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
"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
"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."