Variation in Photoperiod Sensitivity for Time of Flowering and Maturity Among Soybean Strains of Maturity Group III1

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Abstract
In field screenings at Urbana, Illinois, in 1973 and in Puerto Rico in 1974‐75, soybean [Glycine max (L.) Merr.] strains of Maturity Group III exhibited a wide range in sensitivity to photoperiod. Several hundred strains were grown under natural day length, and the same strains were grown nearby under a photoperiod extended with incandescent light to either continuous light or with a 5‐hour interruption of the night. All strains grown under the extended photoperiod eventually flowered, but they showed a wide range in delay compared with those under natural day length. The frequency distribution of strains with respect to the length of delay approached a normal curve. At Urbana, extended photoperiod delayed flowering from ‐2 to 60 days. Of 515 strains, 25 were delayed less than 15 days and 5 were delayed more than 55 days. By first frost, 172 days after planting, 24 lines had matured under extended photoperiod. In Puerto Rico, flowering was delayed from 3 to 85 days by a continuous photoperiod. One strain was delayed less that 10 days and 3 were delayed more than 80 days. When the experiment was terminated at 235 days, 285 of the 439 strains were mature and 7 had matured within 170 days. Thirty‐two strains were identified as having low photoperiod sensitivity, and one of these, PI 317.334B (‘Kitami Shiro’), exhibited almost no photoperiod sensitivity at either location.

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