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 growthlimiting 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 mmol 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. Published by Elsevier B.V.