DURATIONS OF THE PHOTOPERIOD-SENSITIVE AND PHOTOPERIOD-INSENSITIVE PHASES OF TIME TO PANICLE INITIATION IN SORGHUM

The development of sorghum [Sorghum bicolor (L.) Moench] is influenced by genes that control sensitivity to photoperiod, and their interacti on with photoperiod and temperature. While temperature influences deve lopment throughout the Life cycle of plants, photoperiod influences th e vegetative stage (from seedling emergence to panicle initiation). In order to simulate plant development, it is essential to know when sor ghum plants first become sensitive to photoperiod, and how long that p hotoperiod sensitivity persists. Ten cultivars with different levels o f photoperiod sensitivity were grown in pots under natural climatic co nditions both in short days (SD: 8 h day(-1)) and long days (LD: 17 h d(-1)). Plants were transferred at different times after seedling emer gence from SD to LD and vice versa. The time to panicle initiation (PI ) for each transfer treatment was determined. In cultivars that remain ed continuously in SD, the time to PI varied from 16 to 27 d, whereas, in continuous LD it varied from 22 to 37 d. The cultivars started rea cting to photoperiod 4-9 d after seedling emergence, After sensing pho toperiod stimuli, inductive effects among cultivars persisted for 4-14 d in SD, and for 15-33 d in LD depending on their intrinsic photoperi od sensitivity. The sensitivity ended 2-5 d before panicle initiation. This interval, between completion of the photoperiod-inductive phase and the actual observation of PI under the microscope, represents the time required for the photoperiod-inductive stimulus to promote suffic ient cell division and growth at the shoot apex for the morphological change to become visible as a shiny globular structure. We conclude th at photoperiod sensitivity in these sorghum cultivars ends shortly bef ore or at the PI stage. Our results support the assumptions followed i n several crop simulation models that sorghum remains photoperiod-sens itive until the completion of the vegetative stage. (C) 1998 Elsevier Science B.V.