Photorespiration

Abstract

Photorespiration is the light‐dependent release of carbon dioxide initiated by ribulose‐1,5‐bisphosphate carboxylase/oxygenase ( Rubisco ) in oxygen‐producing photosynthetic organisms. It occurs because oxygen can substitute for carbon dioxide in the first reaction of the photosynthetic carbon dioxide‐fixation process, causing the idle synthesis of phosphoglycolate. Phosphoglycolate is scavenged in the photorespiratory C 2 cycle, which is an essential auxiliary metabolic pathway that allows photosynthesis in oxygen‐containing environments. Three out of four misdirected carbon atoms are recovered and the fourth is released as photorespiratory carbon dioxide. Absolute rates vary in different organisms and they also depend on environmental conditions, mainly oxygen, carbon dioxide and temperature. They are highest in C 3 plants and much reduced in other organisms, such as C 4 plants, algae and cyanobacteria. In the presence of oxygen, phosphoglycolate production is unavoidable and cannot be eliminated. Key concepts: Nearly all photosynthetic activity on earth is oxygen‐producing photosynthesis. Dictated by the biochemistry of carbon dioxide fixation, this unavoidably comes with the production of phosphoglycolate. When assessed by mass flow, excelled only by photosynthesis, phosphoglycolate production constitutes the second‐most important process in the land‐based biosphere. The scavenging of phosphoglycolate was a condition for the evolution of cyanobacteria, algae and plants. Oxygenic photosynthesis is only possible on the condition of adequate photorespiratory metabolism and life on earth would look very different without it. Oxygenic photosynthesis is coupled to photorespiratory carbon dioxide losses and this reduces competitiveness of C 3 plants in warm environments. Gene technology‐assisted breeding hopes to reduce these losses in crops.

Composição química dos grãos e da cera foliar de variedade Glycine max (L.) Merrill cv. MG/ BR-46 Conquista cultivada sob atmosfera enriquecida de gás carbônico e temperatura elevada

“…Elevadas temperaturas diminuem a solubilidade do gás carbônico em água, estimulando a função oxigenase da rubisco, estimulando a fotorrespiração e promovendo maior liberação do fosfoglicolato (2PG) ao invés de 3-fosfoglicerato (Bauwe, 2010). Na rota de fotorrespiração, o fosfoglicolato é convertido em glicerato 3-P para seu ingresso no Ciclo de Calvin, liberando CO 2 e NH 3 , entre outros compostos.…”