What was done
The authors sequentially grew well watered and fertilized wheat (Triticum aestivum L. cv. Alcala) in 16-liter pots of perlite (sown at a rate of 35 seeds per pot) in a controlled-environment growth chamber -- first at an atmospheric CO2 concentration of 370 ppm and then at 700 ppm -- from sowing through anthesis, measuring gas exchange in flag leaves at ear emergence in order to obtain the values of various plant physiological parameters required for the biochemical photosynthesis model developed by Farquhar et al. (1980), along with the responses of those parameters to changes in temperature.

What was learned
Alonso et al. discovered that the "photosynthesis response to temperature was negative at low air CO2 concentrations and became progressively positive as CO2 increased," as might have been expected, in their words, "from the increase in photorespiration with temperature and the gradual inhibition of this process as CO2 increases (Long, 1991)." In addition, they report that "at high chloroplastic CO2, photosynthesis in elevated growth CO2 was lower at 15-25°C and higher at 30-35°C, than in ambient growth CO2, implying an enhanced photosynthesis response to temperature in plants grown in elevated CO2."

What it means
When all of their complex analyses were completed, the three Spanish researchers concluded that "growth in elevated CO2 has important additional effects on electron transport capacity and on Rubisco kinetics that can improve photosynthesis temperature response," and that "future increases in atmospheric CO2 may therefore positively affect photosynthesis adjustment to global warming in wheat," which conclusion they reiterated in the final sentence of their abstract, where they state that "the effects of future increases in air CO2 on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming."

References
Farquhar, G.D., von Caemmerer, S. and Berry, J.A. 1980. A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149: 78-90.

Long, S.P. 1991. Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: has its importance been underestimated? Plant, Cell and Environment 14: 729-739.