Production of somatic hybrid citrus rootstocks with potential for wide adaptation and improved resistance to sting nematode

Sting nematode has become a primary factor limiting citrus production in localized regions of the central Florida sandridge citrus production area, making the development of resistant rootstocks a new breeding objective. In efforts to develop a replacement root-stock for sour orange, our focus has been on somatic hybridization of selected mandarin+pummelo combinations, since sour orange has been shown by molecular marker analysis to be a mandarinXpummelo hybrid (Nicolosi et al. 2000). Somatic hybrid plants were produced from four new mandarin+pummelo parental combinations by fusing embryogenic suspension culture-derived protoplasts isolated from selected mandarins with leaf protoplasts of pummelo seedlings previously selected for resistance to the sting nematode (Belonolaimus longicaudatus Rau) as follows: Amblycarpa mandarin+'Liang Ping Yau' (seedling) pummelo seedling SN7; Amblycarpa mandarin+'Hirado Buntan Pink' pummelo seedling SN3; Murcott tangor+pummelo seedling SN3; and Shekwasha mandarin+pummelo seedling SN3. Somatic hybridization was verified by ploidy analysis (via flow cytometry) and RAPD analyses. Mandarin parents were selected for wide soil-adaptation and ability to produce friable embryogenic callus lines. Pummelo seedlings used as leaf parents were identified from a previous screen of large seed populations (200 each) from four pummelos for resistance to sting nematode as follows: 'Hirado Buntan Pink'; 'Red Shaddock'; 'Large Pink Pummelo' and a seedling pummelo of 'Liang Ping Yau'. Seven resistant/tolerant pummelo seedlings were selected from the 800 pummelo seeds planted in the screen for use in fusion experiments. The four new somatic hybrids are being propagated to evaluate their horticultural performance and resistance to the sting nematode. These potential somatic hybrid root-stocks should also have potential to control tree size due to polyploidy.