Control of Photorespiration | Photosynthesis

The below mentioned article provides a notes on Control of Photorespiration.

Photorespiration is apparently a wasteful process because quite a large amount of recently fixed carbon is lost by re-oxidation to CO₂. Lorimer and Andrews in 1973 suggested that oxygenation of RuBP is unavoidable because with the present gaseous composition of the atmosphere, photosynthesis via the C₃ cycle is obligatorily coupled to oxygenase function.

If this is true, then regulation of photorespiration and the control of the associated inhibition of photosynthesis by oxygen would be an interesting and challenging effort for increasing agricultural productivity.

Photosynthetic CO₂ fixation and the rate of dry matter production can be substantially increased by reducing O₂ concentration and by increasing CO₂ of the atmosphere. Many workers have also tried to select individuals from a usual photo respiring population which are deficient in photorespiration.

Mutations and chemicals have been tried to modify the nature of RuBPcase activity to achieve the desired character, viz., increased affinity towards CO₂ and reduced affinity towards O₂. But such a research endeavor seems to be futile because oxygenation is an inevitable and inescapable phenomenon in the reaction mechanism of RuBPcase.

Chemical control of photorespiration has been tried by Zelitch (1966) with α-hydroxy-2-pyridine-methane-sulphonic acid (α-HPMS) which is an inhibitor of glycolate oxidase, whereby the metabolism of glycolate is blocked leading to its accumulation and net photosynthesis is increased. But α-HPMS, which is an anti-transpirant, induces stomatal closure and thus may inhibit RuBPcase with respect to CO₂.

Another deleterious effect may result from high glycolate concentrating to a toxic level in presence of this stomatal inhibitor.

Later, Zelitch (1974) reported that 2, 3-epoxy propionate (glycidate), an analogue of glycolate, inhibitsglycolate formation and photorespiration and stimulates net photosynthesis. But such Stimulation of photosynthetic rates has been noticed in leaf discs and isolated chloroplasts, whereas in intact plants glycidate treatment has not been successful (Chollet and Ogren, 1975).

Another glycolate oxidase inhibitor, 2-hydroxy-3-butyonate and an inhibitor of serine-glycine conversion have been tested by Servaites and Ogren (1977). They found that these chemicals may inhibit the glycolate pathway but are not able to raise the net photosynthesis.

Several suggestions have been made to ascribe useful functions to photorespiration. The primary function of the C₂ cycle is a carbon salvaging mechanism because it returns to the C₃ cycle three- quarters of the carbon that would have been lost as glycolate. This strongly supports the view that photorespiration is a normal component of photosynthetic metabolism and is dependent on the C₃ pathway.

The reaction involving glycine-serine conversion may be coupled to the re-oxidation of NADH via the mitochondrial electron transfer chain and thus ATP is likely to be produced. Some of the intermediates, particularly glycine, serine and glyceric acid may be used in cellular synthesis.

Photo inhibition of photosynthetic capacity occurs when plants are exposed to strong illumination under limited CO₂ supply during diurnal stomatal closure.

Under this circumstance, abundant photochemical energy, which is not used in carbon assimilation, is likely to damage the photosynthetic apparatus. Osmond and Bjorkman (1972) have proposed that sufficient O₂ supply would permit photo respiratory generation of CO₂ and thereby raise the CO₂ compensation point above zero thus allowing CO₂ fixation.

It seems probable that photorespiration serves to protect the photochemical apparatus from light damage by the dissipation of photochemical energy with concomitant CO₂ assimilation by consuming light-generated reductant.