Further Analysis of Geographical Variation of Heading Response to Daylength in Foxtail Millet (Setaria italica P. BEAUV.)

Abstract

Using materials differing in heading response from the previous study (TAKEI and SAKAMOTO 1987), the variations in the effects of daylength and temperature on floral initiation and panicle development in foxtall millet (Setaria italica P. BEAUV.) were investigated. Type I strains, those from Europe and Afghanistan, initiate the floral buds without short-day stimulus. In these strains, floral initiation rather depends on the degree of vegeta-tive growih, although panicle development is accelerated by short-day and high temperature conditions. The number of days to heading depends consequently on temperature conditions. Strains from Okinawa, Batan Is., and Lan-Yu Is. which showed sensitivity to daylength, did not necessarily need short-day stimulus for panicle initiation, but panicle development was accelerated by short day. Type II strains, those from low latitudinal areas have a long basic vegetative growth period and the duration of this period is reduced by high temperature conditions. In addition, these strains need many cycles of short-day to induce the floral buds. The requirement of short-day cycles for floral initiation and morphological development is reduced by aging. The heading of the strain from Halmahera Is. (2°N), the representative of Type II, is accelerated by 10 hrs. of light, but floral initiation does not necessarily take place as a response to the subtle change of daylength of this low latitude locality. The strain from Thailand is sensitive to a wide range of daylength. It should be identified as the fourth type, Type IV, characterized by a long basic vegetative growth period and high sensitivity to daylength. The strains from the Temperate Zone are sensitive to changes of daylength. The strain 403, from Kochi Prefecture, which is a representative of Type III and is an obligate short-day plant, shows a striking response to daylength only one week after sowing, and only a few cycles of short-day stimulus are enough for floral inductlon.