ANATOMY AND HISTOCHEMISTRY OF THE MEXICAN CACAO (Theobroma cacao L.) SEED

Abstract

The present research contributes knowledge of the embryo sac, seed anatomy and histochemistry of the Mexican cacao (Theobroma cacao L.) cv. ‘Carmelo’, and the relationship of these traits to seed recalcitrance. Flowers in anthesis and developing seeds were processed for anatomical (safranin and fast green) and histochemical evaluations of starch, lipids, proteins and tannins. It was found that the ovule is anatropous, crassinucellate and bitegmic with nuclear endosperm, characteristics significantly associated with recalcitrance of 45 dicotyledon families. The incipient growth of the inner tegument forms an endostome and exostome out of alignment. The embryo sac is formed by three antipodal cells, two synergids, one egg cell and one binucleated central cell. The inner integument (two to three layers) becomes thicker at the beginning of development (eight to 10 layers in 1 cm seeds), but afterwards it collapses. The outer integument (three to four layers at anthesis) is multiplicative; the mesotest divides (25 to 27 layers), and then it partially collapses. Mucilage sacs of testa originate from hypodermal cells of the exotesta. The fruit flesh is maintained attached to the testa, a trait associated to recalcitrance. The embryo shows a late growth, and the globular stage (40 μm) is present in seeds of 1 cm. Young cotyledons are straight and foliar (1.5 cm seeds), but at maturity they are thick and folded over the embryo axis. The endosperm is reduced to two layers at maturity. Cotyledons start to accumulate starch and proteins in seeds of 1.5 cm (Stage VI: fruits of 21.0 to 23.9 cm); lipids start when cotyledons folds (Stage VII: fruits of 21.0 to 23.9 cm), and tannins accumulate in idioblasts at the end of seed development.