Histological studies of cellular differentiation during somatic embryogenesis of coconut plumule-derived calli

Since coconut is one of the most recalcitrant species to generate in vitro, it is necessary to study in detail about the cellular changes that occur during somatic embryogenesis to enhance our knowledge about this phenomenon. In the present study, coconut plumular tissues, the shoot meristem including leaf primordia, were used as explants for in vitro regeneration studies. Histological studies were carried out in different stages of plumule culture. No noticeable growth was observed in 15 days old cultures. After 30 days, meristematic cells could be identified. Abundance of meristematic cells, foremost to the development of callus structures, was observed after 45 days. After 75 days, globular friable calli were formed and histological studies revealed the presence of meristematic centers which eventually formed somatic embryos. The histological study of matured somatic embryos formed after 120 days of callus initiation showed a clear meristematic zone of parenchyma cells, surrounded by vascular bundles. Histological studies, carried out for certain abnormalities like compact calli, abnormal somatic embryoids with rudimentary shoots and multiplied roots, revealed the presence of intact cotyledonary leaves which seemed to inhibit the apical meristem development of somatic embryoids. The presence of vascular bundles in the early stages of callus formation might lead to the direct formation of meristemoids. These results could aid future studies leading to enhanced control of the somatic embryogenic process and greater efficiency of somatic embryo and plantlet formation in coconut.