Daylength and Sowing Date Responses of Soybean Lines with “Juvenile” Trait¹

Abstract

Daylength limits adaptation of soybean [ Glycine max (L.) Merr.] genotypes to a narrow range of latitudes and sowing dates by affecting vegetative and reproductive growth and development. Photoperiod and sowing‐date experiments were conducted in Gainesville, FL (29.38°N) during 1984 and 1985 to assess the vegetative and reproductive growth of soybean genotypes previously observed to differ in sensitivity to change in latitude and sowing date. ‘Kirby’ and ‘Improved Pelican’ and two breeding lines, F82‐7656 and F84‐1220, selected for adequate vegetative development under short days (“juvenile” trait), were used. Photoperiods included 14‐h, 18‐h, and natural‐winter daylengths between 22 Jan. and 30 Mar. 1984 in a greenhouse experiment, and 10‐h (covered 14 h) and natural‐summer days in 1985 in a field experiment (Jonesville Taxajunct Soil classified as loamy, mixed, thermic Arenic Hapludalf). Sowing dates in the 1984 and 1985 field experiments were monthly from 1 April through 1 August. Improved Pelican, Kirby, F82‐7656, and F84‐1220 ranged from highly to only moderately responsive to the photoperiod and sowing date treatments. In 1984, under natural‐winter days and 14 h, all plants averaged the R1 stage at 30 and 60 days after planting (DAP), respectively. Plants under 18 h were vegetative at 68 DAP, the termination date. Under 10 h in the 1985 field experiment, Kirby, F82‐7656, Improved Pelican, and F84‐1220 were in the R1 stage in 48, 52, 54, and 57 DAP, respectively. Plant height, node number, and DAP to R1 for F84‐1220 were most constant over the range of photoperiods and sowing dates, indicating efficacy of the juvenile trait in extending adaptability. Improved Pelican in the April and May sowings grew excessively tall (2 m) and aborted its flowers and pods, which indicated not only pre‐, but also postflowering daylength effects. Kirby and F82‐7656 at the August sowing date flowered too early, resulting in severe reduction of vegetative growth and pod number. In the 1 April and 1 August sowings and the 10‐h photoperiod, F84‐1220 was near optimum height and produced more branches, racemes, and pods than the other genotypes. We conclude that the juvenile trait, as demonstrated in line F84‐1220, can improve climatic adaptation of soybean cultivars by buffering photoperiod effects incident to suboptimal latitudes and/or sowing dates.