Frequency and characteristics of nucellar and zygotic seedlings in 12 citrus rootstocks

Abstract

Analysis of 10 isozyme loci was used to distinguish nucellar (apomictic) from zygotic (sexual) seedlings in 12 citrus rootstocks. The percentage of zygotic seedlings was <10 in C-32 citrange, Gomeri and Indio rough lemon, 10–30 in C.P.B. 4475 citrumelo, Cuban shaddock, Volkamer lemon and Yuma Ponderosa lemon, and >30 in Sacaton and Terra Bella citrumelos, Taiwanica sour orange and Yuma citrange. Small seed had lower germination and seedlings were smaller than those derived from normal seed. In all rootstocks except Yuma citrange, the frequency of zygotics in seedlings from small seed was not significantly different from that in populations derived from normal seed. Zygotic seedlings were generally shorter than nucellar seedlings. The distribution of height of zygotics considerably overlapped that of nucellars in most rootstocks, so that roguing by height alone was relatively ineffective. At 14 months after planting, most zygotics could be recognized by morphological characteristics in C.P.B. 4475 and Sacaton citrumelos, but many could not be identified by these criteria in Taiwanica and Volkamer lemon. For the polyembryonic accessions studied, zygotic seedlings are as likely to occur in seeds producing 2 seedlings as in those producing 1 seedling.