Building a Replacement Sour Orange Rootstock: Somatic Hybridization of Selected Mandarin + Pummelo Combinations

Abstract

<jats:p>Sour orange has been a premier citrus rootstock worldwide due to its ability to perform on challenging soils and to produce and hold high-quality fruit. However, increasingly widespread quick-decline isolates of citrus tristeza virus (CTV) have destroyed entire industries on sour orange in some countries, and are in the process of destroying millions of trees on sour orange in Florida. CTV also threatens other citrus locations planted heavy to sour orange, including Texas and Mexico. An acceptable alternative rootstock to replace sour orange is in high demand but has yet to be developed. Molecular analyses have recently determined that sour orange is probably a hybrid of pummelo and mandarin. We report the production of 12 new mandarin + pummelo somatic hybrids produced by protoplast fusion from selected superior mandarin and pummelo parents, in efforts to develop a suitable replacement sour-orange-like rootstock that is resistant to CTV-induced quick decline. Somatic hybrids from all 12 parental combinations were confirmed by a combination of leaf morphology, flow cytometry, and RAPD analyses (for nuclear hybridity). These new mandarin + pummelo somatic hybrids are being propagated by rooted cuttings as necessary to conduct quick-decline resistance assays and to assess horticultural performance in replicated field trials.</jats:p>

Identification of Sour Orange Accessions and Evaluation of Their Genetic Variability by Molecular Marker Analyses

“…It also would permit to isolate, within the best local selections, individuals amenable for setting up new breeding programmes for genetic improvement. These breeding programmes aim to constitute mainly new CTV-resistant hybrid rootstocks having the same good qualities of sour orange (Grosser et al, 2004).…”

Characterization of zygotic and nucellar seedlings from sour orange-like citrus rootstock candidates using RAPD and EST-SSR markers

“…Uniformity of the rootstock genotype is essential for reliable plant performance following budding and orchard establishment. However, most rootstock cultivars presently grown are polyembryonic and produce seeds that may contain both nucellar as well as zygotic embryos (CorazzaNunes et al 2002;Grosser et al 2004). Off-type (zygotic) seedlings are generally eliminated by nurserymen, but they are sometimes difficult to identify at their early stage and could develop into plants with very different agronomic traits.…”

“…The rootstock is a major contributor to tree performance and longevity since it determines the tolerance to soil-borne pathogens and pests; it influences tree size, yield, fruit quality (Davies and Albrigo 1994), and also allows the scion to adapt to a variety of soil conditions. Sour orange (Citrus aurantium L.) has been used as a successful rootstock throughout the world; in fact, it is one of the few rootstocks that can tolerate salinity, high pH, or calcareous soils to some degree (Castle 1987;Grosser et al 2004). However, trees of most commercial scions on sour orange rootstock are most vulnerable to the so-called "quick-decline" disease caused by certain strains of citrus tristeza virus (CTV; Bar-Joseph et al 1981, 1989Yokomi et al 1994).…”

“…However, trees of most commercial scions on sour orange rootstock are most vulnerable to the so-called "quick-decline" disease caused by certain strains of citrus tristeza virus (CTV; Bar-Joseph et al 1981, 1989Yokomi et al 1994). CTV infections have destroyed entire citrus industries on sour orange in many countries, and most of the remaining ∼15 million sour orange rooted trees in Florida, USA are expected to die within the next decade (Grosser et al 2004). A suitable elite rootstock replacement for sour orange is in high demand but remains to be identified.…”

“…Apomictic embryos arising from the nucellar tissue give rise to seedlings that are genetically identical to the seed source tree, thus yielding uniform rootstocks for propagation and planting (Frost 1943;Xiang and Roose 1988;Garcia et al 1999;Ruiz et al 2000). However, citrus rootstock cultivars can produce seeds that contain both nucellar and zygotic (sexual) embryos and therefore uniformity is not always complete (Moore and Castle 1988;Xiang and Roose 1988;Ruiz et al 2000;Oliveira et al 2002;Corazza-Nunes et al 2002;Grosser et al 2004). Although rootstocks can be developed through traditional plant breeding methods, the ability to screen and select for economically important traits in an efficient fashion is limited by the difficulties of screening techniques based on whole plant performance that is time consuming, requiring 4 to 7 years after planting (Ruiz et al 2000).…”