Photorespiration is a metabolic process that occurs in plants when there is an excess of oxygen in the leaf relative to carbon dioxide. This can happen when the stomata, the tiny openings on the leaves, are closed to conserve water, or when the temperature is high.
During photosynthesis, the enzyme rubisco catalyzes the fixation of carbon dioxide from the atmosphere, but it can also catalyze the addition of oxygen to RuBP instead of carbon dioxide, leading to the formation of a two-carbon molecule called phosphoglycolate. This process is called photorespiration and it reduces the efficiency of photosynthesis, as it wastes energy and releases CO2.
The phosphoglycolate produced in the first step of photorespiration is recycled in a complex metabolic pathway known as the photorespiratory cycle. This cycle requires ATP and consumes O2 while producing CO2. Therefore, photorespiration not only reduces the efficiency of photosynthesis, but also leads to the loss of carbon and energy.
The significance of photorespiration lies in its role in regulating the CO2/O2 ratio in the chloroplast. Photorespiration acts as a safety valve that prevents the buildup of toxic intermediates in the chloroplast and protects the plant from oxidative damage caused by excess light. Moreover, photorespiration plays a role in nitrogen and sulfur metabolism, as some of the intermediates produced in the photorespiratory cycle can be used for the synthesis of amino acids.
Despite its regulatory role, photorespiration is still considered a major limitation for plant productivity, especially in C3 plants, which have a low CO2/O2 ratio. Researchers are trying to develop strategies to reduce the impact of photorespiration on plant growth and yield, such as engineering plants with more efficient carbon concentrating mechanisms, or introducing enzymes that can bypass the photorespiratory cycle. These efforts could lead to the development of more resilient crops that can thrive under changing environmental conditions.