Regulation of somatic embryogenesis in celery cell suspensions - 1. Promoting effects of mannitol on somatic embryo development

Embryogenic suspension cultures of celery (Apium graveolens L.) were established from petiole and leaf callus. Suspensions were routinely subcultured in a 'maintenance medium' (with 2.3 μM 2,4-D and 0.88 μM BA). Somatic embryogenesis occurred in this medium, but was considerably improved in a 'regeneration medium' (2.3 μM kinetin, without 2,4-D). Cultures thus maintained, contained embryogenic clumps, aggregated somatic embryos, and few free-floating singular somatic embryos. Addition of mannitol (3-4% w/v) prevented cell lysis, greatly increased the number of singular somatic embryos, improved their normal differentiation, and accelerated torpedo embryo development. Experiments to reveal the nature of the mannitol effect demonstrated that the decreased osmotic potential was an important factor, but not the only one: iso-molar solutions of sucrose alone were not as effective. The mannitol effect could be manifested after a short (2-3 days) exposure period, suggesting a 'trigger' (induction) mechanism. Several pathways of somatic embryogenesis in celery and its regulation by subculturing, with the addition of mannitol, are outlined. Cultures thus maintained resulted in a high rate of normal somatic embryogenesis and production of normal transplantable celery plants.