Photoperiod and Temperature Regulation of Floral Initiation and Anthesis in Soya Bean

Floral development includes initiation of floral primordia and subsequent anthesis as discrete events, even though in many investigations only anthesis is considered. For ‘Ransom’ soya bean [Glycine max (L.) Merrill] grown at day/night temperatures of 18/14, 22/18, 26/22, 30/26, and 34/30 °C and exposed to photoperiods of 10, 12, 14, 15, and 16 h, time of anthesis ranged from less than 21 days after exposure at the shorter photoperiods and warmer temperatures to more than 60 days at longer photoperiods and cooler temperatures. For all temperature regimes, however, floral primordia were initiated under shorter photopenods within 3 to 5 days after exposure and after not more than 7 to 10 days exposure to longer photoperiods. Once initiation had begun, time required for differentiation of individual floral primordia and the duration of leaf initiation at shoot apices increased with increasing length of photoperiod. While production of nodes ceased abruptly under photoperiods of 10 and 12 h, new nodes continued to be formed concurrently with initiation of axillary floral primordia under photoperiods of 14, 15 and 16 h. The vegetative condition at the main stem shoot apex was prolonged under the three longer photoperiods and is suggestive of the existence of an intermediate apex under these conditions. The results indicate that initiation and anthesis are controlled independently rather than collectively by photoperiod, and that floral initiation consists of two independent steps—one for the first-initiated flower in an axil of a main stem leaf and a second for transformation of the terminal shoot apex from the vegetative to reproductive condition.