Changes in water status and proline and abscisic acid concentrations in developing somatic embryos of pedunculate oak (Quercus robur) during maturation and germination.

Somatic embryos of oak (Quercus robur L.) were matured on P-24 media differing in gel strength (0.8, 0.9 and 1.0% (w/v) agar). Viscosity and osmotic potential (Psi(pi,medium)) Of the media were determined. Developing cotyledonary embryos were analyzed at maturity Stages I-III for water content, osmotic potential (Psi(pi,embryo)) and concentrations of abscisic acid (ABA) and proline. Proliferation of embryogenic tissue, germination rates and the number of embryos formed were also determined in order to relate embryo quality to physiological parameters. Viscosity increased with agar concentration, a phenomenon apparently related to water availability. Many Stage III embryos with high germination potentials were obtained on P-24 medium containing 1.0% agar. Embryo water content decreased progressively from 94 to 80% during embryo maturation. Stage I and II embryos that matured on media containing 0.8 or 0.9% agar had similar values of Psi(pi,embryo), whereas Psi(pi,embryo) of Stage III embryos that matured on medium containing 1.0% agar was significantly lower, although T-pi,T-medium was unaffected by gel strength. Stage III embryos showed a nearly 16-fold increase in proline concentration and a 50% decrease in ABA concentration compared with Stage I embryos. We conclude that tissue water status and a complex relationship between ABA and proline concentrations, modulated by medium gel strength, are important factors in the maturation process and the quality of oak somatic embryos.