Reference
Garcia, R.L., Idso, S.B. and Kimball, B.A. 1994. Net photosynthesis as a function of carbon dioxide concentration in pine trees grown at ambient and elevated CO2. Environmental and Experimental Botany 34: 337-341.
What was done
Two Eldarica pine (Pinus eldarica L.) seedlings were grown out-of-doors in a field of Avondale loam at Phoenix, Arizona (USA) within transparent open-top enclosures, one of which was maintained for fifteen months at a mean atmospheric CO2 concentration of 402 ppm and one of which was simultaneously maintained at 788 ppm, after which short-term whole-tree net photosynthetic rates (one-hour averages) were measured at a number of different CO2 concentrations (changed at 1.5-hour intervals) ranging from ambient (360 ppm) to 3000 ppm.
What was learned
Garcia et al. report that "the two trees responded identically to short-term atmospheric CO2 enrichment to about a tripling of the ambient CO2 concentration." However, as the CO2 content of the air was increased further, they found that "the net CO2 assimilation responses of the two trees diverged: the photosynthetic response curve of the low-CO2-grown tree exhibited the classical form of a rate-limiting rectangular hyperbola, while that of the high-CO2-grown tree maintained its linearity to the highest CO2 concentration investigated." At this latter CO2 concentration (3000 ppm), the photosynthetic rate of the low-CO2-grown tree had long since plateaued out at a value approximately five times that of its value at 360 ppm, while the photosynthetic rate of the high-CO2-grown tree was still rising linearly at a value approximately ten times greater than what it had exhibited at 360 ppm.
What it means
The three researchers concluded that "atmospheric CO2 enrichment produces a type of up-regulation of carbon assimilation in Eldarica pine trees, as long-term exposure to elevated CO2 enabled the high-CO2-grown tree to continue to respond to further increases in the CO2 content of the air while the photosynthetic rate of the tree grown in ambient air reached an asymptotic limit." This observation suggests that as the air's CO2 content continues to climb, it may be possible for earth's plants to continuously adapt to - and thereby continuously profit from - its ever-increasing concentration.