POLYETHYLENE-GLYCOL PROMOTES MATURATION BUT INHIBITS FURTHER DEVELOPMENT OF PICEA-ABIES SOMATIC EMBRYOS

A combined application of abscisic acid (ABA) and high molecular mass osmoticum, polyethylene glycol (PEG) has become a routine method for s timulating somatic embryo maturation in some genera of Coniferales. Th e goals of the present study were to clarify how the PEG 4000-attribut ed low osmotic potential (Psi(s)) of the maturation medium affects the yield and morphology of mature somatic embryos as well as subsequent developmental processes during germination and ex vitro plantlet growt h in different genotypes of Picea abies belonging to 3 full sib seed f amilies. Despite high within- and among-family variation, a stimulator y effect of 7.5% PEG (Psi(s) = -0.645 MPa) on somatic embryo maturatio n was recorded for 13 out of 17 cell lines (F = 2.83, P = 0.1). PEG-tr eated somatic embryos were more dehydrated than embryos matured in the absence of PEG. Subsequently, embryos were partially desiccated using a high relative humidity treatment (HRH-treatment). The dynamics of e mbryo water content (WC) during HRH-treatment differed between embryos developed on maturation medium for 5 or 7 weeks. These two patterns r emained unchanged irrespective of the (Psi(s)) of the maturation mediu m. In 5-week somatic embryos, the WC decreased to the lowest level (in the range 25-35%) within the first 8 days of HRH-treatment and was no t further substantially changed. Seven-week embryos also lost water wi thin 8 to 16 days (decrease to 15-25% WC), but this drop was followed by rehydration of embryonic tissues by 24th day of HRH-treatment up to nearly the initial WC. Thus, 7-week embryos experienced both desiccat ion and slow imbibition in the course of the 24-day HRH-treatment. Thi s could account for their increased germinability compared to 5-week s omatic embryos found in the present study. Addition of 7.5% PEG to the maturation medium significantly inhibited somatic embryo germination for the vast majority of genotypes (F = 7.35; P = 0.01). Moreover, eve n after ex vitro transfer, both radicle elongation and lateral root fo rmation were substantially suppressed (F = 3.8; P = 0.03) in those pla ntlets produced from PEG-treated somatic embryos. Alterations both in the organization of the root meristem and in the structure of the root cap were found by histomorphological analysis of PEG-treated somatic embryos. All those embryos possessed massive root caps with numerous i ntercellular spaces in the pericolumn tissue. Cells of the quiescent c enter exhibited clear symptoms of degradation manifested in shrinkage and collapse of the protoplasm. In addition, PEG-treated embryos were of smaller size compared to embryos matured without osmoticum. When gr own in artificial substrate (up to 5 months) the PEG-induced inhibitor y post-effect gradually decreased. At this stage, the duration of matu ration was the only factor separating plantlets into slow- and fast-gr owing categories. Somatic embryos matured for 5 weeks produced plantle ts twice the size of those produced by 7-week embryos (F = 37.8; P < 0 .0001). This trend did not depend on Psi(s) of the maturation medium, nor on the genotype.