RICE RATOONING - IRRI books - International Rice Research Institute

VARIETAL IMPROVEMENT FOR RICE RATOONING 251mentioned that ratooning ability appears to be associated with lateness of the F 2segregants. The question is how much of the genotypic variance for ratooningpercentage was caused by variance for flowering date. In a study of deepwater riceratooning (5), ratooning ability was postulated to be simply inherited (5). Thedistribution pattern of an F 2 population fit a 1:3 ratio of ratooning:no ratooningsegregates. However, ratooning ability was also strongly correlated with (late)flowering, leading to speculation that the ratooning gene is actually a floweringduration gene.Is ratooning ability a genetic trait in its own right, or a trait derived from aninteraction of genotypic and environmental factors? The data and experience appearto support the conclusion that it is a composite trait.Consequently, in genetics studies of ratooning ability, it would be advisable toidentify the trait being measured specifically as one of many traits contributing toratooning success and to measure all conceivably related traits at the same time.BREEDINGBefore a breeder can focus on ratooning ability, he must have some concept of thephysiology of the trait and be confident that it is at least partly genotypicallydetermined. A testing method for the trait must be selected. The desired expressionof the trait should not be linked to unfavorable expressions of other important traits.And sufficient breeding materials must be available.SELECTION CRITERIAThese important heritable supporting traits can be distinguished:Location of ratoonsSome varieties have a tendency to basal ratoons, others to axial upper-nodal ratoons(31). Because not much work has been done beyond this basic finding, it may be bestto express this trait as the average location of ratoons in cm or nodes from theground. Care should be taken after harvest to allow time for photosynthates totranslocate to the lower part of the stems of the main crop.Nutrient reservesThe choice is between several parameters: number of ratoon tillers per hill or perarea, weight of ratoon tillers, height of ratoon tillers, etc. Number of ratoon tillers asa percentage of main crop tillers is a reliable parameter to use (11, 17). Other traitsused could be the most heritable (19) or the ones best correlated with carbohydratereserves (20). Another is the proposed ratoon rating formula (29):(average number ratoon tillers per hill) × {l-(.0l ratoon vigor)}The degree of source or sink limitation, as a varietal trait contributing to ratooningability, could be measured as the response of main crop yield and yield componentsto source-sink manipulations.