LOS ANGELES - So much for a hoped-for bright spot to global warming.

Some biologists had theorized earlier that rising greenhouse gas levels would encourage plant growth over the long term because of the increased amount of carbon dioxide in the atmosphere. Plant physiologists from the University of California, Davis, may have further dashed those hopes.

They've shown that too much carbon dioxide, which plants need for energy, actually can inhibit a plant's ability to assimilate nitrates - nitrogen-based nutrients pulled from the soil that plants use to make enzymes and other essential proteins.

Without those essential proteins, plant health - and food quality - may suffer, the researchers say in a study published online Thursday in the journal Science.

Scientists had previously observed that a rise in carbon dioxide levels - 39 percent globally since 1800, according to the Intergovernmental Panel on Climate Change - would boost photosynthesis, the sunlight-fueled process by which plants make sugar.

But previous studies showed that after an initial spike in sugar-making activity, photosynthesis appeared to level off, even if the carbon dioxide rate remained high.

``Here we have this quandary where we thought rising carbon dioxide levels might actually have some benefit, but it proves to be wrong. ... Over a period of time, be it weeks or years, that stimulation disappears,'' said lead author Arnold Bloom, a professor in the department of plant sciences at UC Davis.

Other studies showed that after plants were exposed to excess carbon dioxide, their protein content also dropped.

In a series of five experiments, Bloom and his colleagues found an explanation. The team exposed plants to high carbon dioxide (or low oxygen), fertilized them with nitrates and tracked how much nitrogen they successfully incorporated into their systems.

In each case, the researchers found that the more carbon dioxide exposure, the less plants were able to assimilate nitrogen. Without enough nitrogen, the plants could not make as many proteins, including those enzymes used in photosynthesis - and thus, would be unable to take advantage of all that extra carbon dioxide in the air anyway.

The findings have significant implications for agriculture, biologists said. They suggest that, as global warming continues and carbon dioxide levels rise, food may become poorer in quality and less nutritious, and farmers may have to worry about lower-quality crop yields that could perhaps be more prone to pest infestations (as plant eaters may have to eat more to get the same nutritional value as before).

Farmers will have to figure out how to fertilize their crops without poisoning them, researchers said, since ammonium (another form of inorganic nitrogen that can be used to feed plants) is not subject to the nitrate inhibition issue but can be toxic if not used wisely.

The study ``has some very important real-world implications,'' said Harvard University plant physiologist Noel Michele Holbrook, who was not involved in the study. ``How do we think about the idea of breeding for more productive crops, and what sorts of attitudes for breeding are going to pay off in the long run? We're facing really important challenges in terms of food production and quality of food.''
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