Zygotic and somatic embryos of cucumber (Cucumis sativus L.) substantially differ in their levels of abscisic acid.

In this work we studied the changes in the level of abscisic acid (ABA) in the somatic embryos (SE) and in the diploid and triploid zygotic embryos (ZE) of the same cucumber line during embryogenesis and seed maturation. Different stages of seed development were selected according to days after pollination (DAP): 21, 24, 28, 35, 42 and 42 plus 14 days of storage for diploid ZE and 35 and 42 plus 14 days of storage for triploid ZE. SE were collected at five growth stages from globular to late cotyledonary. Quantitative analysis of ABA was performed using an enzyme linked immunosorbent assay (ELISA) test. Both types of embryos - somatic and zygotic - essentially differed in their levels of ABA, always being the highest for 2n ZE and lowest for SE. Although the concentration of ABA in ZE of the triploid line was higher when compared with the same DAP, when the comparison was based on embryo development, both the concentration and content of ABA was higher in the diploid line. The pattern of developmental changes in the level of ABA in the diploid ZE was consistent with that known for other species. An increase was observed during embryo development with a peak (51.1 µg g(-1) FW or 0.95 µg per embryo) at the final stage of embryo formation between 21 and 24 DAP. A sharp decrease in the ABA level then took place (more than 3-fold within 4 days) and was followed by a further reduction as the seed matured. The maximal and minimal values for ABA concentration differed about 35-fold. SE differed substantially from their zygotic counterparts not only in that the concentration of ABA was extremely low (0.005-0.011 µg g(-1) FW) but also that no significant changes occurred during embryo development and no peak of ABA concentration was observed. Other tissues of the ovule and ovary also contained ABA and could be a source of ABA for the embryo.