Photoperiod sensitivity of Ppd-H1 and ppd-H1 isogenic lines of a spring barley cultivar. Exploring extreme photoperiods.

Barley is a long-day plant with a major gene (PPD-H1) that determines its sensitivity. Under long days (i.e. 16 h), flowering occurs earlier in sensitive (Ppd-H1) than in insensitive genotypes (ppd-H1), while under short days (i.e. 12 h) both flower late and more or less simultaneously. We hypothesised that (i) the sensitive line should flower later than the insensitive line under very short days (< 12 h), and (ii) both the sensitive and insensitive lines should have similar phenology under very long days (> 18 h). When comparing a pair of spring isogenic lines for sensitive and insensitive PPD-H1 alleles (into the barley cultivar "WI4441"), we found responses fully in line with expectations for the commonly explored range from 12 to 16-18 h. When the responses were extended to very short days sensitivity increased noticeably, and time to flowering of the sensitive line was longer than the insensitive one. Under very long days, the sensitive line did not respond further (it seemed to have reached its minimum time to flowering under a 16 h), while the insensitive line continued shortening its time to flowering until c. 21 h. Consequently, both lines flowered similarly under very long days, which opens opportunities to easily test for differences in earliness per se, as in wheat.