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Interactive Effects of low CO2 and Temperature on Bean
Cowling, S.A. and Sage, R.F. 1998. Interactive effects of low atmospheric CO2 and elevated temperature on growth, photosynthesis and respiration in Phaseolus vulgaris. Plant, Cell and Environment 21: 427-435.

What was done
The authors grew young bean plants for 16 days in growth chambers that received ambient (383 ppm) or sub-ambient (196 ppm) atmospheric CO2 concentrations and moderate (25C) or high (36C) air temperature treatments to investigate their interactive effects on photosynthesis and growth.

What was learned
The authors reported a strong interaction between growth CO2 concentration and air temperature. At moderate temperature, net photosynthesis was 58% greater at ambient CO2 than it was at reduced CO2, and at high temperature, this effect was even greater, with plants grown at ambient CO2 exhibiting photosynthetic rates that were 73% greater than those observed in plants grown with only half as much CO2. These observations are in agreement with other data sets, which show that elevated CO2 concentrations have greater percentage effects on photosynthesis at higher, rather than lower, air temperatures. In fact, the authors showed that the optimal temperature for photosynthesis was about 5C higher in plants grown at ambient CO2 than in plants grown at sub-ambient CO2. Increased photosynthetic rates at ambient CO2 concentration led to the creation of a greater carbon supply for supporting growth and development, as indicated by whole-plant biomass data that showed the plants grown at high CO2 to have approximately 59% and 200% more biomass than plants grown at low CO2 concentration at moderate and high air temperatures, respectively.

What it means
The rising CO2 content of the air from pre-industrial levels to today's concentration has been responsible for significant increases in bean productivity. Furthermore, as the CO2 content of the air rises even higher, bean plants will probably experience still greater productivity increases, in an extension of this historic trend. In addition, if there is any further global warming, higher levels of atmospheric CO2 should allow the optimum temperature for photosynthesis and growth in bean plants to keep up with the increase in air temperature, so there would be no need to shift their current production zone to higher latitudes under such circumstances, although that option would certainly become available in a high-CO2 world of the future.

Reviewed 1 November 1998