Environmental Control of Flowering in Barley (Hordeum vulgare L.). I. Photoperiod Limits to Long-day Responses, Photoperiod-insensitive Phases and Effects of Low-temperature and Short-day Vernalization

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 versaat 4, 8, 16 or 32 d after germination). Ten contrasting genotypeswere examined, including seven spring-sown types-Mona, BGS T16-2,Athenais, Emir, Funza, USDA-016525 and S-37, and three autumn-sowntypes-Gerbel B, Arabi Abiad and Ager. In the latter two alltreatments were repeated on plants grown from seeds which hadbeen vernalized at 2 °C for 42 d. The results suggest that, between the critical photoperiod (belowwhich there is a delay in flowering) and the ceiling photoperiod(below which there is no further delay), there is a linear relationbetween photoperiod and the reciprocal of the time taken toflower (awn emergence). In all genotypes the ceiling photoperiodwas