Plant regeneration from cotyledons and embryonic axes in apple: Sites of reaction and effect of pre-culture in the light

Summary Mature embryos of the cultivar Gloster were isolated from seeds of fruits stored at 1 °C. Embryonic axes and wounded cotyledons were pre-cultured in the light for 0 to 6 days on embryo growth medium (experiment 1) or for 0 to 24 hours on shoot induction medium (experiment 2). After a subsequent dark period of 3 weeks for induction of organogenesis, explants were cultured in the light on shoot elongation medium. Up to 95% of the embryos formed shoots, while the mean shoot number per embryo reached 6.4. Regeneration frequency was influenced by the type of explant: embryonic axes reacted better (75%) than the tops of the cotyledons (29%) or wounds on the cotyledons (9%). The mean total shoot number per embryo comprised means of 2.3 for embryonic axes, 1.8 for tops of cotyledons and 2.3 for wounds on cotyledons (experiment 1). Wounds on the proximal part of the cotyledon reacted much better than those on other parts. Shoot differentiation was highest when cotyledons were cultured with the adaxial side on the medium, suggesting that there exist gradations within the cotyledon for regeneration capacity (experiment 2). The number of shoots per reactive site was not normally distributed, while a high frequency of 6 or more shoots per reactive site was found (experiment 1). A pre-culture of more than 24 hours in the light seemed to have negative effect, but there was no clear evidence whether this was a light and/or a medium effect.