Hereditary control of plant growth is contained largely in the DNA of cell chromosomes which through messenger RNA regulates the kinds of proteins and enzymes synthesized, which in turn control cell structure and plant responses. DNA also is present in mitochondria and chloroplasts, organelles that are important in producing essential proteins [e.g., large Rubisco (ribulose bisphosphate carboxylase/oxygenase) subunits]. Advances in molecular biology have demonstrated that genetic control of plant growth and development is very complex, with large numbers of genes being differentially expressed in various organs. In tobacco, for example, about 70,000 structural genes are expressed through the life cycle of the plant, but 6,000 of the genes transcribed in stem tissues are not transcribed in other parts of the plant (Kamaly and Goldberg, 1980, cited in van Loon and Bruinsma, 1992). Similarly, the gene coding for the small subunit of Rubisco is expressed in aerial parts of the plant but not in roots (Fluhr et al., 1986, cited in Hutchison and Greenwood, 1991). The role of gene expression in plant growth and development is discussed further in Chapter 9.
Several causes of genetic variation in plants have been identified. The long-term and ultimate source of variation is mutation, which may involve a change in a single gene by alteration of DNA molecules or a loss or addition of one or more chromosomes or parts of chromosomes. The main short-term source of variation is genetic recombination, in which different combinations of genes are created in each generation of plants by segregation, assortment, and recombination of alleles and chromosomes. The level and distribution of genetic variation within a population are affected by the breeding system, with inbreeding increasing total variation but reducing it within families, whereas outcrossing concentrates it within families. Genetic variation is increased by migration of alleles ("gene flow") through dispersal of seeds and pollen. Genetic variations within and among plant populations differ appreciably with their mating systems and gamete dispersal mechanisms. Plants with high potential for gene movement (e.g., wind-pollinated species with winged seeds) show less genetic variation among populations than do species with low mobility.
Genetic variation has been effectively exploited to meet growing demands for wood. Large increases in wood production have been achieved by combinations of parent selection, clonal selection, and interspecific hybridization (Zobel and Talbert, 1984; Stettler et al., 1988; Lugo et al., 1988). Much attention has been given throughout the world to planting genetically improved, fast-growing forest trees in close spacings along with using intensive cultural practices. Such trees commonly are harvested at short rotations, usually at 3 to 10 years. Early yields of wood from short-rotation plantations are suitable for timber and pulp, and the yields often are at least double those from conventional plantations (Zobel and van Buijtenen, 1989). This is particularly true for pines that are suitable for selective breeding (Kozlowski and Greathouse, 1970; Burley and Styles, 1976). In Brazil, closely spaced, high-yielding Eucalyptus species are grown for charcoal to operate steel mills, for pulp, and for fuelwood. Under optimal conditions yields are 40 to 50 tons ha-1 year of dry matter, but the average is closer to 25 tons (Eldridge, 1976; Hall and de Groot, 1987). Sachs et al. (1988) obtained data on potential yield of Eucalyptus trees on good sites near Davis, California. Productivity of biomass of selected river red gum (Eucalyptus camaldulensis) in an intensively managed plantation (1,100 trees ha-1) was 25 to 27 tons acre-1 for the third and fourth year after planting. Because the rate of growth did not increase after the third year, a three-year rotation was indicated.
Genetic variations in growth and development of woody plants differ among species, populations within a species, and individual plants. Genetic variations are particularly evident in species with large population sizes and extensive ranges. This is because the number of new mutations arising in each generation increases with population size, the rate of genetic drift varies inversely with population size, and environmental variation usually increases with the geographic range of a species (Mitton, 1995). There is a voluminous literature on genetic variation in woody plants (e.g., Wright, 1976; Burley, 1976; Zobel and van Buijtenen, 1989; Adams et al., 1992; Mitton, 1995). Here we cite only a few examples.
In Douglas fir, genetic variations were reported in biomass partitioning, wood density, crown width, stem diameter growth, branch diameter and length, and needle size (St. Clair, 1994a,b). The variability in biomass partitioning and wood density indicated that genetic gains may be expected from selection and breeding of desirable genotypes. Some of the crown structure traits showed promise as potential ideotype traits. Large trees that grew vigorously in their growing space had tall, narrow crowns, large leaf areas, and preferential partitioning of carbohydrates to leaves over branches.
