Osmoregulation is the passive regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of electrolytes (salts in solution) to keep the fluids from becoming too diluted or concentrated. The immediate and most common response by the different organs of a plant to water stress is decrease in turgor. This may be partially or fully adjusted by accumulation of solutes.

1. Stress and Osmoregulation
in plants
BY HAIDER ALI MALIK

2. Stress
 Biological stress is not easily defined but it implies adverse
effects on an organism
 Like all other living organisms, the plants are subjected to
various environmental stresses such as water deficit and
drought, cold, heat, salinity and air pollution etc.

3.  Stress is any change in environmental conditions that might reduce or adversely change
plant’s growth and development (Levitt, 1972)
 Adverse force or influence that tends to inhibit normal systems from functioning (Jones,
1989)
 Any situation where the external constraints limit the rate of dry matter production of all or
part of the vegetation below its ‘genetic potential’ (Grime, 1979)
 Therefore, most practical definition of a biological stress is an adverse force or a
which inhibits the normal functioning and well being of a biological system such as

4. Stress terminology
 Stressor/Stress factor:
Any factor that causes injury or stress stimulus
 Stress response:
Stress stimulus with ensuing state of adaptation
 Eustress:
It is an activating, stimulating stress that increase the physiological activity of a plant and thus
a positive element for plant development.

5.  Distress:
It is a severe and a real stress that causes damage and thus has a negative effect
the plant and its development.
 Zero stress:
The stress that is just insufficient to produce a plastic strain.
 Stress resistance:
Ability of the plant to survive under adverse environmental condition is termed as
stress resistance (adaptation, avoidance and tolerance).

6.  Elastic resistance:
Ability of the plant to prevent reversible or elastic strain (physical or chemical change) when
exposed to a specific stress
 Adaptation
It refers to heritable modifications in structure or function that increase the fitness of the
organism in the stressful environment. It is also called protection. e.g. CAM plants to desert
 Acclimation
It refers to non-heritable physiological modifications that occur over the life of an individual.
modifications are induced by gradual exposure to the stress. The process of acclimation is known
hardening

7.  Damage/Stress injury:
It is the result of too high a stress which can not be compensated.
 Dehydration :
The loss of water from a cell. Plant cells dehydrate during drought or water
deficit.
 Desiccation :
The extreme form of dehydration. Denotes the process whereby all free water
lost from the protoplasm.

8.  Homoiohydry : Water economy strategy whereby plants strive to maintain a
high water potential under water limiting conditions. Homoiohydric plants
drought avoidance.
 Poikilohydry : Water economy strategy whereby plants lack the ability to control
water loss to the environment. Poikilohydric plants must be drought tolerant.
 Poikilotherms: Plants that tend to assume the temp. of their environment i.e they
must develop temp. tolerance

9. Effect of stress on plants
 There are no specific osmoregulatory organs in higher plants, the stomata are the only important
structures rake part in regulating water loss through evapotranspiration, and on the cellular level
the vacuole is crucial in regulating the concentration of solutes in the cytoplasm.
 Strong winds, low humidity and high temperatures all increase evapotranspiration from leaves.
 Abscisic acid is an important hormone in helping plants to conserve water—it causes stomata to
close and stimulates root growth so that more water can be absorbed.
 Plants share with animals the problems of obtaining water but, unlike in animals, the loss of water in
plants is crucial to create a driving force to move nutrients from the soil to tissues. Certain plants
have evolved methods of water conservation.

10.  The immediate and most common response by the different organs of a plant to
water stress is decrease in turgor. This may be partially or fully adjusted by
accumulation of solutes.
 Plants growing under conditions of high salinity accumulate various solutes as a
result of alterations in intermediary and secondary metabolism of nitrogen or of
carbon (Greenway and Munns, 1980; Stewart and Larher, 1980).
 This results most probably from an imbalance in the inorganic ion status ultimately
causing a malfunctioning of the enzymes involved.
 Amides, free amino acids, proline, amines, quaternary ammonium compounds and
sugars are some of the organic solutes that show a change in their accumulation
under condition of stress (Hsiao, 1973; Stewart and Larher, 1980).

11. It has been suggested that high concentrations of organic solutes in the
cytoplasm play a double role (Greenway and Munns, 1980):
 They can contribute to the osmotic balance when electrolytes are lower in
the cytoplasm than in the vacuole and
 They can have a protective effect on enzymes in the presence of high
electrolytes in the cytoplasm. However, there remains speculation about
the primary roles of these solutes, viz., whether it is one of storage of
reduced carbon and/or nitrogen, or in the osmotic balance of the cell as a
whole (Greenway and Munns, 1980).

12. Stress
Osmoregulation is the passive regulation of the osmotic
pressure of an organism's body fluids, detected by
osmoreceptors, to maintain the homeostasis of the organism's
water content; that is, it maintains the fluid balance and the
concentration of electrolytes (salts in solution) to keep the
fluids from becoming too diluted or concentrated.

13. Introduction
 The protoplasm of living organisms has a high percentage of water, so
without water, living organisms would die.
 Plants living in water, or those in hot, arid conditions where water is not
readily available all the time, or in which there is a high concentration of
solutes such as occurs in/near sea water, must adapt their structure and/or
their various functions – or both – to ensure the conservation of needed
water and prevent the upset of the osmotic balance of cell contents.
Without the right osmotic balance – the plant dies!

14. Surviving the salt
These Mangroves grow in wet,
muddy soil at the sea -water's
edge. If you look at the leaves,
salt crystals are excreted on to
their surfaces, and if you taste
the sap – it’s very salty!

15. Surviving the salt
Some mangroves are almost covered by salty sea water!
Most trees cannot survive in
water that has too much salt
in it, but mangrove trees
have a unique adaptation for
dealing with the sea's
salinity.

