Why tropical sorghum sown in winter months has delayed flowering and modified morphogenesis in spite of prevailing short days

Sowing certain late photoperiod-sensitive tropical sorghum varieties under short-day conditions in November–January in the northern hemisphere can result in a pronounced delay in flowering and prostrate, high-tillering forms similar to the rosettes in winter small-grain cereals. The cause of this phenomenon in sorghum has long been questioned and is often attributed to the effects of low night temperatures during this period. Monthly sowings, from December to March, in greenhouse compartments with two contrasting night temperatures and under field conditions with contrasting soil conditions were conducted over two years near Bamako, Mali, with three contrasting sorghum varieties. Dates of panicle initiation, rates of leaf primordia initiation at the stem apex, and rates of leaf appearance were observed. In the greenhouse, colder night temperatures were found to have no effect on prolonging vegetative-phase duration when measured in thermal time and on plant development. Although a prolonged vegetative phase and prostrate forms were observed under field conditions, these were not observed in the greenhouse experiments despite having similar temperature conditions. Although the higher soil fertility of the potting soil increased the development rates slightly, and reduced the vegetative phase relative to natural soil conditions in the field, this difference between greenhouse and field growth remained when a comparison was made between plants grown in pots. Thus, it appears that the greenhouse glass had masked the UV component of sunlight, which would mediate the cue responsible for the delayed flowering and modified morphogenesis. This cue, which has strongly contrasting effects between October and November sowings, could be the daily changes in the sunrise and sunset hour that have negative values from November to July.

► In the greenhouse, cold night temperature did not lengthen the vegetative phase. ► The greenhouse glass masked UV radiation that caused flowering delays in fields. ► Flowering of the photoperiod-sensitive variety was delayed by daily earlier sunrises and sunsets. ► Sunrise and sunset time variations must be further evaluated as a co-factor with daylength.