Somatic embryogenesis in citrus spp.: Carbohydrate stimulation and histodifferentiation

Somatic embryogenesis from nucellus-derived callus cultures of five cultiva rs, including three (Caipira, Seleta Vermelha, and Valencia) of sweet orang es (C. sinensis L. Osbeck), Rangpur lime (C. limonia L. Osbeck), and Cleopa tra mandarin (C. reticulata Blanco) (lines I and II), were studied. Callus lines maintained on MT medium supplemented with 50 g l(-1) sucrose were tra nsferred to MT medium supplemented with different carbohydrate sources: gal actose, glucose, lactose, maltose, or sucrose at 18, 37, 75, 110, or 150 mM , or glycerol at 6, 12, 24, 36, or 50 mM. Globular embryos were observed af ter approximately 4 wk, in several treatments. Cultures of Valencia and Cai pira sweet oranges and Cleopatra mandarin (line I) showed high numbers of e mbryos on medium containing galactose, lactose, and maltose. Histological s tudies showed somatic embryos in all developmental stages with a normal his todifferentiation pattern. The other two cultivars (Rangpur lime and Cleopa tra mandarin, line II) formed very few embryos, which did not develop furth er following the globular stage. Some of the abnormalities observed were la ck or dedifferentiation of protoderm and absence of apical meristems and pr ocambial strands. Embryos that followed the normal sequence of development were easily converted into plants. Non-embryogenic cultures continued as pr oliferating callus cultures, eventually forming a few embryos which did not convert into plants. Statistical analyses of the callus response to carboh ydrate treatments was done using an overdispersion Poisson model.