Effects of abscisic acid and high concentrations of PEG on Hevea brasiliensis somatic embryos development

The effects of polyethylene glycol (PEG) and abscisic acid (ABA) were analysed on Hevea brasiliensis somatic embryos development. The presence of osmoticum greatly reduced the phenomenon of secondary embryogenesis and improved the conversion of proembryonic masses (PEMs) into torpedo-shaped embryos, while the addition of exogenous ABA favoured only the formation of globular-stage embryos. The development of embryos with a desirable morphological appearance (regular shape with a smooth and well-defined epidermis) was only observed in the presence of PEG in the solid culture media. The endogenous level of ABA was lower with these culture conditions than in control one (less than 10 nmol/g), indicating that PEG would not act through an endogenous ABA synthesis. However, the addition of 10(-5) M of ABA in the presence of the osmoticum increased its endogenous level (higher than 150 nmol/g) and the conversion into torpedo-shaped embryos but did not modify their morphological appearance. This effect of ABA was PEG-dependent and the best development occurred in the presence of 140 g/l PEG and 10(-5) M ABA. Further, the use of sucrose or PEG as osmotica has been discussed and an original method to achieve solid media with high concentrations of PEG has been proposed. (C) 1997 Elsevier Science Ireland Ltd.