Background
The authors note that, in general, "photosynthesis can function without harm between 0 and 30°C in cold-adapted plants," that in plants from equitable habitats "photosynthesis operates safely between 7 and 40°C with no apparent problem," and that in plants from hot environments "photosynthesis operates between 15 and 45°C with no apparent problem." But what happens if CO2-induced global warming turns out to be as great as is currently predicted by state-of-the-art climate models and it results in temperatures that significantly exceed the upper limits of these three types of environment?

What was done
Sage and Kubien review the current state of our understanding of this important subject, focusing primarily on C3 plants, which comprise the vast bulk (about 95%) of earth's terrestrial vegetation.

What was learned
First of all, the two researchers report that "with changes in growth conditions," such as an increase in mean growing-season temperature, the optimum temperature range for photosynthesis "can shift," and that it typically shifts upward "by one-third to one-half the number of degrees as the shift in growth temperature," which represents a substantial degree of acclimation. But what about the other half to two-thirds of the temperature increase? Is there anything that can compensate for it?

Based upon modeled responses that they say "are qualitatively similar to responses determined on a wide range of C3 species, including bean, Eucalyptus, soybean, chili pepper, tomato, Populus fremontii and Scrophularia desertorum, rice, alfalfa, oak, Abutilon theophrastii, Chenopodium album, pine, grape, pima cotton, and sweet potato," for each of which plants they cite separate scientific studies, Sage and Kubien find that a 320-ppm increase in the atmosphere's CO2 concentration (from an initial value of 380 ppm representative of today's concentration to a future value of 700 ppm) increases the optimum temperature for photosynthesis from approximately 32°C to about 38°C, as best we can determine from the graphical representations of their results. In addition, the net photosynthetic rate at the higher optimum temperature in the 700-ppm-CO2 air is about 85% greater than the rate corresponding to that of the lower optimum temperature in the 380-ppm-CO2 air. On the other hand, they find that a 200-ppm decrease in the air's CO2 concentration (from an initial value of 380 ppm to a value of 180 ppm representative of late-Pleistocene conditions) decreases the optimum temperature for photosynthesis from approximately 32°C to 22°C, while the net photosynthetic rate at the lower optimum temperature in the 180-ppm-CO2 air is about 55% smaller than the rate corresponding to that of the higher optimum temperature in the 380-ppm-CO2 air.

What it means
Earth's terrestrial plants appear to be well-equipped indeed to live in a CO2-enriched and warmer world, due to both warming-induced and CO2-induced increases in the optimum temperature for photosynthesis and the absolute rate of photosynthesis. What is more, Sage and Kubien note that "through breeding and genetic engineering, humans could get a jump on climate change by directly selecting for traits that will preadapt species to warmer, CO2-enriched environments."

Clearly, it is not doom and gloom that the future portends, it is biological bounty, if we will only prepare for it and not fight against it.