Spontaneous tetraploidy in apomictic seedlings ofCitrus

Abstract

Populations of apomictic seedlings of clones ofCitrus species,Citrus hybrids, andPoncirus in the sub-family Aurantioideae were examined for spontaneous tetraploids as a source of materials for use in breeding experiments. Diagnostic features found useful in identifying nucellar tetraploids were leaf shape, petiole blade shape, leaf blade thickness, leaf color, comparative size differences in leaf venation, oil glands, and stomata, stem thickness, and relative size and developmental pattern of the root system. In older or bearing-age plants, nucellar tetraploids may be identified by differences in growth habit, vigor, size, time of growth initiation and bloom, and flower and fruit characteristics. Data are given for tetraploid frequency in glasshouse-grown, first-year nucellar seedlings of 42 populations of 32 clones of different genetic and seed origin and for tetraploid frequency in commercialnursery nucellar seedlings of the Carrizo rootstock clone in two consecutive years. Comparative data are given for quantitative development of roots, stems, and leaves of tetraploids and similar diploid nucellar seedlings. The data suggest that the ability to produce tetraploid apomictic seedlings is a variable genetic trait present in all or nearly all clones able to reproduce by adventitious embryony. Aspects of tetraploid nucellar seedlings that might warrant their testing as tree-size-controlling rootstocks in commercial citrus growing are discussed.