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Photoperiodically Sensitive Interval in Time to Flower of Soybean

Dep. of Crop Science, Box 7620, North Carolina State Univ., Raleigh, NC 27695
Dep. of Agric. Engineering
Dep. of Agronomy, Univ. of Florida, Gainesville, FL 32611

^* Corresponding author.

In order to accurately model the flowering process, it is essential to determine which environmental factors are affecting development at any point in time. The purpose of this research was to determine when different soybean [Glycine max (L.) Merr.) genotypes first become sensitive to photoperiod and how long photoperiod continues to influence the appearance of the first flower under optimal temperature conditions. Controlled-environment experiments were conducted in which plants were switched at intervals between emergence and flowering from long-day (22 h) to short-day (9 h) treatments and from short- to long-day treatments. AH photoperiod chambers were maintained at 26 °C both day and night. Six cultivars from six maturity groups were used: ‘Dawson’, ‘Williams’, ‘Ransom’, ‘Forrest’, ‘Davis’, and ‘Jupiter’. Two nearly isogenic breeding lines differing in response to inductive conditions were also evaluated. All cultivars tested were sensitive to photoperiod by the time the unifoliolate leaf was fully expanded. The experimental line bred for a lengthened juvenile phase exhibited apparent insensitivity to photoperiod for 11 d longer than any of the other genotypes. After flowering was induced, further inductive nights hastened flowering. for all cultivars, photoperiod during the last 6.3 to 8.7 d prior to expression of the first flower had no effect on time to first flower. Thus, for modeling purposes, the interval between emergence and first flower can be divided into four phases: (1) a purely vegetative phase (absent in most of the cultivars tested); (2) a photoperiod-sensitive inductive phase; (3) a photoperiod-sensitive post-inductive phase; and (4) a photoperiod-insensitive post-inductive phase.

Received for publication November 27, 1987.

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