16. Surviving the salt
 When they’re submerged in sea water, warty growths on
mangrove roots filter out most of the salt as they take
water in through their roots.
 Some mangroves concentrate extra salt in old leaves
(which turn yellow and die), and some are able to get rid
of the salt by secreting it through the pores of special
glands.

17. Surviving drought
In contrast to mangroves, plants, such as
these cacti and Acacia that live in places
like along the Palisadoes strip or in the
Hellshire area, grow in limited, dry,
sandy soil, with little rainfall, a very high
temperature and a hot, dry wind.

18. Some water conservation methods
Succulent plant stem
(Cactus)
Succulent leaves of Sesuvium & Aloe

19. OSMOREGULATORY ADAPTATIONS
 The plants shown on the previous slides have
adaptations that ensure osmoregulation.
 Osmoregulation is the active regulation of the osmotic
pressure of an organism’’s fluids to maintain the
homeostasis (or constant unchanging balance) of the
organism’s water content; that is, it keeps the
organism's fluids from becoming too diluted or too
concentrated.

20. OSMOREGULATORY ADAPTATIONS
 Plants such as mangroves
develop structural and
physiological adaptations to
regulate the osmotic balance of
their cell contents – i.e to carry
out osmoregulation.
 The cacti and other plants living
along the hot, dry scrubland of
the Palisadoes strip also develop
special adaptive features for
osmoregulation.

21. OSMOREGULATION AND STRESS PARADIGM
IN PLANTS
 Drought and salinity stress are the major causes of historic and modern
agricultural productivity losses throughout the world.
 Both drought and salinity result in osmotic stress that may lead to
inhibition of growth. Salinity causes additional ion toxicity effects mainly
through perturbations in protein and membrane structure.
 In contrast to animals, which rely on Na1/K1-ATPases for the expulsion of
osmotic plants rely on plasma membrane and endosomal ATPase activities
to generate proton gradients to drive ion extrusion and intracellular
sequestration.

22. OSMOREGULATORY MECHANISMS IN
PLANTS
 Consequently, most angiosperms, including all major crop species, have a
diminished capacity for Na1 transport and tolerance to high salinity.
 The chemiosmotic regulatory systems of plant and fungal cells differ
fundamentally from those found in animal cells.
 Animal cells rely on a primordial Na1 chemiosmotic circuit consisting of
Na1/K1-ATPase ‘‘pumps’’ to drive the efflux of 3Na1 and influx of 2K1 coupled
to ATP hydrolysis.
 This active Na1 extrusion creates an electrochemical Na1 gradient across the
plasma membrane to drive secondary symport and antiport carriers that, in
turn, regulate nutrient uptake and pH.

23.  In contrast, plants appear to lack plasma membrane Na1/K1- ATPases. Thus,
plants utilize H1-ATPases for primary extrusion or sequestration of protons to
generate H1 electrochemical gradients, which drive secondary ion and nutrient
transport processes via H1-symport/ antiport systems. These H1-ATPase pumps
also modulate both intracellular and extracellular pH.
 Except in the case of extreme halophytic archaebacteria, viable cellular processes
in animals, fungi, and plants depend upon the maintenance of low cytoplasmic
Na1 and Cl2 concentrations and a high K1/Na1 ratio, because K1 counteracts the
inhibitory effects of Na1 (and Li1).
 Like animal cells, most plant cells maintain cytosolic K1 concentrations in the
range of 100–200 mM and Na1 values in the low mM range (1– 10 mM) up to a
maximum of 100 mM.

24.  In contrast to K1, an essential cation for maintaining biochemical interactions of
the cytoplasm, Na1 is not essential for, but does facilitate, volume regulation and
growth in most plants. However, at high concentrations Na1 limits growth.
 Ironically, the productivity of irrigated agricultural regions is generally many times
greater than non-irrigated areas, yet irrigated crops are most susceptible to
detrimental salinity effects.
 Therefore, genetic engineering of crop plants to improve their capacity for Na1
transport and sequestration is an important goal for meeting the future food and
fiber demands of a rapidly growing human population.

25.  Many plants, such as extreme halophytes, display Na1 dependence for optimal
growth and development and have developed specialized structures such as salt
glands and bladders to accommodate high salt concentrations in tissues. Others
have developed whole plant strategies for avoiding stress such as accelerated
completion of ontogeny.
 However, these specialized adaptations are lacking in most major crop species.
Furthermore, the precise impact of osmotic and ionic effects on cell growth,
division, phytohormone balance, and death in the context of the whole plant are
complex and require further investiga

26. Osmoregulatory adaptations – Types of
plants
 Depending on their habitat, plants can be grouped into four different types
according to the osmoregulatory adaptations that they show either in their
structure, functions, or both.
 Groups are as follows:
 Halophytes
 Hydrophytes
 Xerophytes
 Mesophytes

28. Examples
(a) mangrove = halophyte
(b) Catus = xerophyte
(c ) an ackee tree = mesophyte
(d) water lily = hydrophyte
The leaves float on the water surface and numerous stomata are present on the
upper surface of the leaves facing the atmosphere to promote loss of water. The
surface area of these leaves is very large to enable excessive water loss by
transpiration

29. Why is osmoregulation important to plants?
1) Enables the plant to grow, develop, carry on respiration, photosynthesis and survive, even if:
 the habitat is dry, hot and desert-like.
 sandy/rocky soil does not hold much water.
 rainfall is scarce or only at certain times.
 adequate water is not available for photosynthesis and
 hydration of the cell contents.
 habitat is completely aquatic.
 salinity of the habitat is high.
2) It regulates and balances the uptake and loss of water and solutes so maintains homeostasis.

30. THEEND