Researchers reveal benefits of photorespiration

DAVIS, California, July 27, 2004 (ENS): Plant photorespiration has significant benefits and should not be engineered out, according to researchers with the University of California at Davis.
The researchers have found that the biological process - once thought useless and wasteful - is necessary for healthy plant growth and if impaired could inhibit plant growth, particularly as atmospheric carbon dioxide rises as it is globally.

Photorespiration was thought to be wasteful because it undoes much of the work of photosynthesis by converting sugars in the plant back into carbon dioxide, water and energy.

"Photorespiration is a mysterious process that under present condition dissipates about 25 percent of the energy that a plant captures during photosynthesis," said Arnold Bloom, a professor in UC Davis' vegetable crops department and lead researcher on the study.

Believing that photorespiration is a consequence of the higher levels of atmospheric carbon dioxide in ages past, many scientists concluded that photorespiration is no longer necessary.

In a bid to make more productive and efficient crop plants, some have set about to genetically engineer crop plants so that the activity of the enzyme that initiates both the light-independent reactions of photosynthesis and photorespiration would favor photosynthesis to a greater extent and minimize photorespiration.

The new UC Davis study suggests that attempts to minimize its activity in crop plants would be ill advised.

"Our research has shown that photorespiration enables the plant to take inorganic nitrogen in the form of nitrate and convert it into a form that is useful for plant growth," Blooms said.

The UC Davis team found that when plants are exposed to elevated levels of atmospheric carbon dioxide or low levels of oxygen - both conditions that inhibit photorespiration - nitrate assimilation in the plant's shoot slows down.

Eventually, a shortage of nitrogen will curtail the plant's growth.

"This explains why many plants are unable to sustain rapid growth when there is a significant increase in atmospheric carbon dioxide," said Bloom. "And, as we anticipate a doubling of atmospheric carbon dioxide associated with global climate change by the end of this century, our results suggest that it would not be wise to decrease photorespiration in crop plants."

The findings are published this week in the "Proceedings of the National Academy of Sciences."

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