the single most important environmental factor affecting plant growth. Accordingly, plants have evolved with complex physiological strategies for regulating water use, including, but not limited to, minutes timescale regulation of

stomatal

apertures in response to sudden changes in environmental conditions. In cropping situations

Worldwide, water deficits constitute the single largest cause of crop failure.

Slide26

Diffusion

The net, random movement of individual

molecules from one area to another. The

molecules move from [hi] → [low], following

a concentration gradient.

Another way of stating this is that the molecules move

from an area of high free energy (higher concentration)

to one of low free energy (lower concentration).

The net movement stops when a

dynamic equilibrium is

achieved.

Slide27

Some studies indicated that diffusion directly across the lipid bilayer was not sufficient to account for

observed rates

of water movement across membranes, but the

evidence in

support of microscopic pores was not compelling.

This uncertainty was put to rest with the recent

discovery of

aquaporins

.

Aquaporins

are integral membrane proteins that form water-selective channels across the membrane. Because water diffuses faster through such channels than through a lipid bilayer, aquaporins facilitate water movement into plant cells

.

Water can cross plant membranes by diffusion of individual water molecules through the

embrane

bilayer

, as shown on the left, and by microscopic bulk

flow of

water molecules through a

water- elective

pore formed by integral membrane proteins such as

aquaporins

as shown in the diagram

Slide28

Slide29

Osmosis

Water potential is a measure of the energy state of water.

Determines the direction and movement of water.

Unit for water potential

MegaPascal

Mpa

Ψ pure water at 1

atm

= 0

Mpa

Ψ=Ψs + ΨpΨ= water potentialΨs=solute potential (osmotic potential) –Always negativeΨp=(pressure potential)Water molecules move from higher water potential to lower water potential

The spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides –dynamic equilibrium.

Slide30

ABSORPTION OF WATER

Water

in the soil is mostly and abundantly, under

normal conditions

, is available in the form of Capillary water

. W

ater plays a crucial role

in the life of the plant. For

every gram

of organic matter made by the plant, approximately 500 g of

water is

absorbed by the roots, transported through the plant body and lost to the atmosphere.In the soil the space in between soil particle forms a network of spaces, which normally is filled with water. The

water that is present

in such

spaces is called capillary water

.

Land plants absorbs water by

(1) An

extensive root

system to

extract water from the soil;

(

2) A

low-resistance

pathway through

the xylem vessel elements and

tracheids

to

bring water

to the leaves;

(

3) A

hydrophobic cuticle covering

the surfaces

of the plant to reduce evaporation

;

(

4) M

icroscopic stomata

on the leaf surface to allow gas exchange; and

(5) Guard

cells to regulate the diameter (and

diffusional

resistance) of

the

stomatal

aperture

.

Slide31

In plants, following two pathways are involved in the water movement. They are(1) Apoplastic

pathway

(2)

Symplastic

pathway

(3)

Transmembrane

pathway

1

.

Apoplastic

pathway : The apoplastic movement of water in plants occurs exclusively through the cell wall without crossing any membranes. The cortex receive majority of water through apoplastic way as loosely bound cortical cells do not offer any

resistance. But the movement of water in root beyond cortex

apoplastic

pathway is blocked by

casparian

strip present in the endodermis.

Slide32

2. Symplastic pathway

The movement of water from one cell to other cell through the

plasmodesmata

is called the

symplastic

pathway

of water movement. This pathway comprises the network of cytoplasm of all cells inter-connected

by

plasmodermata

.

Slide33

3. The transmembrane pathway is the route

followed by

water that sequentially enters a cell on one

side, exits

the cell on the other side, enters the next in

the series

, and so on. In this pathway, water crosses

at least

two membranes for each cell in its path (

the plasma

membrane on entering and on exiting).

Transport across the tonoplast may also be involved.

Slide34

Translocation and conduction of water and minerals

Some aspects of phloem translocation

have

been

well established

by extensive research over many years.

