Expression of stylar incompatibility in the Andean clonal tuber crop oca (Oxalis tuberosa Mol., Oxalidaceae)

Long-, mid-, and short-styled clonal accessions of oca (Oxalis tuberosa) we re intercrossed in a complete diallelic design. Pollen tube growth in style s was monitored in all diallelic crosses. Pollen fertility was estimated by two tests: staining of pollen grains with aceto-carmine and detection of b eta-galactosidase activity by the substrate X-Gal. The two methods of polle n fertility estimation were equally useful to detect fertility levels. Poll en originating from short stamens had the highest fertility (85%) and polle n from long stamens had the lowest fertility (70%). Pollen fertility was hi gh throughout, but its degree varied with the stylar morph on which the pol len was formed. Long-styled accessions had the highest rates of fertile pol len. Differences in pollen fertility at different anther levels in the same style morph were also apparent. Pollen grain diameter of the six morph-ant her level combinations was inversely correlated with pollen fertility. Poll en grains from long stamens were the largest and pollen grains from short s tamens were the smallest. Neither pollen fertility nor pollen grain size ha d an influence on pollen tube growth in the style or on fruit and seed set. Pollen tubes growing within the styles were inhibited at a different level for each of the Is cross combinations in the dialler. Although legitimate crosses had greatest pollen tube growth, some of the illegitimate inter- an d intramorph crosses had equally high scores. Of all illegitimate crosses, mid-styled seed parents had the lowest level of stylar incompatibility. Fru it and seed set were highly correlated with the extent of pollen tube growt h in the style. The number of pollen tubes entering ovules in a flower was in good agreement with the number of seeds produced per fruit. Therefore, i t is concluded that stylar incompatibility is the major determinant of limi ted seed formation in oca even in the most successful legitimate cross comb inations.