Flowering response of rice to photoperiod and temperature: a QTL analysis using a phenological model.

In this study we have attempted to quantify the thermal and photoperiodical responses of rice (Oryza sativa L.) flowering time QTLs jointly by a 'date-of-planting' field experiment of a mapping population, and a 'phenological model' analysis that separately parameterizes the two responses, based on daily temperature, daily photoperiod and flowering date. For this purpose, the 'three-stage Beta model', which parameterizes the sensitivity to temperature (parameter alpha), the sensitivity to photoperiod (parameter beta), and earliness under optimal conditions (10 h photoperiod at 30 degrees C) (parameter G), was applied to 'Nipponbare' x 'Kasalath' backcross inbred lines that were transplanted on five dates. QTLs for the beta value were detected in the four known flowering time QTL (Hd1, Hd2, Hd6 and Hd8) regions, while QTLs for the G value were detected only in the Hd1 and Hd2 regions. This result was consistent with previous reports on near-isogenic lines (NILs) of Hd1, Hd2 and Hd6, where these loci were involved in photoperiod sensitivity, and where Hd1 and Hd2 conferred altered flowering under both 10 and 14 h photoperiods, while Hd6 action was only affected by the 14 h photoperiod. Hd8 was shown to control photoperiod sensitivity for the first time. Interestingly, Hd1 and Hd2 were associated with a QTL for the alpha value, which might support the previous hypothesis that the process of photoinduction depends on temperature. These results demonstrate that our approach can effectively quantify environmental responses of flowering time QTLs without controlled environments or NILs.