EFFECT OF ABA AND SLOW DRYING ON DNA-REPLICATION IN CARROT (DAUCUS-CAROTA) EMBRYOIDS

Addition of abscisic acid (ABA) at the torpedo-shaped stage of develop ment and slow dehydration are two parameters necessary to produce comp letely desiccation-tolerant carrot (Daucus carota L.) embryoids. The m ode of action of these parameters is still largely unidentified. Emplo ying flow cytometry we investigated their effect on DNA replication an d cell cycle activity of the developing embryoids. DNA replication was determined as percentage of 4C nuclei. Addition of ABA did not alter DNA replication and cell cycle during embryoid development in vitro, i n spite of the putative quiescent state of the torpedo-shaped embryoid s. In contrast, during slow drying the nuclei were preferentially arre sted in the presynthesis G(0)/G(1)-phase and the amount of G(2) nuclei decreased. Dry zygotic carrot embryos do not contain any G(2) nuclei and are completely desiccation tolerant. The decline of G(2) nuclei in dry somatic embryoids seems to coincide with the increase in desiccat ion tolerance, which is incomplete compared to zygotic embryos. Our re sults suggest that in order to withstand anhydrobiosis, DNA replicatio n may be controlled during the embryoid developmental program and slow dehydration, but not by the plant growth regulator ABA.