"From Simmonds and Dumbroff (1974).
"From Simmonds and Dumbroff (1974).
Even though some enzymes are active in dormant seeds, there generally is a dramatic increase in enzyme synthesis and activity after germination processes are set in motion. Changes in enzyme activities during and following germination have been studied in many woody plants. Examples include pines (Noland and Murphy, 1984, 1986; Gifford et al, 1989; Groome et al., 1991), spruce (Gifford and Tolley, 1989), tamarack (Pitel and Cheliak 1986), elm (Olsen and Huang, 1988), and hazel (Gosling and Ross, 1981; Li and Ross, 1990). The increased activity of some enzymes is the result of their conversion from an inactive to active form. However, many new enzymes also are synthesized after seeds imbibe water. Concurrently, some existing enzymes are degraded; hence, enzyme turnover is very great. The types of enzymes vary greatly among seeds and are related to the types of substrates present. As germination proceeds, the activity of enzymes involved in digestion of starch, lipids, proteins, hemicelluloses, and phosphates rapidly increases. Most of the enzymes involved in carbohydrate conversions become active because of synthesis, but some are mobilized by activation or release. The lipid bodies in seeds contain or acquire lipases that are involved in conversion of lipids to fatty acids and glycerol. Seeds also contain many proteolytic enzymes that are synthesized in the protein bodies or originate elsewhere in the cell and are translocated into the protein bodies (Mayer and Poljakoff-Mayber, 1989).
Phosphorus-containing compounds in seeds include nucleotides, nucleic acids, phospholids, phosphate esters of sugars, and phytin. There is consid erable interest in the phosphate metabolism of germinating seeds because of the relationship between phosphorus and energy transfer in metabolism. When cherry embryos were chilled to break dormancy, phosphate was translocated from the cotyledons and appeared in the embryo axis as sugar phosphates, high-energy nucleotides, and nucleic acids. In unchilled embryos, however, only inorganic phosphates accumulated in cells (Olney and Pollock, 1960). Such experiments have been interpreted as showing that the breaking of seed dormancy is accompanied by phosphate metabolism and an increase in available energy to the embryo.
Germination of seeds involves a drastic reversal of metabolic processes in the food storage tissues. Cells that initially synthesized insoluble starch, protein, and lipids during seed development suddenly begin to hydrolyze these materials. During seed development there is transport into the storage tissues, but during germination the soluble products of hydrolysis are translocated out of storage tissues to the meristematic regions of the seedling. This reversal of translocation involves considerable activation and deactivation of enzymes and conversion of "sinks" into "sources" (Dure, 1975). As a result the dry weight of storage tissues is reduced. For example, the dry weight of the megagametophyte of ponderosa pine seeds decreased greatly as it was depleted of reserves during germination (Fig. 2.10).
Carbohydrates During the initial stages of germination, insoluble starch and reserve sugars are converted to soluble sugars, and the activities of amylases and phosphorylases increase. The soluble sugars are translocated from endosperm to cotyledon tissues to growing parts of the embryo. Cells of cotyledon tissues from dormant black oak acorns contain many starch granules, whereas cotyledons of germinating acorns contain few starch granules (Fig. 2.11). In seeds of Araucaria araucana, starch degradation is initiated by a-amylase and phosphorylase in the megagametophyte (Cardemil and Varner, 1984). The cotyledons of Araucaria can take up sucrose as well as glucose, fructose, and maltose (Lazada and Cardemil, 1990).
Lipids Reserve fats in seeds are first hydrolyzed to glycerol and fatty acids by the action of lipases. These enzymes generally are not substrate specific and may hydrolyze different triglycerides to the same extent. Some of the hydrolyzed fatty acids are reused in synthesis of phospholipids and gly-colipids that are needed as constituents of organelles and membranes. Most of the fatty acids, however, are converted to acetyl coenzyme A and then to sugar by reversal of the glycolytic pathway. In some species there is little or no conversion of lipids to carbohydrates. In oil palm, for example, most seed lipids are consumed by respiration during seed germination (Oo and Stumpf, 1983a,b). In seeds of jojoba wax esters serve as food reserves during germination (Moreau and Huang, 1977).
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