Are annual plants adapted to the current atmospheric concentration of carbon dioxide?

The concentration of carbon dioxide [CO2] in the atmosphere has risen from about 280 mu mol mol(-1) in 1870 to about 370 mu mol mol(-1) currently, and this concentration continues to increase rapidly. In planning for future, higher atmospheric [CO2], the question arises whether genetic modifications of crop plants are required in order to fully exploit the increased availability of this often growth-limiting resource, as does the question of whether genetic changes are likely to result from natural selection in non-crop species as atmospheric [CO2] rises. Based on the concept that adaptation to a given resource level is reflected in how resource-use efficiency changes with the availability of that resource, we examined various aspects of plant growth response to [CO2] from 90 mu mol mol(-1) below to 90 mu mol mol(-1) above the current atmospheric [CO2] in four annual weedy herbaceous species. By several measures, the efficiency at which plants used carbon dioxide decreased abruptly just above the current atmospheric concentration of carbon dioxide. For example, total plant leaf area increased up to, but not above, the current [CO2], and leaf area per unit of plant dry mass was constant up to the current [CO2] and decreased at higher [CO2]. Down-regulation of photosynthesis occurred in three of the four species when grown above the current [CO2]. These patterns occurred for two different growth light regimes. These responses indicate that these annual weedy species are adapted to the current atmospheric [CO2], but not to higher concentrations.