Morphogenetic Competence and Somaclonal Variation in Oca (Oxalis Tuberosa M.)

Abstract

Shoot regeneration has been achieved by culturing petiole and internode explants of oca in several nutrient media and hormone combinations. A modified B5 medium supplemented with 3 mgl-1 each of zeatin and naphthaleneacetic acid (regeneration medium) was the most effective one for obtaining greater numbers of single shoots per explant. Induction of numerous buds leading to shoots was possible by culturing explants on modified B5 medium supplemented with 3 mgl-1 benzyladenine and 4 mgl-1 naphthaleneacetic acid (rescue and regeneration (RR) medium) Deep-red smooth-surfaced compact callus was morphogenic while white and loose callus with filamentous surface cells was non-morphogenic Cells of morphogenic callus were highly cytoplasmic, less vacuolated and had higher number of starch grains whereas non-morphogenic cells were highly vacuolated and had few starch grains The morphogenic capacity of callus was short-lived when cultured in the regeneration medium whereas the morphogenic capacity could be retained for an indefinite period (more than 2 years) when cultured on the RR medium. This allowed the production of as many plants as required from a single morphogenic callus. When cultured on the regeneration medium morphogenic calli gave rise to roots with a swollen base. Meristemoids developed inside the swollen root bases led to the development of shoots. Multiple shoots were produced from morphogenic callus through meristemoid development, when cultured in the RR medium, without formation of any swollen roots. Shoot regeneration ability using these cultures varied with genotype as well as with the source of explant used. Weak growing genotypes gave more shoot regeneration than the vigorous growing genotypes. Sections from the 4th to 6th internodes (counted from the apical fully observable internode) were more regenerative than sections from the other internodes. In addition sections of the petiole sheath gave higher regeneration than sections of the petiole stalk. Leaf as well as tuber explants failed to regenerate any shoots in these media. Regenerated shoots could easily be rooted by growing them on basal MS or MB5 medium. Gamma-irradiation of morphogenic calli, while grown on the RR medium produced a drastic effect on the subsequent root formation of those regenerated shoots. Evaluation of 852 plants regenerated from different genotypes of oca was performed under both laboratory and field conditions. Considerable variation was recorded among the regenerants for plant colour, height, days to flower, tuber colour, tuber type, yield and chromosome number. Regenerants with chromosome doubling induced through tissue culture were associated with a dwarf phenotype characterised mainly by shortened internodes and dark green leaves. Aneuploidy (2n = 48-52) was associated with lower shoot vigour but otherwise normal leaves and internodes. Ninety seven percent of the regenerants from explants of the 'white' genotype (a spontaneous mutant from the 'red' genotype) produced red tuber types, corresponding to their higher yield and with nutritional properties similar to the 'red' genotype. In addition 95% of. the regenerants from explants of the 'golden yellow' genotype (another spontaneous mutant from the 'red' genotype) showed reversion to the 'red' genotype characteristics by producing red tubers. From these high reversion figures, it appears that the regeneration process activates routinely the already inherited but inactive gene(s) in the, ‘white’ and ‘golden yellow’ genotypes. However, plants regenerated through direct morphogenesis of the flowers were all true to the parental type. Some improved somaclones including one early flowering (by two weeks) and a higher yielding type (by 37% compared with the parent) have been selected in their SC-2 generation. In the SC-3 generation the earliness character was retained and the higher yielding type gave 15% higher yield than the parent genotype ‘red’ or the existing best yielding type. In addition, in the latest yield trial conducted in 1987-88 season, two somaclones (WRN-4 and WRN-18) from the 'white' genotype have given significantly higher yields in their SC-2 generation compared with either 'red' (best yielding type) or 'white' (donor) genotypes. The yield increase was 41 and 30% compared to the 'red' genotype. Thus induction of somaclonal variation in oca can potentially broaden the germplasm variability. Moreover, selection of some improved types from this work showed potential for release as new cultivars, which may lead to the conclusion that application of tissue culture techniques can contribute to the improvement of this neglected crop.