| The embryo of white spruce (Picea glauca (Moench.) Voss.) is not dormant but dormancy is imposed upon certain seeds by the mechanical restraint of surrounding tissues. Even a carbon source, isolated embryos from dormant and non-dormant seeds with or without a dormancy breaking treatment (moist chilling) complete germination and elongate.; Energy for the synthesis of cellular components and for germination senso stricto in white spruce seeds is probably obtained from the catabolism of sugars of the raffinose family, which constitute from 8-18% of embryo dry weight and from 1-3% of megagametophyte dry weight, depending on the seed source. These sugars are not metabolized in seed parts from seeds not completing germination, or during moist chilling. Sucrose contents remain about the same in both seed parts up until radicle protrusion at which time they increase, at least temporarily, probably due to the extensive catabolism of lipid reserves. Glucose and fructose contents also increases at this time and reflect increasing hydrolysis of sucrose after the completion of germination.; Radicle protrusion from white spruce seeds is dependant on weakening of the megagametophyte and perisperm, possibly through the degradation of mannan hemicelluloses. A new enzyme assay for endo-{dollar}beta{dollar}-mannanase indicates that at least 3 isozymes of this hemicellulose are present in all parts of mature, desiccated seeds. Mannanase activity in the megagametophyte peaks one day prior to testa splitting for the majority of seeds. Mannanase activity in the embryo increases after radicle protrusion and is associated with rapid embryo elongation. Increases in mannanase activity correlate with weakening of the megagametophyte and perisperm as determined by Instron analysis of the force necessary to puncture these tissues, and treatments repressing enzyme activity maintain tough tissues. |