These include

the following:

The

pathway of

translocation- Sugars

and other organic materials are conducted throughout the plant in the phloem, specifically in cells called sieve elements. Sieve elements display a variety of structural adaptations that make them well suited for transport.Patterns of translocation. Materials are translocated

in

the

phloem from sources (areas of

photosynthate

supply

) to sinks (areas of metabolism or storage of

photosynthate

). Sources are usually mature

leaves. Sinks

include organs such as roots and

immature leaves

and fruits.

Materials

translocated in the phloem. The

translocated

solutes

are mainly carbohydrates, and sucrose is

the most

commonly translocated sugar.

Phloem

sap

also contains

other organic molecules, such as

amino acids

, proteins, and plant hormones, as well as

inorganic ions.

Slide35

Rates of movement. Rates of movement in the phloem are quite rapid, well in excess of rates of diffusion. Velocities average 1 m h–1, and mass transfer ratesrange

from 1 to 15 g h–1 cm–2 of sieve elements.

Other aspects of phloem translocation require

further investigation

, and most of these are being studied

intensively at

the present time. These aspects include the following:

Phloem

loading and unloading. Transport of sugars

into

and

out of the sieve elements is called sieve element loading and unloading, respectively.Mechanism of translocation.Photosynthate allocation and partitioning.

Slide36

Guttation

Guttation

is the appearance of drops

of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses.

Guttation

is not to be confused with dew, which condenses from the atmosphere onto the plant surface Secretion of water on to the surface of leaves through specialized pores, or

hydathodes

.

The main cause of

guttation

in plants is root pressure, during night when root pressure is high sometimes den due to this pressure watery drops ooze out with the assistance of special structures which help in

guttation called the hydathodes.

Slide37

Slide38

TranspirationDuring the plant’s lifetime, water equivalent to 100 times the fresh weight of the plant may be lost through the leaf surfaces. Such water loss is called

transpiration.

Transpiration is an important means of dissipating the heat input from sunlight. Heat dissipates because the water molecules that escape into the atmosphere have higher than- average energy, which breaks the bonds holding them in the liquid.

For

a typical leaf, nearly half of the net heat input from sunlight is dissipated by transpiration

.

Latent heat of vaporization is

the energy needed to separate molecules from the liquid phase and move them into the gas phase at constant temperature—a process that occurs during transpiration.

For water at 25°C, the heat of vaporization is 44 kJ mol–1—the highest value known for any liquid. Most of this energy is used to break hydrogen bonds between water molecules.

Slide39

1. Stomatal transpiration: Transpiration

that occurs

through stomata

called

stomatal

transpiration

. This type

of transpiration

only occurs in

its presence

of sunlight (in daytime).

Because stomata open in the present of sunlight and close in the darkness. In this method plants give out 80-90% water in the form of vapor.

Slide40

2. Cuticular transpiration:

Transpiration that

occurs through

the cuticle or cracks

of thin

cuticle layer of leaves

and stems

is said to be

cuticular

transpiration

.

This is a day-night process. In this process, 5-10% water is given out in the form of vapor.

Slide41

3. Lenticular transpiration:

Sometimes transpiration

occurs through

lenticels, the

small opening

in the corky

tissue covering

stems and twigs,

and this

type of transpiration is

said to

be the lenticular transpiration. In this process, only 0.1% water is given off of the forms of vapor.

Slide42

Transpiration is a process similar to evaporation.It is a part of the water cycle, and it is the loss of water vapor from parts of plants (similar to sweating), especially in leaves but also in stems, flowers and roots.

Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants they are more numerous on the undersides of the foliage.

The stomata are bordered by guard cells that open and close the pore.

Leaf transpiration occurs through stomata

.

An Increase in Guard Cell

Turgor

Pressure Opens the Stomata

Guard cells function as multisensory hydraulic valves.

environmental

factors such as light intensity and quality,

temperature, relative humidity, and intracellular CO2 concentrations are sensed by guard cells, and these signals are integrated into well-defined stomatal responses. If leaves kept in the

dark are illuminated, the light stimulus is perceived

by the

guard cells as an opening signal, triggering a series

of responses

that result in opening of the

stomatal pore.

