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A biological process in plants, thought to be useless and even wasteful, has
significant benefits and should not be engineered out -- particularly in the
face of looming climate change, says a team of UC Davis researchers, July
2004 by Andy Fell
The researchers have found that the process, photorespiration, is
necessary for healthy plant growth and if impaired could inhibit plant
growth, particularly as atmospheric carbon dioxide rises as it is globally.
Their findings are published this week in the Proceedings of the National
Academy of Sciences.
Over the past two hundred years, scientists have come to understand that
plants are amazing biochemical factories that harness energy from sunlight
to convert water and carbon dioxide into sugars that fuel the plant, while
giving off oxygen.
Though elegantly simple in concept, this process, known as photosynthesis,
is remarkably complex in detail. And for years, researchers have been
puzzled by another process, photorespiration, which seems to have annoyingly
associated with photosynthesis down the evolutionary pathway.
Photorespiration has appeared to be downright wasteful because it virtually
undoes much of the work of photosynthesis by converting sugars in the plant
back into carbon dioxide, water and energy.
Believing that photorespiration is a consequence of the higher levels of
atmospheric carbon dioxide in long past ages, many scientists concluded that
photorespiration is no longer necessary. Some have even 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
The result, they have thought, would be more productive crop plants that
make more efficient use of available resources.
But the new UC Davis study suggests that there is more to photorespiration
than meets the eye and any attempts to minimize its activity in crop plants
would be ill advised.
"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. "But 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."
The UC Davis team used two different methods to demonstrate in both wheat
and Arabidopsis, a common research plant, 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 UC Davis study was supported by the National Science Foundation, the
U.S. Department of Agriculture and an Israel Binational Agricultural
Research and Development Fund fellowship.
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