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 availa bility of this often growth-limiting resource, as does the question of whet her genetic changes are likely to result from natural selection in non-crop species as atmospheric [CO2] rises. Based on the concept that adaptation t o a given resource level is reflected in how resource-use efficiency change s with the availability of that resource, we examined various aspects of pl ant 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 spe cies. By several measures, the efficiency at which plants used carbon dioxi de decreased abruptly just above the current atmospheric concentration of c arbon 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 cons tant up to the current [CO2] and decreased at higher [CO2]. Down-regulation of photosynthesis occurred in three of the four species when grown above t he current [CO2]. These patterns occurred for two different growth light re gimes. These responses indicate that these annual weedy species are adapted to the current atmospheric [CO2], but not to higher concentrations.