Germination and conversion of somatic embryos derived from mature Quercus robur trees: the effects of cold storage and thidiazuron

Abstract

Somatic embryos of Quercus robur L. belonging to lines derived from mature trees were germinated in experiments designed to determine the effects of pregermination chilling and the temporary inclusion of thidiazuron (TDZ) in the germination medium. Germination response was evaluated in terms of the percentages of embryos exhibiting root-only elongation, shoot-only elongation, or both (conversion to plantlets). Conversion rates depended on genotype and were lower than those of oak zygotic embryonic axes germinated in vitro. Two months’ storage of somatic embryos at 4°C between maturation and germination treatments did not influence their conversion frequency, although root-only germination was promoted in the three embryogenic lines investigated. Addition of 0.01–5.0 µM TDZ to the germination medium for 7 or 18 days induced multiple shoot formation in the epicotyl region of germinating embryos, although the 18-day treatment was detrimental with TDZ concentrations of 0.5–5.0 µM. Exposure to 0.05–0.1 µM TDZ for 7 days increased the frequency of shoot-only elongation and decreased that of root-only elongation in embryogenic lines with both high and low conversion ability, but whereas the conversion rate of the hard-to-convert line Sainza was significantly increased, that of the readily converted line B-17 was negatively affected. As a result, with 0.1 µM TDZ the total potential plant recovery of Sainza (the conversion rate plus the percentage of embryos exhibiting shoot-only germination) was 64%, approximately the same as that of B-17. Shoots excised from shoot-only germinating embryos can be elongated and rooted by proven micropropagation procedures. An additional result was the finding that adventitious bud regeneration was induced on the cotyledons of TDZ treated embryos (69% with 0.5 µM TDZ), which may prove useful for genetic transformation. This study is the first to have shown that TDZ can increase the germination efficiency of oak somatic embryos.