Endogenous hormone levels in habituated nucellar Citrus callus during the initial stages of regeneration

Embryogenic nucellar callus cultures of different Citrus species and cultiv ars growing in hormone-free medium were transferred to medium containing ei ther sucrose or glycerol as the only carbohydrate source. Glycerol has been reported to induce further development of Citrus somatic embryos, while in the presence of sucrose they continue to proliferate in an 'undifferentiat ed' manner. The endogenous hormone levels of the cultures were evaluated af ter 2 and 5 days to characterise the initial steps of embryo development. I n most cases, differences among treatments were observed only after 5 days of culture. Higher cytokinin levels were found in most of the cultures tran sferred to the glycerol-containing medium. The effect of ageing sweet orang e cultures on their endogenous hormone levels was determined by leaving the m in the original culture medium without subculturing for 60 days. While no changes were observed in the free indoleacetic acid and gibberellin conten ts. lower levels of abscisic acid and cytokinins were found in the aged cul tures than in those transferred at the normal interval. every 30 days. The endogenous hormone contents of Citrus callus of different genotypes were co mpared. Significant differences were observed in the levels of all hormones evaluated, even when the in vitro ontogeny of the different genotypes was very similar.