Plants commonly respond to elevated CO2 by increasing their rates of photosynthesis. Higher rates of photosynthesis usually result in higher accumulations of carbon-rich carbohydrates. Furthermore, the increased atmospheric CO2 levels mean that stomatal conductance is reduced because plants can get sufficient atmospheric CO2 into their leaves even when their stomates are closed more often. A reduction in stomatal conductance results in greater efficiency of water use by plants, because less water is lost through transpiration. Both these factors have important effects on plant chemistry.
First, increased carbon uptake by plants results in higher plant growth rates, with leaf area index, woody biomass, and below-ground biomass sometimes increased by as much as 25 to 50%. Despite the increase in plant growth, there is usually no increase in the availability of soil nutrients, particularly nitrogen, and these nutrients must be spread further among the available plant biomass. The usual result is a decrease in total plant nitrogen because nitrogen is diluted over the entire plant. Herbivore growth is most often limited by nitrogen rather than by carbon, so that plants grown in atmospheres of elevated CO2 become poorer quality forage. Plant—water content usually affects digestibility, so that the poorer quality diet is partly offset by an increased ease of digestion.
The second major change in plants grown under conditions of elevated CO2, namely, is a change in the ratio of carbon to nitrogen (C:N), as described in the preceding section. This has major implications for the concentration of defensive compounds in the leaves, the so-called secondary chemicals. Carbon-based secondary chemicals often increase and deter insect feeding. The overall effect of increased CO2 on insect herbivores is to decrease plant palatability because of decreases in nitrogen levels and increases in secondary chemicals.
For secondary chemicals, the increases seem to be greatest for soluble phenolic compounds, especially condensed tannins, which are found in a variety of trees, especially oaks. These compounds are known to negatively affect many herbivorous insect species. Yet for other defensive compounds, such as linear fura-nocoumarins, found in celery, and monoterpenes and sesquiterpenes, found in peppermint, little increase has been noted.
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