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Somatic embryogenesis in Hevea is stimulated when the embryogenesis induction medium contains maltose, rather than glucose, fructose, or sucrose, in equimolarity (Blanc et al., 1999). Kinetic analyses were carried out on various physiological and biochemical indicators over the 8 weeks that the induction phase then expression of somatic embryogenesis can take. Embryogenesis induction in the presence of glucose, fructose or sucrose revealed strong callus growth in the first 3–4 weeks, associated with a high intra‐ and extracellular hexose content, a high starch content and a substantial decline in protein synthesis. In the presence of maltose, callus growth was slow and only half that seen with sucrose. This morphogenetic behaviour is associated with a drop in endogenous hexose and starch contents, and an increase in protein synthesis in the first three weeks of culture. The induction of embryogenesis in the presence of maltose was uniform and twice as fast as with sucrose supply. At the end of culture, peroxidase activity, antioxidant and membrane protein contents increased in these calluses; these characteristics may be associated with somatic embryo organization and with the maintenance of effective membrane integrity within a nutrient environment that has become limiting. These new results tally with data in the literature on the roles of sugars, and provide some precise information with regard to the ‘carbohydrate deficit’ hypothesis usually put forward to explain maltose action. An analysis of these results led to the hypothesis that regulation of endogenous hexose contents at a low level, through slow maltose hydrolysis, was a key element of the biochemical signal leading this callus towards somatic embryogenesis. [ABSTRACT FROM PUBLISHER]

To improve somatic embryogenesis in Hevea brasiliensis, the water and plant growth regulator status of the culture medium was studied. Induction of embryogenic tissue from the internal integument of immature seeds was clearly favored by stabilizing the water potential of the culture medium at -0.7 MPa, by using low and decreasing concentrations of 3 ,4-dichlorophenoxyacetic acid and benzyladenine or by incorporating 10-7 M abscisic acid in the medium. Each of these changes in the medium favored a specific water status in the callus, namely a high relative water content (93 to 95%) and an elevated water potential (-0.9 MPa). These characteristics were apparently important for initiating somatic embryogenesis, and their decrease corresponded to the loss of embryogenic potential in the callus. Thus, the relative water content and water potential of callus appear to be good markers of its embryogenic state. [ABSTRACT FROM AUTHOR]