Correlation of cold-weather tolerance with pubescence color and flowering time in yellow hilum soybeans in Hokkaido

Brown-hilum soybeans often have cold-weather tolerance and growth vigor superior to that of yellow hilum soybeans. However, Japanese consumers do not prefer brown-hilum soybeans. In this paper, we investigated the correlation of cold-weather tolerance with pubescence color and flowering time using near-isogenic lines (NILs). The gene for the former is one of the genes, which controls hilum color and that for the latter is related to growth vigor. First, the NILs for pubescence color (tawny: T and gray: t) of yellow-hilum soybeans were grown under various temperature conditions, and their growth and seed yield were examined. There was no significant difference in the seed yield between tawny and gray lines in a warm region for eastern Hokkaido, but the seed yield in tawny lines was significantly higher than that in gray lines in cooler regions. And dry matter production at the flowering stage was also higher in tawny lines than in gray lines in cooler regions. Next, the effects of cooling treatment on the pod number and seed yield of the NILs for a combination of pubescence color (T/t) and flowering time (E1/e1), te, tE, Te and TE, were examined. The cooling treatment [four weeks, at 18degreesC/13degreesC (day/night)] from the beginning of flowering slightly reduced the pod number and seed yield in the tawny lines (Te, TE) and in the late-flowering gray line (tE), but greatly reduced them in the early flowering gray line (te). Then, gray pubescence lines were grouped into early, mid and late flowering groups from two crosses, and they were exposed to low temperature mentioned above. The rates of reduction in the pod number and seed yield by the cooling treatment were highest (cold-weather tolerance was lowest) in the earlier flowering group and lowest (cold-weather tolerance was highest) in the later flowering group. These results show that selecting yellow-hilum soybeans with tawny pubescence and with later flowering time are both useful for developing yellow-hilum soybeans with cold-weather tolerance.