Slide43

In guard cells the microfibril organization is different.Kidney-shaped guard cells have cellulose

microfibrils

fanning out

radially

from the pore (

Fig. A

). Thus the

cell girth

is reinforced like a steel-belted radial tire, and

the guard cells curve outward during

stomatal opening In grasses, the dumbbell-shaped guard cells function like beams with inflatable ends.As the bulbous ends of the cells increase in volume and swell, the beams are separated from each other and the slit between them widens (

Fig. B).

Guard cell

Slide44

Mechanism of Transpiration

The

Cohesion–Tension Theory

Explains

Water. Transport

in the

Xylem. In

theory, the pressure gradients needed to move

water through

the xylem could result from the generation of

positive pressures

at the base of the plant or negative pressures at the top of the plant.It is mentioned previously that some roots can develop positive hydrostatic pressure in their xylem—the so-called root pressure.However, root

pressure is

typically less than 0.1

MPa

and disappears when

the transpiration

rate is high, so it is clearly inadequate to move water up a tall tree.

Instead, the water at the top of a tree develops a

large tension

(a negative hydrostatic pressure), and this

tension

pulls

water through the xylem. This mechanism, first

proposed

toward

the end of the nineteenth century, is

called the

cohesion–tension theory of sap ascent because

it

requires

the cohesive properties of water to sustain

large tensions

in the xylem water

columns.

Slide45

Root pressure theoryRoot pressure is most likely to occur when soil

water potentials

are high and transpiration rates are low.

When transpiration

rates are high, water is taken up so

rapidly into

the leaves and lost to the atmosphere that a

positive pressure

never develops in the xylem.

Plants that develop root pressure frequently produce

liquid droplets

on the edges of their leaves, a phenomenon known as guttation.

Slide46

Water in the plant can be considered a continuous hydraulic system, connecting the water in the soil with the water

vapor in the atmosphere.

Transpiration

is

regulated principally

by the guard cells, which regulate the

stomatal

pore

size to meet the photosynthetic demand for

CO2 uptake

while minimizing water loss to the atmosphere.

Water evaporation from the cell walls of the leaf mesophyll cells generates large negative pressures (or tensions) in the apoplastic water. These negative pressures are transmitted to the xylem, and they pull water through the long xylem conduition.

Although aspects of the cohesion–tension theory of sap ascent are intermittently debated, an overwhelming body of evidence supports the idea that water transport in the xylem is driven by pressure gradients.

When transpiration is high, negative pressures in the xylem water may cause cavitation (embolisms) in the xylem.

Such embolisms can block water transport and lead to severe water deficits in the leaf.

Water deficits are commonplace in plants, necessitating a host of adaptive responses that modify the physiology and development of plants.

Water balance and Stress

Slide47

Water deficit can be defined as any water content of a tissue or cell that is below the highest water content

exhibited at

the most hydrated state.

When

water deficit

develops slowly

enough to allow changes in

developmental processes

, water stress has several effects on growth,

one of

which is a limitation in leaf expansion. Leaf area

is important because photosynthesis is usually proportional to it. However, rapid leaf expansion can adversely affect water availability.Adaptation and acclimation to environmental stresses result from integrated events occurring at all levels of organization, from

the anatomical and morphological level

to the

cellular

, biochemical

, and molecular level. For

example, the wilting of leaves in response to water deficit reduces both

water loss from the leaf and exposure to incident

light, thereby

reducing heat stress on leaves

.

At the biochemical level, plants alter metabolism

in various

ways to accommodate environmental

stresses, including

producing

osmoregulatory

compounds such

as proline

and

glycine

betaine

to adapt

and acclimate to water

deficit, salinity

, chilling and freezing, heat, and oxygen

deficiency in

the root

biosphere.

Slide48

ReferencesPlant Physiology, 3rd

ed

,

Lincoln

Taiz

and Eduardo

Zeiger

, Publisher

:

Sinauer

Associates; 3 edition (2002).Fundamental of biochemistry by D Voet, J.GVoet and C.W Pratt, John Wiley & Sons, Inc., New York, 2nd edition, 2005 Principles of Biochemistry by Albert Lehninger, W.H. Freeman & Company; 3rd edition (February 2000), ISBN-10: 1572591536

Botany by A. K Nanda