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Photorespiration - breathing life into plants

The simple view of photosynthesis is that it involves carbon dioxide (CO2) uptake and release of O2. It is not that simple. Even during photosynthetic CO2 uptake (‘fixation’), some CO2 is simultaneously released by the process of photorespiration. Photorespiration is a consequence of the high oxygen content of the air, which leads to a competing oxidation reaction at the same site as fixation, resulting in loss of carbon and energy from the plant.

Wasteful process or safety valve?

There are contrasting views of the role of photorespiration. One opinion is that it is an unavoidable process leading to wasteful loss of energy from the plant. Another opinion is that it provides a safety valve allowing the light-capturing part of photosynthesis to continue even when demand for carbon by the plant is saturated.

Hot and dusty does it

Hot, dry, sunny conditions promote photorespiration, so it is particularly important in the Australian context. Under these conditions the plant faces a dilemma - closing the pores (stomata) on the underside of the leaves

  • will limit water loss
  • but will also cause build up of oxygen (from photosynthesis) in the leaf
  • and will restrict uptake of adequate CO2 from the atmosphere.

So, in coping with heat, intense sunlight and dust, the ratio of O2 to CO2 inside the leaf progressively increases and photorespiration accelerates.

Energy organelles in concert

The first step in photorespiration is oxygenation of a 5-carbon photosynthetic precursor, ribulose 1,5-bisphosphate, leading to release of a 2-carbon product called phosphoglycolate. This product is recycled in a multistep metabolic pathway involving all three energy organelles. Some of the carbon and energy from phosphoglycolate is recovered in the form of glycerate, but some is lost as CO2.

Damage limitation

High light and temperature can lead to excess excitation energy capture from the sun. This excess energy can cause oxidative damage to the photosynthetic apparatus if it is not dissipated (see Oxidative> Stress). The photorespiratory pathway may allow such dissipation by releasing chemical energy from phosphoglycolate as CO2, without coupling this to ATP synthesis.

The research


(click to enlarge image).
The metabolic pathway depicted is incompletely understood. We aim to discover how it works and whether it is possible either to reduce photorespiration or to make the recycling of energy more efficient. We also strive to understand the impact of photorespiration upon the light-capturing reactions in the chloroplast. The Centre has established a coordinated research program in photorespiration in which nearly all groups participate. Such a concerted multidisciplinary approach has the potential to lead to significant breakthroughs where individual groups and single organelle focus could not.