Seed Composition

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Seeds contain variable quantities of foods in the form of carbohydrates, fats, and proteins. Seed starches are stored as granules in the endosperm, megagametophyte, and cotyledons. Lipids occur in fat bodies of seeds as fats and oils, depending on the proportions of saturated and unsaturated fatty acids present. Three-fourths or more of the seed proteins are stored in protein bodies, organelles 1 to 20 fim in diameter that contain a pro-teinaceous matrix and are bound by a single membrane. In beech cotyledons, the protein bodies originate by gradual subdivisions of vacuoles in which reserve proteins are deposited (Collada et al, 1993). In addition to storing proteins, the protein bodies may contain mineral nutrients and crystals (White and Lott, 1983). Some seed proteins are distributed in nuclei, mitochondria, proplastids, microsomes, and cytosol (Ching, 1972).

Starch is the principal and most widespread storage carbohydrate in angiosperm seeds, but seeds of many gymnosperms store little starch (Ching, 1972; Kovac and Kregar, 1989b). Sucrose is the major stored sugar, but other sugars are found in varying amounts. Trehalose occurs in beech seeds; stachyose and raffinose are in honey locust seeds. Mannans (un-branched polymers of mannose) (see Chapter 7 of Kozlowski and Pallardy, 1997) occur in significant amounts in seeds of date palm and coffee (Math-eson, 1984). Hemicellulose occurs in some seeds and is metabolized during germination, as in persimmon, coffee, and date palm (Crocker and Barton, 1953; De Mason and Thomson, 1981).

The seeds of many species store some or most reserves as fatty acids, with oleic, linoleic, and linolenic acids most common. Other fatty acids occur as glycerides. Among these are acetic, butyric, palmitic, stearic, lauric, and myristic acids. Other lipid compounds include esters of alcohols, sterols, phospholipids and glycolipids, tocopherols, and squalene (Mayer and Polj akoff-Maybe r, 1989).

Although some seed proteins, such as enzyme proteins and nucleopro-teins, are metabolically active, a large proportion is inactive. In addition to proteins, the nitrogenous material in seeds includes free amino acids and amides (see Chapter 9 of Kozlowski and Pallardy, 1997). Other seed constituents include variable quantities of minerals, phosphorus-containing compounds (phosphates, nucleotides, phospholipids, nucleoproteins), nucleic acids, alkaloids, organic acids, phytosterols, pigments, phenolic compounds, vitamins, and hormonal growth regulators (e.g., auxins, gib-berellins, cytokinins, abscisic acid (ABA), and ethylene).

Higgins (1984) divided seed proteins into storage proteins and "housekeeping" proteins, the latter essential in maintaining normal cell metabolism. The storage proteins consist of relatively few species of proteins, whereas housekeeping proteins are composed of small amounts of many protein species. Storage proteins tend to be high in asparagine, glutamine, and arginine or proline. However, the composition of seed proteins varies among plant species. Whereas seeds of chestnut and beech accumulated globulin, those of oaks accumulated glutelin (Collada et al., 1988, 1993). In Hopea odorata and Dipterocarpus alatus, both members of the Dipterocar-

paceae, the amino compounds in seeds varied somewhat. In both species glutamic acid and glutamine were the major amino compounds present (Huang and Villanueva, 1993). Hopea also contained large amounts of aspartic acid, asparagine, serine, threonine, arginine, and alanine. Seeds of Dipterocarpus contained high levels of alanine, arginine, and threonine.

The proportions of carbohydrates, fats, and proteins vary greatly in seeds of different species of plants, with carbohydrates or lipids usually predominating. Seeds of white oak, silver maple, horse chestnut, and American chestnut have high carbohydrate contents (Table 2.1). Examples of seeds with very high lipid contents include those of English walnut, butternut, pecan, coconut, oil palm, and macadamia (Table 2.2). Seeds of eastern white pine and longleaf pine have high lipid and protein contents but are low in carbohydrates (Table 2.1). There often is appreciable variation in the composition of seeds of different species in the same genus. For example, seeds of sugar maple have high carbohydrate contents, but lipids and proteins predominate in seeds of box elder.

Although the chemical composition of seeds is genetically determined, the relative amounts of seed constituents are influenced by the environmental regime at the seed source and nutrition of the parent tree. For example, Durzan and Chalupa (1968) showed considerable variation in the chemical composition of the embryos and megagametophytes of jack pine seeds collected from different geographical sources. The climate at the seed source influenced the degree to which metabolism of carbon and nitrogen compounds proceeded before and during incipient germination.

Table 2.1 Carbohydrate, Fat, and Protein Contents of Some Tree Seeds"

Percentage of air-dried seed mass

Percentage of air-dried seed mass

Table 2.1 Carbohydrate, Fat, and Protein Contents of Some Tree Seeds"

Species

Carbohydrates

Fats

Proteins

Silver maple

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