Cellular responses of leaf explants of Cocos nucifera L. in vitro

Leaf explants of Cocos nucifera L. (coconut palm) were studied in vitro in order to establish whether or not rapid cellular changes contribute to the well known recalcitrance of coconut cells in tissue culture. Segments from the base of immature leaves were cultured on modified Eeuwens' medium at 30°C in darkness. The mitotic index, nuclear DNA amounts, cell and nuclear size were measured both before and during culture (from 0 to 70 days). There was no basipetal gradient of cell division in immature coconut leaves; the mitotic index never exceeded 2% and showed neither a positional nor temporal relationship with leaf development. Moreover the vast majority of cells were in G1 of the cell cycle. This cell cycle pattern was maintained for most of the period in culture although at 70 days there was an increase in the proportion of cells in S- and G2-phases consistent with low rates of callus formation. The nuclear: cell size ratio was constant in cells within the immature leaf irrespective of developmental age. However upon transfer to culture media, cell size but not nuclear size increased. We suggest that this uncoupling of cell and nuclear size disrupts cell co-ordination and is a key contributor to recalcitrant cellular behaviour of this species in vitro.