Effects of elevated [CO2] on photosynthesis and seed yield parameters in two soybean genotypes with contrasting water use efficiency

The predicted increase in atmospheric CO2 concentration [CO2] is expected to enhance photosynthesis and seed yield in crops such as soybean [Glycine max (L.) Merr.]. However, future breeding for high water use efficiency (WUE) could interfere with the amount of carbon (C) fixed by leaves and seed mineral composition under elevated [CO2] due to lower stomatal conductance (g(s)). In the present study, two genotypes with contrasting WUE were grown in open top chambers (OTC) under ambient (410 ppm; a[CO2]) and elevated (610 ppm; e[CO2]). In order to test performance of both cultivars to changing CO(2 )conditions, growth, photosynthetic performance (leaf and canopy level) and seed mineral composition were analyzed. The low WUE genotype had a greater response to e[CO2] in terms of leaf daily photosynthetic C gain due to greater g(s) which was compensated in the high WUE genotype by an increase in leaf area (LA). However, in the low WUE genotype, improved daily photosynthetic C gain did not translate into greater biomass or seed yield [CO2] response compared to the high WUE genotype, suggesting better assimilate partitioning by the high WUE genotype. In terms of seed composition, the high WUE genotype generally had lower mineral concentrations at e[CO2] compared to a[CO2], but greater total amounts of nutrient (due to higher seed yield) under e[CO2] compared to the low WUE genotype. Findings presented here highlight importance of genetic variation in soybean response to future atmospheric [CO2] which should be considered when breeding for future climates.