Relations between biochemical characteristics and conversion ability in Hevea brasiliensis zygotic and somatic embryos

The morphological, hydric, and biochemical characteristics were analysed on zygotic embryos at different stages of their development and on cotyledonary somatic embryos in Hevea brasiliensis (M ll. Arg.). The minimum level of maturity required for a good conversion ability was defined. Sixteen weeks after anthesis, the zygotic embryos had acquired a conversion ability equivalent to that of fully mature embryos (24 weeks after anthesis) and to that of somatic embryos. The conversion rates fluctuated between 50 and 60%. The water content, osmotic potential, and turgor pressure of somatic embryos were comparable with those of 16-week-old zygotic embryos. The water potential was comparable with that of immature 13-week-old embryos. Partial dehydration of somatic embryos and 16-week-old zygotic embryos was compatible with a good conversion ability. In zygotic embryos, accumulation of reserves began early in development (13th week for proteins and 15th week for starch). In the 16th week, they had virtually acquired all their reserves. The somatic embryos had equal protein and starch concentrations compared with fully mature zygotic embryos. However, they were much smaller than zygotic embryos (ratio of 1:30 for dry weight), and reserves were around 20 times lower than those of zygotic embryos. This characteristic was linked to the low vigor of plantlets formed in vitro and to the long time taken for full plantlet development.