RICE RATOONING - IRRI books - International Rice Research Institute

178 RICE RATOONINGBREEDING POTENTIALSRatooning ability and related charactersSeveral reports of varietal differences in ratooning ability have been exhaustivelyreviewed (4). Samson (27) suggested a rating scale for ratoon vigor. Differentvarieties produce ratoon tillers of different kinds (30). Genotypes differ inproportion of basal and nodal tillers (1, 6, 32); high basal tillers were found desirable.Late leaf senescence of the main crop is considered favorable for the ratoon crop (4).Main crop root vigor and stem thickness (9) generate better regrowth.Therefore more basal tillers with uniform flowering and late leaf senescenceand stem and root characters can be considered as important breeding objectives.Reports of successful ratoon crops from early-maturing (3), medium-duration(35), and late-maturing (11, 24, 25) varieties suggest that breeding for ratooningability should emphasize the length of maturity needed for specific growingconditions. Breeding lines of less than 100 d growth duration have been reported topossess high ratooning ability (4), but other reports contradict this.Mahadevappa observed about 700 genotypes at IRRI and many more atKarnataka experimental stations. He found an apparent correlation between ratoongrowth period and ratoon yield (10). Ratooning ability, as a genetically controlledcharacter, can be incorporated into any maturity group. Biometrical studies of arange of material could confirm and elaborate this observation.Studies on the genetics of ratooning ability are limited and contradictory (2, 5,29). The total inability of some genotypes to initiate regrowth, even under the mostfavorable conditions; the ability of certain other genotypes to ratoon vigorously,despite adverse environments; and the existence of moderate ratooning typesindicate that ratooning ability is influenced by a number of genes. Hybrids in generalseem to have higher ratooning ability than the parents, revealing the heterotic natureof the genes governing ratooning ability inheritance.Resistance to biotic and abiotic stressesOne reason for the nonadoption of rice ratoon cropping systems, despite manyattempts, is the lack of recognition of the importance of resistance, especially to thesystemic diseases (viruses, microplasma-like organisms) and their vectors found inmost rice growing countries. Ratoon rice is more likely to be damaged by virus andMLOs since a main crop infection is also manifested in the ratoon crop. Many of thegood ratooning genotypes selected in the International Rice Cold Tolerance Nurseryat Karnataka and in other locally constituted trials at Bangalore had to be rejectedbecause of their susceptibility to viruses. Of 338 genotypes, only 86 were unaffected,including B2983-b-853-2-4, IR15579-135-3, IR18476-55-2, IR19743-25-2-2, IR9202-25-1-3, and NR10073-167-2-1-1. These are under further evaluation. The varietiesIntan, being adapted for ratooning in the hill region of Karnataka, and Mingolo,now under prerelease farm trial, have been showing high resistance to viruses andMLOs. Breeding even more good ratooning and disease-resistant genotypes isnecessary. It is risky to depend on only one or two varieties which may breakresistance, as happened for blast resistance in Intan in the same region.