THE INFLUENCE OF SOURCE TREE ON SOMATIC EMBRYOGENESIS FROM EASTERN BLACK WALNUT (JUGLANS NIGRA) IMMATURE COTYLEDONS

Abstract

The influences of stage of fruit development, media treatments and source tree (genotype) on somatic embryogenesis in Juglans nigra L. were studied over two years. Beginning at 12 weeks and continuing until 15 weeks postanthesis, 1 cm 2 explants from cotyledons of immature seeds were excised from eastern black walnut fruit during 1999 and 2000. Fruits from different trees were tested each year because of the alternate bearing habit of this species. Explants were placed on WPM, DKW, or LP medium with 1 g L -1 casein hydrolysate, 30 g L -1 sucrose, 5.0 μM thidiazuron (TDZ), 0.1 μM 2,4-D, and 7 g L -1 Sigma agar. The explants were under 16-hour photoperiod and 25 °C for four weeks. Explants were then transferred to basal media with 1 g L -1 casein hydrolysate and no plant growth regulators and incubated in darkness. Each year, the experimental design was a completely randomized design with 20 replications per treatment with factorial combinations of media salts, source tree and number of weeks post anthesis. During 1999, there was a significant interaction between genotype and media with LP being a consistently good medium for all three genotypes, with a range of 1.0 to 1.9 somatic embryos per explant and 24 to 30 percent of the explants producing somatic embryos depending on genotype. During 2000, there was a significant interaction between genotype and medium, and between genotype and number of weeks postanthesis. Explants from the best source tree produced 2.8 somatic embryos per explant plus 34 to 38% of the explants were embryogenic when DKW or LP salts were used. Medium had little effect on explants from one of the other two genotypes; the third genotype produced the most embryos when LP medium was used. Explants tended to be the most embryogenic 12 weeks postanthesis with 0.8 to 4.2 embryos per explant depending on source tree.