What was done
The authors grew well watered and fertilized sweet potato (Ipomoea batatas L.) plants in 20-L pots of soil in a greenhouse, periodically measuring light-saturated rates of net photosynthesis in new but fully-expanded leaves in response to short-term changes in air temperature and atmospheric CO2 concentration.

What was learned
In response to an approximate 370-ppm increase in the air's CO2 concentration, the optimum leaf temperature for net photosynthesis in sweet potato, i.e., the leaf temperature at which net photosynthesis proceeds at its maximum rate, rose by approximately 4.5°C, while its maximal rate of net photosynthesis rose by about 75%.  Viewed another way, the doubling of the air's CO2 concentration had no impact on net photosynthesis at a leaf temperature of 15°C, but it boosted it by 28% at 21°C, by 43% at 27°C, by 56% at 33°C, and by 70% at 39°C.  And viewed yet another way, in order for the net photosynthetic rate of sweet potatoes growing in air of 740 ppm CO2 to drop below the maximum rate exhibited by plants growing in air of 370 ppm CO2 (which occurred at a leaf temperature of 30°C), leaf temperature would have to rise by a full 12°C to a value of 42°C.

What it means
These results demonstrate the strong synergism that exists between rising atmospheric CO2 concentration and temperature, and the positive impact these increases make on the net photosynthetic rate of sweet potato.  This phenomenon is also observed in many other plants, as noted in our major report The Specter of Species Extinction, where a graphical presentation similar to that observed in the present study is displayed in the middle of Section II.B, which is found on page nine if viewed in pdf format.  And since a picture is worth a thousand words, as the saying goes, we suggest you visit that document to gain a better appreciation of this phenomenon.