Background
Electron transport through photosystem II, which is often measured and reported as the maximum photochemical efficiency of photosystem II, is the most heat-sensitive component of the entire photosynthetic process.  Any reductions in electron transport through this thylakoid membrane-bound protein complex invariably lead to reductions in photosynthetic carbon uptake and reduced growth potential for plants.  Thus, it is important to determine if elevated levels of atmospheric CO2 impart any thermal protection to this temperature-sensitive component of the photosynthetic process.

What was done
The authors conducted several experiments in controlled environment chambers, greenhouses, and FACE plots to examine the photosynthetic responses of various plants to acute heat stress under ambient and elevated CO2 concentrations ranging from 550 to 1000 ppm.  The plants studied included representatives of herbaceous, woody, monocot, and dicot species, which allowed the authors to make a preliminary assessment the degree of universality of any detected responses.

What was learned
Of sixteen plant species grown in elevated CO2, all but one displayed greater photochemical efficiencies of photosystem II than their ambiently-grown counterparts when exposed to high air temperatures.  In fact, the air temperatures that caused a 50% reduction in the maximum efficiency of photosystem II were nearly one degree Celsius higher for plants grown in elevated CO2 than they were for plants grown at ambient CO2.  In other words, elevated CO2 --almost universally-- allowed more electrons to flow through photosystem II, thereby laying the foundation for greater photosynthetic rates to occur.  In an extended experiment, in fact, rates of net photosynthesis measured at 40°C in CO2-enriched cucumbers were 3.2 times greater than those exhibited by ambiently-grown control plants exposed to the same air temperature.

What it means
As the atmospheric CO2 concentration increases, it is likely that nearly all plants will be endowed with an enhanced thermal tolerance to high temperature stress, thereby enabling them to exhibit greater rates of net photosynthesis and growth than they do today.  When applied to natural ecosystems, these results indicate, once again, that earth's vegetation will not need to migrate poleward to escape to cooler climates if temperatures rise, regardless of the reason for any temperature rise that may occur.  Instead, plants will likely stay within their current ranges and grow even better than they did previously, thanks to higher air temperatures driving biological growth processes and the added thermal protection provided by the rising CO2 content of the air.