**6. Enhancing outcrossing in hybrid rice seed production**

Various natural and artificial methods are employed to increase outcrossing rate in hybrid rice seed production. Among the artificial ones are flag leaf clipping, gibberellin application and supplementary pollination [14]. Besides, there are several traits that naturally contribute to hybrid rice seed production efficiency. These include days to heading, pollen load, pollen longevity, and morphological traits of floret such as size of stigma and style, stigma exertion, stigmatic receptivity and spikelet opening angle. Stigma exertion rate is emphasized as a key factor for efficient hybrid rice seed production [14]. The extent to which the stigma is exerted in the female parent (male sterile line) increases the chances of outcrossing thereby increasing hybrid seed set. Stigma exertion is a genetic trait and not all male sterile lines possess high expression of it. It is therefore possible to enhance the trait through specific breeding efforts using appropriate donors. Both qualitative and quantitative modes of inheritance have been reported for stigma exertion. Besides several quantitative trait loci (QTL) controlling stigma exertion have been mapped to different rice chromosomes [22] with molecular markers and can easily be introgressed into other parents.
