Variation of the vascular bundle system in Asian wild rice, Oryza rufipogon and its relationship to rachis branch number and growth habit

Accessions of Asian wild rice, Oryza rufipogonGriff., vary in phenology, growth habit, reproductivesystem, panicle architecture and rachis branchnumbers, and in habitat preferences. In this paper 86accessions of annual, perennial and intermediategrowth habit variants have been examined for variationin the numbers of rachis branches in the panicle andvascular bundles in their subtending peduncles.Accessions of annual habit, which regenerate from seedand are adapted to shallow and temporary swamps,developed fewer rachis branches (mean = 6.0) thanthose of perennial habit (mean = 7.2) which largelyregenerate vegetatively and are adapted to stable deepwater habits. In both cases variation within growthhabit groupings was narrow. Variation in vascularbundle numbers, which has not been previouslyreported, was similar (10.1 to 10.3), but morevariable within annuals. As a result the V/R ratio (ofvascular bundles: rachis branches) was higher inannuals (mean = 1.71) than among perennials (mean =1.46). Accessions of annual habit, and adaptedto a wide range of habitats, varied considerably inboth rachis branch (mean = 9.5) and vascular bundlenumbers (mean = 14.0), with V/R ratios similar tothose of perennial growth habit (mean = 1.49).Corresponding measures for both indica and japonica of cultivated rice (O. sativa) variednarrowly and were substantially greater for bothrachis branches (mean = 11.6 and 13.8, respectively)and vascular bundles (mean = 19.1 and 14.8,respectively), with V/R ratios of 1.67 for indica and similar to accessions of O.rufipogon of annual habit, and 1.07 for japonica and lower than accessions of O.rufipogon of both perennial and intermediate habit.Accessions of O. rufipogon from the India andIndochina regions were significantly lower in rachisbranch, but not vascular bundle numbers thanaccessions from China; with the V/R ratio higher amongaccessions from India than found in other geographicregions of origin. The possible role of O.rufipogon accessions of intermediate growth habit inthe evolution of cultivated rice is discussed,although it is speculated that accessions ofintermediate habit with high numbers of rachisbranches and vascular bundles may have resulted frominterspecific hybridization with O. sativacultivars.