Changes in the phenology of flowering in soybeans caused by long-term growth at elevated CO 2 may be important to the responses of seed yield to elevated CO 2 . Here we utilized near-isogenic lines of soybeans differing in three genes influencing photoperiod sensitivity to determine whether these genes affected the response of flowering time to elevated CO 2 . Six isolines of Harosoy 63 were grown at ambient (380 μmol•mol −1 ) and elevated (560 μmol•mol −1 ) CO 2 concentrations in the field

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... g free-air CO 2 enrichment systems, in air-conditioned glasshouses with natural summer photoperiods, and in indoor chambers with day lengths of 11, 13, 15, and 17 hours. The effect of CO 2 concentration on flowering time varied with genotype, and there was also an interaction between CO 2 and photoperiod in all genotypes, as indicated by ANOVA. Elevated CO 2 accelerated flowering in some cases, and delayed it in other cases. For all three of the isolines with single dominant genes, elevated CO 2 decreased the days to first open flower at the longest photoperiod. At the shortest photoperiod, elevated CO 2 delayed flowering in all but one isoline. The all-recessive isoline had slower flowering at elevated CO 2 at both the shortest and the longest photoperiods, and also in the field and in the glasshouse. Delayed flowering at elevated CO 2 in the field and glasshouse was associated with an increased final number of main stem nodes. It is concluded that the E1, E3, and E4 genes each influenced how the time to first flowering was affected by CO 2 concentration at long photoperiods.