Effects of photoperiod, temperature and asynchrony between thermoperiod and photoperiod on development to panicle initiation in sorghum

The duration of the vegetative phase (i.e. days from sowing to panicle initiation) in sorghum [Sorghum bicolor (L.) Moench] is affected by photoperiod and temperature. Plants of several contrasting genotypes of sorghum were grown in controlled-environment growth cabinets with either synchronous or asynchronous photoperiods and thermoperiods. Apical development was recorded. Diurnal asynchrony between photoperiod and thermoperiod reduced durations to panicle initiation when the temperature warmed after lights went on and cooled after lights went off, but increased these durations when the temperature warmed before lights went on and cooled before lights went off. These effects were shown in the maturity lines 60M and SM100 and also in the USA cv. RS610 and the Sudanese landrace IS22365, but their magnitude varied with genotype, photothermal regime, and the degree of asynchrony. The greatest effect was detected in IS22365 grown at 30/21 degrees C (12 h/12 h) with a 12 h d(-1) photoperiod when the temperature warmed 2.5 h before lights went on and cooled 2.5 h before lights went off, when the duration from sowing to panicle initiation was 69 d compared with 37 d in the control (synchronous photoperiod and thermoperiod in each diurnal cycle). Reciprocal transfers of plants of IS22365 between short and long days revealed that asynchrony principally affected the duration of the photoperiod-insensitive pre-inductive phase of development; i.e. asynchrony affected the time (age) at which the plants were first able to respond to photoperiod. In that investigation in controlled-environment growth chambers, the subsequent photoperiod sensitive inductive phase continued until panicle initiation. Subsequent reciprocal transfer experiments in controlled-environment glasshouses in four different alternating temperature regimes employed synchronous photoperiods and thermoperiods in short (11 h) days with temperature warning 1.5 h after the beginning of the day in long (12.5 h) days. In those investigations, photoperiod sensitivity ended some time before (2.5-8.1 d, mean 5.7 d) panicle initiation in IS22365, Naga White and Seredo. Moreover, whereas the duration of the photoperiod-insensitive pre-inductive phase was affected by temperature, the durations of the photoperiodsensitive inductive and the photoperiod-insensitive post-inductive phases were not. (C) 1997 Annals of Botany Company.