What have we come to know about the nutrition of embryo from in vivo & in vitro studies
The nutrition of embryo in vivo studies
The young proembryo derives its nutrition from ovular tissues with the help of suspensor. As the embryo develops it's suspensor degenerates. Later the chief source of nutrition of the embryo attains a late globular state, & its suspensor becomes dysfunctional, the endosperm is generally already a cellular tissue surrounding the embryo on all sides. The developing seed is a powerful sink for nutrients. Food materials received through the funicular vascular supply, are absorbed by the endosperm & passed on the embryo for its growth & development. The central cell wall has transfer cells for absorption of nutrients. Sometimes persistent antipodal cells or special structures such as hypostases & testament of the ovule help in translocated of food to the endosperm. After the embryo is fully grown, the endosperm persists in a large number of plants, particularly monocotyledons, storing starch or oil or protein or all the three. These are used up during seed germination.
The nutrition of embryo in vitro studies
Studies involving culture of embryos excised of various stages of development (i.e., early globular proembryo, late globular proembryo or heart shaped & torpedo shaped embryos) in media containing various combinations/concentrations of nutrients & growth requirements of embryos.
Experiments using embryos of Datura, Capsella & a few of other plants have shown that the mature embryo can develop into a normal seeding when cultured in a nutrient medium. Torpedo-shaped embryos require salts of essential minerals dextrose & certain amino acids, vitamins & PGRs for successful growth. Heart shaped embryos can be cultured only on addition of coconut water to above medium. So far it has not been possible to isolate & grow the zygote or a few celled proembryo on an artificial medium. Young proembryos when cultured become formless & do not acquire the typical structure of a mature embryo.
Two phases of embryo development based on nutritional requirements are:-
(i) Heterotrophic phase: During this phase, which may last up to the globular stage, the proembryo is dependent upon the endosperm.
(ii) Autotrophic phase: This may begin at the late heart-shaped stage when the embryo becomes fairly independent for its nutrition.