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(Received 13 October 1999; accepted 3 May 2000; editor S. S. Korban)
SUMMARY
A method has been developed to facilitate shoot formation from leaf explants of almond. Leaves were dissected from micropropagated shoot cultures of the commercial cultivars Nonpareil and Ne Plus Ultra, and sections incubated on Almehdi and Parfitt's (1986) basal medium (AP) with varied plant growth-regulator conditions. Three auxins, 2,4-- dichlorophenoxyacetic acid (2,4-D), alpha-naphthaleneacetic acid (NAA), and indole-3-butyric acid (IBA), in combination with two cytokinins, benzylaminopurine (BA) and thidiazuron (TDZ), were tested at various concentrations along with casein hydrolysate (CH) to determine the conditions most conducive to adventitious shoot regeneration. Response to the tested plant growth-regulator conditions varied with genotype. Of the three auxins tested, NAA and IBA induced adventitious shoots from Ne Plus Ultra explants, but only IBA was effective for Nonpareil. For the cytokinins, shoot development from Ne Plus Ultra occurred in the presence of either BA or TDZ, whereas for Nonpareil only TDZ was effective unless CH was incorporated in the basal medium. The inclusion of CH (0.1% w/v) improved callus morphology, and increased regeneration frequencies for both cultivars. Maximum regeneration frequencies for Ne Plus Ultra (44.4%) and Nonpareil (5.5%) were achieved on AP basal salts supplemented with CH, IBA (9.8 (mu)M), and TDZ at 22.7 and 6.8 (mu)M, respectively.
Key words: adult explant; casein hydrolysate; organogenesis; Prunus; thidiazuron; woody plant.
INTRODUCTION
Conventional breeding of woody fruit species is a slow and difficult process due to high levels of heterozygosity and long generation cycles (Sriskandarajah et al., 1994). For this reason it is important to develop gene-transfer methods for fruit crops, to accelerate the breeding process and broaden the germplasm sources available for crop improvement. One of the prerequisites for plant transformation is a method for efficiently regenerating plants in vitro (De Bondt et al., 1994). However, fruit trees are amongst the most recalcitrant for in vitro culture, and regeneration of adventitious shoots from adult explants has proven difficult (Miguel et al., 1996; Singh and Sansavini, 1998). For the Rosaceae, whilst protocols have been established for regenerating adventitious shoots from leaf tissue of Males (Pawlicki and Welander, 1994; Sriskandarajah et al., 1994) and Pyrus (Chevreau et al., 1989; Lane et al., 1998), there are only...