Effect of Temperature During Early Growth Period on the Time of Double Ridge Stage and Terminal Spikelet Stage in Three Wheat Cultivars with Different Degree of Winter Habit

The effects of temperature during the early growth period in Yamaguchi Prefecture, influenced on the time of two spike differentiation stages, double ridge stage and terminal spikelet stage were examined from 1998 to 2011 using three wheat cultivars, Norin 61, Iwainodaichi and Akitakko whose degree of winter habit were II, IV and V, respectively. The seeds were sown in early October (early sowing) and in November to early December (conventional sowing). The period from sowing to double ridge stage and that to the terminal spikelet stage were shorter in early sowing than in conventional sowing in Norin 61, longer in Akitakko, and nearly the same in both early and conventional sowing in Iwainodaichi. The cumulative temperature was greater in early sowing than in conventional sowing in all cultivars, but the difference was small in Norin 61 and large in Akitakko. In spring wheat such as Norin 61, growth in the early stage was promoted by early sowing due to the higher temperature in the early growth stage resulting in reduced days to spike differentiation. However, in winter wheat such as Akitakko, spike differentiation was not completed (terminal spikelet did not differentiate) before March in early sowing, because a longer period was required for vernalization. In moderate winter wheat such as Iwainodaichi, the plants were vernalized before the severe cold season in early sowing resulting in earlier terminal spikelet stage than in conventional sowing.