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Barley plants were grown at a mean diurnal temperature of 15 °C and reciprocally transferred between different photoperiods (from 16 h d−1 to 8, 10 or 13 h d−1 or vice versa at 4, 8, 16 or 32 d after germination). Ten contrasting genotypes were examined, including seven spring-sown types-Mona, BGS T16-2, Athenais, Emir, Funza, USDA-016525 and S-37, and three autumn-sown types-Gerbel B, Arabi Abiad and Ager. In the latter two all treatments were repeated on plants grown from seeds which had been vernalized at 2 °C for 42 d. The results suggest that, between the critical photoperiod (below which there is a delay in flowering) and the ceiling photoperiod (below which there is no further delay), there is a linear relation between photoperiod and the reciprocal of the time taken to flower (awn emergence). In all genotypes the ceiling photoperiod was ≲, 10 hdd−1; the critical photoperiod was always > 13 hdd−1 often > 14 hdd−1, and sometimes > 16 hdd−1. All genotypes were initially insensitive to long days. At 15 °C this pre-inductive phase typically lasted for 8–10 d after germination in spring-sown types and in vernalized autumn-sown types, but continued for more than 32 d in non-vernalized autumn-sown types. It was followed by an inductive phase, the duration of which depended on photoperiod, being longer in shorter days. Finally, there was a photoperiod-insensitive, post-inductive phase, which probably began about 2 weeks before awn emergence. The low-temperature seed-vernalization treatment considerably hastened awn emergence in Arabi Abiad and in Ager; in Arabi Abiad low-temperature vernalization could be partly replaced by treating young plants with short days (8 or 10 h dd−1). Both low-temperature and short-day vernalization advanced flowering by advancing ear initiation (reducing the duration of the pre-inductive phase), whereas long days stimulated the rate of development following ear initiation.
Roberts et al. (Mon,) studied this question.