Background
Long-term exposure of plants to elevated atmospheric CO2 concentrations often results in the acclimation or down-regulation of Rubisco (the primary carbon fixation enzyme found in leaves) in terms of both the enzyme's content and activity; and the authors speculate that if the process of resource optimization is involved, "the down-regulation of carbon gain may cause up-regulation of the capacity for carbon utilization and export."  Hence, they conducted an experiment to test this hypothesis.

What was done
Well watered and fertilized kidney beans (Phaseolus vulgaris L.) were grown from seed in controlled environment chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm and daytime maximum/nighttime minimum air temperatures of 28/18, 34/24 and 40/30°C for a total of 42 days after planting, whereupon photosynthetic rates of individual attached leaves were measured and leaf samples acquired for analyses of the key enzymes involved in carbon utilization or the synthesis of sucrose and starch: sucrose-phosphate synthase (SPS) and adenosine-5'-diphosphoglucose pyrophosphorylase (AGP).

What was learned
In the words of the authors, "overall elevated growth CO2 enhanced kidney bean leaf photosynthesis by 57% across all temperatures."  However, they note that "as growth temperature increased from 28/18 to 40/30°C, there was a small but significant decrease in leaf photosynthetic rates at both CO2 treatments."  Nevertheless, they report that "midday leaf photosynthetic rates of elevated CO2-grown plants at the highest temperature were still 35% greater than those of the ambient CO2 plants at the lowest temperature," so that "even with a down-regulation, the activity and content of Rubisco protein were still adequate to maintain greater photosynthesis at elevated CO2."

The researchers also found that "elevated CO2 significantly increased leaf sucrose and total soluble sugars by 40% when averaged across 28/18, 34/24 and 40/30°C growth temperatures," and that the extra CO2 increased the activities of both SPS and AGP across the entire temperature range investigated.  Consequently, they did indeed establish that "growth at elevated CO2 and/or temperature leads to an up-regulation of the carbohydrate metabolizing enzymes SPS and AGP to accommodate for export of the excess carbohydrates resulting from increased photosynthetic rates."

What it means
Prasad et al. conclude that "the up-regulation of leaf carbohydrate metabolism enzymes under elevated CO2 plus temperature would be beneficial for growth and productivity of kidney bean in future climates."  Hence, as they note that "previous studies on rice (Hussain et al., 1999), soybean (Vu et al., 2001) and orchids (Li et al., 2002) also showed that down-regulation of Rubisco was associated with increased SPS activity and sucrose production under elevated CO2," it would appear that their conclusion may be applicable to many other plants as well.

References
Hussain, M.W., Allen Jr., L.H. and Bowes, G.  1999.  Up-regulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature.  Photosynthesis Research 60: 199-208.

Li, C.R., Liang, Y.H. and Hew, C.S.  2002.  Response of Rubisco and sucrose-metabolizing enzymes to different CO2 in a C3 tropical epiphytic orchid Oncidum Goldiana.  Plant Science 163: 313-320.

Vu, J.C.V., Gesch, R.W., Pennanen, A.H., Allen Jr., L.H., Boote, K.J. and Bowes, G.  2001.  Soybean photosynthesis, rubisco, and carbohydrate enzyme function at supraoptimal temperatures in CO2.  Journal of Plant Physiology 158: 295-307.