A comparison between Theobroma cacao L. zygotic embryogenesis and somatic embryogenesis from floral explants

Summary In order to improve the late phases of Theobroma cacao L. embryogenesis from tissues of maternal origin, zygotic embryogenesis and somatic embryogenesis were compared, with respect to morphological, histological, and physiological parameters. Zygotic embryogenesis could be divided into three steps: (a) embryogenesis sensu stricto, (b) a growth period in which cotyledonary embryos reached their final dimensions, and (c) a maturation period in which embryos accumulated protein and starch reserves, dehydrated to a water content equal to 30%, and underwent a modification in soluble sugar composition. Monosaccharides and sucrose contents decreased to the benefit of the oligosaccharides raffinose and stachyose. The formation of somatic embryos by use of basic protocols was studied to define the limiting factors that could lie behind their poor development. Morphological abnormalities of somatic embryos, which represented 80% of the total population, were described. A histological study showed that somatic embryos lacked starch and protein reserves; moreover, their water content was much higher than that of their zygotic counterparts. Introducing a growth period into the culture protocol made for better embryo development. Adding sucrose and abscisic acid to the maturation medium was effective in increasing reserve synthesis and resulted in higher germination, conversion, and acclimatization rates.