An ultrastructural study on early callus development from immature embryos of the maize strains A188 and A632

Immature embryos of two maize inbred lines (A 188 and A632) have been examined during the first three days of in vitro culture using light and electron microscopy. The first day of culture is characterized by an increase in the amount of organelles, changes in vacuolation and in nucleolar morphology. These ultrastructural events take place in both inbred lines and are not influenced by the presence of 2,4D. After one day of culture variation in developmental patterns occurs, which is dependent on genotype and culture conditions. It is found, that in the scutellum of A188 embryos proliferation is activated strongly within two days of culture, followed by the appearance of a broad scutellar meristematic zone, A second meristematic region is observed around the coleorhiza. In A632 embryos only a coleorhizal meristematic zone developed, while in the scutellum proliferation was hardly observed. Therefore we have subdivided the first period of maize embryo culture into two phases. The first phase is called the shock response and takes about one day, while the second phase starts after one day and is called growth response. In concurrent cultures of A188, which were prolonged for several weeks, white compact callus developed accompanied by somatic embryos, while in the A632 cultures non-embryogenic callus was produced.