Photoperiod Requirements for Flowering and Flower Production in Soybean

Abstract Photoperiod has been recognized as playing a major role in soybean [ Glycine max (L.) Merr.] growth and development. The objective of these studies was to determine the number of short days (13.5 h) representative of early spring daylengths in the southeastern United States necessary to cause anthesis and adequate flower production in soybean. Growth chamber studies were conducted at the Phytotron at North Carolina State University, during the first 26 wk of 1985. The soybean cultivar Tracy‐M and the delayed flowering genotype D77‐12480 were subjected to treatments in which they were transferred either from short‐day (13.5‐h) to long‐day (16‐h) or long‐day to short‐day treatments at different times of their development. The number of days to first flower and the rate of flower production were taken. Results indicated that Tracy‐M and D77‐12480 appeared to become responsive to short‐day induction at 8 to 12 and 8 to 16 d after emergence, respectively. While flowering occurred in Tracy‐M with as few as eight short days, optimal flower production required short‐day treatment up to and past first flowering. In D77‐ 12480, application of long days before first flowering inhibited any flower production. As in Tracy‐M, short days were required throughout the flowering period for maximum flower production. Results indicate that under field conditions, diminished flower production can be expected if long days occur before or during the flowering period, or both. Short‐day treatment sufficient to cause anthesis was not necessarily adequate for ample flower production.