Contrasting responses of seed yield to elevated carbon dioxide under field conditions within Phaseolus vulgaris

The rising concentration of carbon dioxide [CO2] in the atmosphere represents an increase in a growth-limiting resource for C3 crop species. Identification of lines or characteristics of lines which have superior yield at elevated [CO2] could aid in adaptation to this global change. While intraspecific variation in responses to elevated [CO2] has been found in several species, intraspecific differences in crop yield responses to elevated [CO2] under field conditions have seldom been documented. In this 4-year study, the responses of photosynthesis, growth, pod number, seed number and size, and seed yield to the elevation of [CO2] to 180 μmol mol−1 above the current ambient concentration were examined in four varieties of Phaseolus vulgaris in the field, using open-top chambers. There was a significant variety by [CO2] interaction for seed yield, with seed yield at elevated [CO2] ranging from 0.89 to 1.39 times that at ambient [CO2] (mean 1.17×) in the different varieties, when averaged over 4 years. The highest yielding variety at elevated [CO2] was not the highest yielding variety at ambient [CO2]. The varieties with the largest and smallest yield responses both had an indeterminate growth habit. Down-regulation of photosynthesis at elevated [CO2] only occurred in the two indeterminate varieties, and there was no significant correlation between the response of single leaf photosynthetic rate and the response of seed yield to elevated [CO2] among varieties, nor between the responses of stem mass and seed yield. The change in the number of pods at elevated [CO2] was the primary determinant of the response of seed yield. These results indicate that significant variation in the response of seed yield to elevated [CO2] under field conditions does exist among varieties of P. vulgaris, and that variation in the response of pod and seed number may be more important than variation in photosynthetic response.