Photoperiod X temperature interactions in contrasting wheat genotypes: Time to heading and final leaf number

This paper describes the effects of photoperiod (9, 12, 15, 17, 19 and 21 h) and temperature (21/17 and 16/12 degrees C maximum/minimum) on time to heading in wheat. Two spring wheats (a non-segregating awned selection of Sunset, and Condor), a semi-winter wheat (Rosella), and a winter wheat (Cappelle Desprez) were used. The last three genotypes respond to vernalisation and so were exposed to 4 degrees C for 60 d before transfer to the photoperiod and temperature treatments. Increases in both photoperiod and temperature always reduced time to heading, although genotypes differed in the magnitude of the responses. However, the responses to temperature were markedly affected by photoperiod. Thus, in the three genotypes that were vernalised, thermal time to heading under short photoperiods was longer at 21/17 than at 16/12 degrees C, but under long photoperiods thermal time was shorter at 21/17 degrees C. Also, the optimum photoperiod was always longer at 21/17 degrees C than at 16/12 degrees C. Most importantly, the magnitude of the response to photoperiod was dependent on temperature, and varied among genotypes. A numerical description of wheat development to heading is proposed which includes the effects of photoperiod not only on thermal time but also on base temperature. Differential responses to very short photoperiods were evident among genotypes. Cappelle Desprez did not head when plants were grown under photoperiods shorter than 12 h (i.e. a qualitative response to photoperiod), whereas Sunset, at the other extreme, had a clear quantitative response with a progressive delay in heading as photoperiod decreased. Condor and Rosella had intermediate quantitative responses, reaching heading even under the shortest photoperiod, but responding to very short photoperiods much more dramatically than to intermediate photoperiods. This suggests that for a genotype, more than one degree of sensitivity to photoperiod is possible. The genotype X temperature interaction was responsible for changes in time to heading of more than 2 weeks under a 21-h photoperiod. Final leaf number on the main culm increased with shorting photoperiod, but was unaffected by temperature. Although time to heading was always linearly related to final leaf number, the pattern of response varied. The optimum photoperiod was much shorter for final leaf number than for time to heading, the relative sensitivity to photoperiod was less, and the change in sensitivity at very shea photoperiod was not evident in final leaf number. These results suggest that photoperiod acted at least partially independently on the timing of heading and on final leaf number.