Photosynthesis and associated aspects under abiotic stress

Zakriya alviPhD

University of Agriculture Faisalabad.

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.

introduction

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.

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Regulation of photosynthesis under different abiotic stresses, such as changes in temperatures, water stress conditions, salinity, elevated CO2 and O3, and its associated aspects.

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Photosynthesis Under Change in Temperaturesa)- Photosynthesis Under High Temperaturesb)- Photosynthesis Under low

Temperatures Photosynthesis Under Water Stress

a)- Photosynthesis Under Waterlogging Conditionsb)- Photosynthesis Under Water-Deficit Conditions

Contents

Photosynthesis Under Saline Conditions Photosynthesis Under Elevated CO2 Photosynthesis Under Elevated CO2

Concentrations.

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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.

Photosynthesis Under Changein Temperatures

(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.

Photosynthesis Under High Temperatures

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.

Photosynthesis Under High Temperatures

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.

Photosynthesis Under High Temperatures

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.

Photosynthesis Under low Temperatures

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.

Photosynthesis Under low Temperatures

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.

Photosynthesis Under WaterloggingConditions

Stomatal conductance is the major factor affecting photosynthesis under waterlogging conditions in plants.

Photosynthesis Under WaterloggingConditions

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).

Photosynthesis Under Water-DeficitConditions

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.

Photosynthesis Under Water-DeficitConditions

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.

Photosynthesis Under SalineConditions

High salt concentrations accumulating in the soil solution create high osmotic potentials, which reduce the availability of water to the plants.

Photosynthesis Under SalineConditions

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.

Photosynthesis Under ElevatedCO2 Concentrations

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.

Conclusion

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

Conclusion

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