Plants grown at higher C02 levels generally have a higher carbon-to-nitrogen ratio. It seems that because they have more carbon to work with, they end up producing more of the carbon-containing structural molecules of cell walls, such as cellulose and lignin. They may also store up excess carbon as starch reserves inside their cells. It is uncertain what implications these changes might have for ecosystem functioning; for example, they might decrease the suitability of the plant as food for herbivores, and the decomposition rate of dead plant tissues when they end up in the soil. There are also concerns that crops might become less nutritious since they have less protein; people could simply fill up on starch.
Plants grown at high C02 also tend to have a greater mass of roots relative to shoots. Alternatively, the rate of growth and turnover of the small roots that gather nutrients may increase (as in the sweetgum plots in Tennessee). From the plant's point of view, having more carbon as a result of growing in high C02 means that nutrients from the soil are more limiting to its growth, so investing in roots is a good way to gather more nutrients. Getting more nutrients can then mean that it puts on the maximum amount of growth, and produces more offspring.
Another effect, detectable only at the microscopic level, is that plants grown at higher C02 levels have fewer stomata on their leaves, presumably because fewer are needed to allow C02 in to the plant when C02 levels are higher. Perhaps stomata leak water a bit, or use up energy unnecessarily, so that it is advantageous for the plant to have no more stomata than it really needs (Figure 8.9).
0ne rather strange effect of increased C02 levels is that respiration rates of plant tissues tend to be lower. If wasteful burning of carbon was being decreased, then this could be a good thing, allowing the plant to accumulate more carbohydrate for useful tasks. However, much of the respiration that goes on in a plant has a purpose, such as the building and repair of tissues. If this sort of respiration is cut down, plants might not be repairing tissues as thoroughly, with possible long-term consequences.
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