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 p hotoperiod and temperature. Plants of several contrasting genotypes of sorghum were grown in controlled-environment growth cabinets with eit her synchronous or asynchronous photoperiods and thermoperiods. Apical development was recorded. Diurnal asynchrony between photoperiod and thermoperiod reduced durations to panicle initiation when the temperat ure warmed after lights went on and cooled after lights went off, but increased these durations when the temperature warmed before lights we nt on and cooled before lights went off. These effects were shown in t he maturity lines 60M and SM100 and also in the USA cv. RS610 and the Sudanese landrace IS22365, but their magnitude varied with genotype, p hotothermal 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 w ent 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 contr ol (synchronous photoperiod and thermoperiod in each diurnal cycle). R eciprocal transfers of plants of IS22365 between short and long days r evealed that asynchrony principally affected the duration of the photo period-insensitive pre-inductive phase of development; i.e. asynchrony affected the time (age) at which the plants were first able to respon d to photoperiod. In that investigation in controlled-environment grow th chambers, the subsequent photoperiod sensitive inductive phase cont inued until panicle initiation. Subsequent reciprocal transfer experim ents in controlled-environment glasshouses in four different alternati ng temperature regimes employed synchronous photoperiods and thermoper iods in short (11 h) days with temperature warning 1.5 h after the beg inning of the day in long (12.5 h) days. In those investigations, phot operiod sensitivity ended some time before (2.5-8.1 d, mean 5.7 d) pan icle initiation in IS22365, Naga White and Seredo. Moreover, whereas t he duration of the photoperiod-insensitive pre-inductive phase was aff ected by temperature, the durations of the photoperiodsensitive induct ive and the photoperiod-insensitive post-inductive phases were not. (C ) 1997 Annals of Botany Company.