Much attention has been given to genetic variation in poplars, largely for three reasons (Dickmann and Stuart, 1983): (1) poplars produce more biomass in short-rotation intensive culture than most other deciduous or evergreen species; (2) wide genetic variability among poplars offers opportunity for genetic improvement and high yield of wood; and (3) there is a strong demand for use of poplar for pulp, lumber, and energy through combustion or conversion to alcohol or other fuels. Large increases in productivity of poplars have been made by combinations of interspecific hybridization, parent selection, and clonal selection (Weber et al., 1985; Stettler et al., 1988).
Genetic variations have been shown among poplars in photosynthesis and enzymatic traits (Weber and Stettler, 1981; Ceulemans et al., 1987; Rhodenbaugh and Pallardy, 1993); partitioning of photosynthate (Bongarten and Teskey, 1987); stomatal size and frequency (Siwecki and Kozlowski, 1973; Pallardy and Kozlowski, 1979a; Kimmerer and Kozlowski, 1981); phenological characteristics such as timing of seasonal growth cessation and bud set (Weber et al., 1985; Milne et al., 1992; Ceulemans et al., 1992); stem and crown form (Nelson et al., 1981; Dickmann, 1985; Karki and Tigerstedt, 1985); growth rate (Phelps et al., 1982; Dunlap et al., 1992; Heilman and Stettler, 1985; Rogers et al., 1989; Milne et al., 1992); wood properties such as specific gravity, fiber length, energy content, and chemical composition (Sastry and Anderson, 1980; Reddy and Jokela, 1982); rooting capacity (Pallardy and Kozlowski, 1979b; Ying and Bagley, 1977); and response to environmental stresses such as drought (Pallardy and Kozlowski, 1979c; Mazzoleni and Dickmann, 1988) and air pollution (Kimmerer and Kozlowski, 1981).
Characteristics associated with superior growth of poplar hybrids include favorable leaf expansion rates, leaf cell size and number, leaf mesophyll structure, stomatal conductance, stomatal responses to drought, canopy architecture, leaf retention, and resistance to disease (Stettler et al., 1988). Enzymatic and stomatal frequency traits are associated with variations in growth rates of poplars (Weber and Stettler, 1981; Ceulemans et al., 1984, 1987).
In contrast to the high genetic variability in Douglas fir and poplars, variability in red spruce is lower than that of most north-temperate woody species. Genetic variability is particularly low in discontinuous southern populations of red spruce, presumably because of genetic drift followed by inbreeding (Hawley and DeHayes, 1994).
Exploitation of genetic material is crucial for growing fruit trees and forest trees in seed orchards. Improvement of fruit trees has been documented since the time of recorded history. The best available wild edible species have been collected (e.g., blueberry, grape, plum, and walnut), or barely edible plants have been improved by selecting individuals with larger than average fruits and of better quality (e.g., apple and pear). A major objective in fruit-tree breeding is selection of early-flowering genotypes that produce high fruit yields (Alston and Spiegel-Roy, 1985; Sedgley and Griffin, 1989). Selection and breeding programs also have been directed toward producing plants with low chilling requirements, improved reproductive capacity in areas of low spring temperatures, enough cold hardiness to extend growing of fruit trees to colder regions, pest and disease resistance, and high fruit quality. Much attention has been given to selection of new rootstocks for controlling fruit size and production (Alston and Spiegel-Roy, 1985; see also Chapter 8).
For production of forest tree seeds it is essential to upgrade the genetic quality of planting stock by planting new seed orchards as improved clones become available. Desirable traits of introduced clones are high fecundity, synchrony of male and female flowering, and production of large seeds (Sedgley and Griffin, 1989). Seed orchards usually are designed to include large numbers of clones (25 to 30), each with similar opportunity for interbreeding. Many clones usually have been planted because (1) at the time of orchard establishment information often is lacking about the genetic quality of the clone and (2) high levels of outcrossing are maintained by clonal diversity despite variations among clones in seed yield (Griffin, 1982, 1984).
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