2. Abiotic stress causes changes in soil-
plantatmosphere and is responsible for
reduced yield in several major crops.
Prolonged exposure to these abiotic stresses
results in altered metabolism and damage to
biomolecules.
Plants show a range of responses and
adaptations that help bring about abiotic
stress tolerance.
3. Plants activate a specific and unique stress
response when subjected to a combination of
multiple stresses
Photosynthesis is the most important process
in the world which involves a chain of events
where light energy is converted into chemical
energy by the plants through chemical
reactions with water and carbon dioxide.
Photosynthesis is the source of organic
carbon and energy required by plants for
their growth, biomass production, and yield.
4. Regulation of photosynthesis under different
abiotic stresses, such as changes in
temperatures, water stress conditions,
salinity, elevated CO2 and O3, and its
associated aspects.
5. Photosynthesis Under Change in
Temperatures
a)- Photosynthesis Under High Temperatures
b)- Photosynthesis Under low
Temperatures
Photosynthesis Under Water Stress
a)- Photosynthesis Under Waterlogging
Conditions
b)- Photosynthesis Under Water-Deficit
Conditions
6. Photosynthesis Under Saline Conditions
Photosynthesis Under Elevated CO2
Photosynthesis Under Elevated CO2
Concentrations.
7. Photosynthesis and transpiration share a
common pathway through the stomatal
opening regulated by the guard cells.
Photosynthesis is a temperature-dependent
process.
Temperature affects all biochemical reactions
of photosynthesis as well as membrane
integrity in chloroplasts.
8. (a) Optimal temperature
(b) Respiration rates
(c) Membrane-bound electron transport processes
Optimal temperature is the point at which the
capacities of the various steps of photosynthesis are
optimally balanced, with some of the steps becoming
limiting as the temperature decreases or increases.
High temperatures adversely affect plant growth and
survival in a number of ways, but the impact of heat
stress on the photosynthetic apparatus is considered
to be of particular significance because
photosynthesis is often inhibited before other cell
functions are impaired.
9. High temperatures affect photosynthesis by
altering the excitation energy distribution by
changing the structure of thylakoids and by
changing the activity of the Calvin cycle and
other metabolic processes such as
photorespiration and product synthesis.
The diffusion of CO2 and and the affinity for
carboxylation of the Rubisco enzyme have been
proven to be affected by increasing
temperatures.
Under conditions of high solar irradiance, leaf
temperature can increase several degrees
centigrade above air temperature.
10. Besides affecting the membranes, heat stress
has been shown to inhibit the export of
photo-assimilates.
Changes that occur in the properties of
enzymes caused by changes in temperature
contribute to the regulation of carbohydrate
metabolism.
11. lower stomatal conductance, photochemical
efficiency of the photosystem (PS) II, thylakoid
electron transport rate, enzyme activity and
carbon metabolism, as well as the photosynthetic
pigment complex systems.
The photosynthesis is strongly reduced below 18
C while temperatures around 4 C dramatically
depress photosynthetic performance and yield.
After a period of exposure to low temperature,
the photosynthetic assimilation acclimates to low
temperatures which results in an increase in the
capacity of photosynthesis.
12. At low temperatures, photosynthesis can also be
limited by other factors such as phosphate
availability at the chloroplast.
If the rate of triose phosphate utilization in the
cytosol decreases, phosphate uptake into the
chloroplast is inhibited and photosynthesis
becomes phosphate limited.
Starch synthesis and sucrose synthesis decrease
rapidly with temperature, reducing the demand
for triose phosphates and causing the phosphate
limitation at low temperatures.
13. Stomata closure
reduction of transpiration
inhibition of photosynthesis are common
responses that can occur in hours or days.
When waterlogging is prolonged, waterlogging-
susceptible plants drastically reduce their
physiological activities and are often killed in a
short time(Malik et al. 2001; Zaidi et al. 2004) .
whereas in waterloggingtolerant plants, the same
parameters could even be enhanced or have less
effect due to the ability of roots to acclimate to
waterlogging, such as by the ability to produce
adventitious roots and aerenchyma formation.
14. Stomatal conductance is the major factor
affecting photosynthesis under waterlogging
conditions in plants.
15. In case of C3 plants, the photosynthesis is negatively
affected by water stress measured as changes in leaf
water potential or relative water content.
If water stress persists and leaf RWC falls below 70 %,
the loss of photosynthetic activity becomes
increasingly less responsive to high CO2, and
assimilation rate fails to recover to prestress values
following the removal of water stress.
For instance, although photosynthesis may decrease
up to 100 % becoming totally impaired under severe
water shortage, the respiration rate may either
increase or decrease under stress, but may never
become totally impaired (Flexas et al. 2005).
16. If soil water becomes scarce, plant water
status worsens, leading to cascading effects
that can be severely adverse at both the leaf
and plant scales.
To reduce these risks, plants under water
deficit reduce transpirational water losses by
reducing stomatal conductance.
Water stress hinders leaf internal transport of
CO2, enzyme activity, and hence
photosynthetic capacity.
17. Salinity has many direct and indirect effects on
photosynthetic processes.
Photosynthetic rates are usually lower in plants
exposed to salinity and especially to sodium chloride.
Salt effects on photosynthetic processes fall into two
major categories:
(i) Stomatal closure, the usual response of stomata to
salinization of salt-sensitive plants
(ii) Effects on the capacity for CO2 fixation apart from
the altered diffusion limitations.
The reduction in the photosynthesis rate of plants
exposed to salinity usually depends on two aspects of
salinization.
18. High salt concentrations accumulating in the
soil solution create high osmotic potentials,
which reduce the availability of water to the
plants.
19. The concentration of CO2 gas in the atmosphere is
very trace, presently accounting for about 0.038 %.
The elevated CO2 concentrations in the boundary
layer atmosphere of leaves may cause changes in
stomatal aperture are water availability, light, and
CO2 concentration.
Stomata close in response to low water availability in
the air or soil and in most species open in response
to light and close in the dark.
Stomatal response to CO2 and the way this response
affects photosynthesis and transpiration will have
effects on plant water regulation and growth, since
virtually all of the CO2 used by the plant passes
through stomata.
20. Plants get affected by their regular metabolism
under either influence of single abiotic stress or
combinations of two or more alterations in the
photosynthetic activities in plants under abiotic
stress conditions that hinder their growth,
development, and yield.
Stomatal closure plays by far the main role in the
decline in leaf photosynthesis and photosynthetic
machinery remains intact, thereby allowing the
leaf to respond rapidly to changes in vapor
pressure deficit.
Increase in the concentrations of CO2 and O3 in
the atmospheric air causes several effects on
photosynthetic processes.
21. The decrease in photosynthesis may be related to
several factors:
• Reduction of the CO2 supply because of hydro-
active closure of the stomata
• Changes in the leaf temperature required for
photosynthesis
• Dehydration of cell membranes, which reduce their
permeability to CO2
• Salt toxicity
• Enhanced senescence induced by salinity
• Changes in enzyme activity induced by changes in
cytoplasmic structure
• Negative feedback by reduced sink activity
