Light-cytokinin interaction in shoot formation in cultured cotyledon explants of radiata pine

Previous studies utilizing cotyledon explants of radiata pine ( Pinus radiata D. Don) revealed that cytokinin was required for shoot formation. This was confirmed and extended in the present study, which also showed that light was required. As little as three days of exposure to 16 h photoperiod light at a photon fluence rate of ca 80 μmol m^?2 s^?1 was sufficient to give some meristematic tissue formation. Longer exposure to light increased this formation. While cytokinin must be present during the first three days in culture for shoot formation, the exposure of the cotyledons to light could be delayed at least until day 10, but after 21 days in darkness, transfer to light did not permit shoot formation. Anatomical examination of the cotyledons confirmed the morphogenetic interactions of light and cytokinin in shoot formation. The data suggest that cytokinin is directly involved in the induction of shoot initiation and both light and cytokinin are required for the development of meristematic tissue and subsequent shoot formation.