Media and Explant Trials Aimed at Inducing Somatic Embryos from Commercially Important Walnut Cultivars

Attempts were made during the 1990 growing season to induce somatic embryos from non-zygotic tissues of five economically important walnut cultivars. The following explants were employed: fertilized and unfertilized ovules; immature and mature catkins; abnormal "spike" flowers; and in vitro-grown leaves and roots. Modifications of the standard induction media were also examined. These included increased sucrose concentration, ethylene action inhibition, and balance and type of growth regulator. Callus was obtained from all explant types but only ovules produced somatic embryos. Somatic embryo cultures were initiated from interior tissues of two 'Sunland' ovules isolated from bagged unpollinated flowers and cultured on basal DKW medium supplemented with glutamine, casein hydrolysate, 4.6 uM zeatin, 17.1 uM lAA and 0.45 uM thidiazuron. Another somatic embryo, initiated on the exterior surface of a 'Sunland' ovule, developed to the torpedo and heart stages but then callused. Immature embryos from four suspected apomictic 'cisco' fruits were also induced to grow in vitro and two of these produced somatic embryos. Isozyme analysis showed that all of these clones, except one from each cultivar, carry the same zymotype as the maternal tissue. However, restriction fragment length polymorphism analysis demonstrated that all the clones originated from zygotic rather than maternal tissues.