Rising C[O.sub.2] levels prove a mixed blessing

Rising levels of carbon dioxide in the atmosphere could be a boon for agricultural crops, as this greenhouse gas helps plants grow and reproduce more, suggests a study from Ohio State University, Columbus. However, that boon comes with a price, warns Peter Curtis, professor of evolution, ecology, and organismal biology. Greater growth and reproduction may hurt the nutritional value of crops.

"If you're looking for a positive spin on rising C[O.sub.2] levels, it's that agricultural production in some areas is bound to increase Crops have higher yields when more C[O.sub.2] is available, even if growing conditions aren't perfect. But there's a tradeoff between quantity and quality. While crops may be more productive, the resulting produce will be of lower nutritional quality."

Nutritional quality declines because, while the plants produce more seeds under higher C[O.sub.2] levels, the seeds contain less nitrogen. "The quality of the food produced by the plant decreases, so you've got to eat more of it to get the same benefits," Curtis indicates. "Nitrogen is a critical component for building protein in animals, and much of the grain grown in the United States is fed to livestock. Under the rising C[O.sub.2] scenario, livestock--and humans--would have to increase their intake of plants to compensate for the loss."

To understand the role that rising C[O.sub.2] levels may play on plant growth, Curtis and his colleagues conducted a meta-analysis--a technique in which researchers pull together data from a large number of similar studies (159, in this case) and summarize the results. Curtis says that this is the first time that researchers have used the meta-analysis technique to determine the effects of climate change on plant reproduction. They analyzed eight different ways plants respond to higher C[O.sub.2] levels: number of flowers; number of fruits; fruit weight; number of seeds; total seed weight; individual seed weight; the amount of nitrogen contained in seeds; and a plant's reproductive allocation, a measurement of its capacity to reproduce.

Plants grown at higher C[O.sub.2] levels had more flowers (an average increase of 19% in the species studied); 16% more seeds; four percent greater individual seed weight; 25% higher total seed weight; and 14% lower concentration of nitrogen in the seeds than those grown at current levels of atmospheric C[O.sub.2]. Under higher C[O.sub.2] levels, crop plants showed a notable increase in reproduction while wild plants did not.

While crop plants and wild plants had similar increases in total growth (a 31% jump), crops allocated the additional weight to reproduction, while wild plants seem to funnel much of it to other tasks, "Wild plants are constrained by what they can do with increased C[O.sub.2]," Curtis points out. "They may use it for survival and defense rather than to boost reproduction. Agricultural crops, on the other hand, are protected from pests and diseases, so they have the luxury of using extra C[O.sub.2] to enhance reproduction."

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