Introductory Crop Physiology

Photorespiration

• Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light.
• Like ordinary respiration, this process also releases carbon from organic compounds in the form of CO₂ but does not produce ATP.
• Thus, apparently it seems to be a wasteful process, but it must have some functions which are still unknown.
• This process occurs in C₃ plants and to some extent in C₄.
• Photorespiratory substrate is glycolate. RuBisco instead of combining with CO₂ it combines with O₂.
• This type of oxidation of O₂ to RuBP molecule is known as photosynthetic C-oxidation cycle or glycolate pathway or C₂ photorespiratory carbon oxidation cycle.
Generally photorespiration is expressed by the term called CO₂ compensation point and it is defined as CO₂ concentration at which rate of uptake will be equal to the rate of photosynthetic respiratory CO₂ released. Photorespiration is around 80-100 ppm in case of C₃ plants and 0-10 ppm in C₄ plants.
• As alternative substrates for RuBisco, CO₂ and O₂ compete for reaction with ribulose-1,5-bisphostate because of carboxylation (if CO₂ reacts with RuBP then carboxylation takes place) and oxygenation (if O₂ reacts with RuBP then oxygenation takes place) occur within the same active site of the enzyme.
• Photorespiration occurs usually when there is high concentration of O₂. under such circumstances RuBP carboxylase (or RuBisCo), the enzyme that joins CO₂ to RuBP, functions as oxygenase. As a result oxygen, instead of CO₂, gets attached to the binding site of the enzyme and the latter (ie RuBP) is oxidized.
• On oxidation, RuBP releases one molecule of C₃ compound, PGA (which enters C₃ cycle) and one molecule of a C₂ compound phosphoglycolate.
• The latter almost immediately changes to glycolate. The glycolate leaves the chloroplast and enters a membrane bound sac of enzymes called peroxisme.
• Here the glycolate is oxidized into glyoxylate which is aminated into glycine.
• Further condensation of glycine takes place inside the mitochondria where two molecules of glycine interact and give rise to a molecule of serine and CO₂ each. The later is then released in photorespiration
Factors controlling photorespiration

• CO₂ concentration: Higher the CO₂ concentration lesser will be the photorespiration.
• O₂ concentration: More the O₂ concentration more will be the photorespiration.
• Temperature: Optimum temperature is 25-35^oC. More the temperature less will be the photorespiration.
• Reasons for lack of photorespiration in C₄ plant.
• They have low CO₂ composition point.
• No or less CO₂ evolution in light.
• Less response to elevated CO₂ levels with regards to growth rate.
• Due to lack of synthesis of photorespiratory substrate.
• Photorespiration occure in bundle sheath chloroplast where, RuBisCo present.
• Due to CO₂ enrichment mechanism higher CO₂ is maintained in bundle sheath cells. Higher CO₂ is maintained due to rapid decarboxylation of malate and aspertate and transferred these from mesophyll cells. Due to higher CO₂ concentration photorespiration is inhibited.
• So in C₄ plants there will no or less photorespiration.
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