Consequence and further analysis of the minor role of daylength in the floral initiation of sorghum crops

Background and Aims Flowering time is assumed to be primarily controlled by daylength, but field observations disagree. For instance, mid-early sorghum varieties sown at the same time at tropical and temperate latitudes have similar flowering dates. We previously proposed an alternative framework integrating daily changes in sunrise and sunset times (dSR and dSS). Methods Variety trials of mid-late varieties from the Northern Hemisphere were carried out in Madagascar and in Argentina. Series of monthly sowings of 28 varieties were carried out in Mali between 2000 and 2009. Heading date and, in Mali, panicle initiation (PI) were recorded. Key Results In Madagascar and in Argentina mid-late varieties headed late or very late when sown in November and December, consistent with the higher dSR/dSS values of the southern hemisphere summer. In Mali, the high interannual variability in PI time in sowings between November and February revealed that up to four photoperiodic responses - two quantitative and two qualitative - may coexist within a single late photoperiod sensitive variety. Additionally, the qualitative responses may be triggered by an internal dSR/dSS phasic coincidence, which is set at the onset of the photoperiod-sensitive phase. These hypotheses were used to design a research model that incorporated only dSR and dSS cues. The model fitted the dataset well when two sets of parameters were calibrated for the first quantitative response, which covered two periods of over and under twelve hours per year. After daylength was added to the first qualitative response, the model also accurately predicted the duration to PI observed in three varieties sown at tropical and temperate latitudes. Conclusions The flowering time of sorghum crops mainly relies on dSR and dSS cues, while daylength plays a minor role. Two new hypotheses now require confirmation through further evidence, which is likely to come from genetic analyses.