What was done
Winter wheat and barley were grown in field plots for two consecutive years under ambient CO2 conditions and midday temperatures ranging from less than 10°C to over 30°C in order to obtain photosynthetic data for comparison with a biochemical model depicting C3 plant photosynthesis.  These data were obtained from photosynthetic measurements made on leaves that were briefly exposed to air of 350 or 700 ppm CO2 by adjusting the amount of CO2 within the measurement cuvette.

What was learned
Plants grown in the field exhibited CO2-induced photosynthetic stimulations that typically rose with increasing air temperature.  At a temperature of 10°C, for example, wheat and barley exhibited CO2-induced photosynthetic enhancements of approximately 63 and 74%, respectively; while at a temperature slightly above 30°C, they experienced enhancements on the order of 115 and 125%.  The photosynthetic stimulations observed at air temperatures greater than 20°C were in close agreement with the predictions of the biochemical model; but the stimulations observed at temperatures less than 20°C generally exceeded those predicted by the model, indicating that plant responses to CO2 enrichment may be greater at lower air temperatures than what has previously been thought to be the case.

What it means
As the CO2 content of earth's atmosphere continues to rise, wheat and barley should experience increases in photosynthesis over a wide range of air temperatures.  Growth at high air temperatures should increase in accordance with the predictions of state-of-the-art biochemical models, while growth at low air temperatures should also increase, possibly even exceeding the predictions of the biochemical models.  As a result, the cultivation of wheat and barley could well expand into agricultural regions that are currently both too warm and too cool for successful production under earth's current atmospheric CO2 concentration.