THE CHARACTERIZATION OF A DAYLENGTH-NEUTRAL TRAIT IN SOYBEANS (GLYCINE MAX (L.) MERRILL)

Abstract

In effort to breed for daylength-neutral (DNP) soybean germplasms (Glycine max (L.) Merrill), selected longday cultivars (LDP) from Maturity Group 00 were crossed to local adapted shortday cultivars (DP) from Maturity Group VI. In the segregating populations there were many new hybrid combinations, some of which flowered and set pod as early as the Group 00 parents, however, they were larger in size and matured normally. These lines were considered daylength-neutral plants (DNP), and were evaluated in the field nursery in biweekly date of planting experiments from early May to late July during 1978 and 1979. Selected LDP and local adapted SDP cultivars were utilized as check lines. It was observed that LDP cultivars flowered and set pods normally. However, they did not mature normally; the pods ripened and shattered while the stem and leaves remained green and these plants were short and unproductive. Local SDP were the most sensitive plants in response to the change in planting date and daylength. Number of days to flowering, pod setting, and maturity as well as plant heights had decreased sharply in response to the decrease in daylength of later planting dates. In contrast, DNP lines flowered, set pod and matured normally on large vigorous plants in approximately the same period of time regardless of planting date or the daylength during the growing season. To study the inheritance of the daylength-neutral trait in soybeans, crosses were made between DNP lines and local SDP cultivars. These were extremely wide crosses. Segregating populations from these crosses were tested under three light treatments of 12, 18 and 24 hours. Only DNP plants flowered and set pod normally under the long photoperiod treatments of 18 and 24 hours. The magnitude and continuous nature of the frequency distribution of the segregating populations as well as the low heritability estimates of each trait imply that this response is under polygenic control.