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Plant metabolism - Is photorespiration an effective mechanism for protecting against photoinhibition?

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Introduction

Louise Weston Plant metabolism Is photorespiration an effective mechanism for protecting against photoinhibition? CO2 and O2 are competitive substrates for the enzyme Rubisco (Ribulose bisphosphate carboxylase/oxygenase). Photorespiration is the process where by the enzyme Rubisco fixes molecular oxygen as apposed to carbon, which ultimately leads to the evolution of CO2 from plants. The process occurs concurrently with photosynthesis in green leaves of C3 plants. A high CO2 and low O2 atmospheric concentration favours the carboxylase action, whereas a high O2 and low CO2 concentration favours the oxygenase action. Therefore due to the existence of photorespiration, photosynthesis is inhibited by oxygen. When the release of CO2 by photorespiration equals CO2 fixation by photosynthesis, a concentration of CO2 is reached called the CO2 compensation point. When the light is extinguished, continued CO2 production from photorespiration results in a post illumination burst (PIB). The enzymic pathways for CO2 production from oxygen fixation are known as the photosynthetic carbon oxygen cycle (PCO). It involves the coordinated activities of the chloroplast, peroxisome and the mitochondria as well as transport of the compounds through the cytoplasm between these organelles and it is shown below; The cost of fixing one molecule of O2 ...read more.

Middle

Non-assimilatory electron transport via oxygenase photorespiration stimulates photon utilization and thereby mitigates chronic photoinhibition under natural conditions this was observed when transgenic tobacco plants were studied having twice the normal amount of GS2 (plastidic glutamate synthase - a key enyzyme in photorespiration) had an improved capacity for photorespiration and an increased tolerance to high light intensity, where as those with a reduced amount of GS2 had an diminished capacity for photorespiration and were photoinhibited more severely by high light intensity compared with control plants. An additional reaction found to mitigate chronic photoinhibition is the Mehler reaction (the water-water cycle), arguably an additional type of photorespiration whereby the photoreduction of O2 at PSI occurs. This photoreduction produces superoxide radicals (O2-�), which disproportionate to H2O2 and O2 with the aid of superoxide dismutase. The H2O2 is rapidly detoxified to water by the ascorbate peroxidase pathway. Since the electron flow from water in PSII to water in PSI occurs in this process, it has been termed the water-water cycle The water-water cycle not only scavenges free radicals and peroxide, but also generates a pH gradient (?pH) ...read more.

Conclusion

When the water-water cycle and photorespiration were suppressed by low O2 partial pressure the rate of photoinactivation failed to increase indicating that both processes contributed little to photoprotection under low temperature and high light conditions. The water-water cycle was more temperature dependent that photorespiration and was reduced to negligible proportions below 15?C. Therefore photorespiration is not an effective mechanism for protecting against photoinhibition at low temperatures. However, it is more efficient at high temperatures. Under water stress, where the supply of CO2 to the chloroplasts is restrictesd in the light because of stomatal closure, the generation of CO2 in the green cells by photorespiration maintains carbon in the intermediates of the Calvin cycle. Otherwise because of insufficient diffusion of external CO2 into the leaves there would be excess photons. However, although in this incidence photorespiration is increased relative to carbon assimilation, it is decreased in absolute terms. The photorespiration path uses energy even at low photon flux densities thus lowering C assimilations and so the whether it has a unique protective function can be doubted and in this respect it seem a not particularly effective mechanism for protecting against photoinhibition as can not be regulated. However, it is at low Co2 concentrations and high O2 concentrations when its energy dissipation qualities are required. ...read more.

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