Growth Potential of Outplanted Seedlings

Growth of planted seedlings varies greatiy among species and genotypes. Many conifer seedlings grow much more slowly than seedlings of broad-leaved trees as shown by comparisons of dry weight increment and relative growth rates (Jarvis and Jarvis, 1963). Marked differences in productivity were attributed to species variations in photosynthesis, leaf area index, leaf longevity, and partitioning of photosynthate within the plant (Bond, 1986).

The very low rate of growth of conifer seedlings confers a competitive disadvantage on them when they are grown together with broad-leaved trees, shrubs, or herbaceous plants (Kozlowski et al., 1991). Grasses restrict growth of conifer seedlings, often as a prelude to mortality of the conifers. For example, competing grasses severely reduced growth of 3 year old pines. After two growing seasons, shoot length, root collar diameter, and needle length were reduced by 40, 54, and 20%, respectively, compared to seedlings growing without such competition. Growth reduction was associated with multiple environmental stresses induced by the presence of grasses (Caldwell et al., 1995). In competitive situations, conifers often grow best where the environmental conditions are poor for growth of broad-leaved seedlings. Hence, conifers often are excluded from sites where supplies of light, water, and mineral nutrients are high (Bond, 1989; Woodward, 1995).

Summary

Seeds are fertilized ovules that contain an embryo, stored food, and a protective coat. The embryo consists of cotyledons, an embryonic bud, a stem portion, and a rudimentary root. Mature seeds contain various amounts of carbohydrates, fats, proteins, enzymes, minerals, phosphorus-containing compounds, nucleic acids, alkaloids, organic acids, sterols, pigments, phenolic compounds, and hormones.

Seed germination involves resumption of embryo growth resulting in seed coat rupture and emergence of the young plant. Important environmental factors controlling seed germination are water supply, temperature, light (including light intensity, day length, and wavelength), oxygen, salinity, and plant litter. Because of wide differences among seedbeds in physical characteristics, temperature, and availability of water and mineral nutrients, seed germination and seedling establishment vary greatly in different seedbeds. A variety of naturally occurring chemicals (allelochems) and applied chemicals, including pesticides, herbicides, and growth retardants, may arrest plant establishment by suppression of seed germination, toxicity to young seedlings, or both.

Major events in germination include (1) seed hydration, (2) increase in respiration, (3) enzyme turnover, (4) increase in adenosine phosphates,

(5) increase in nucleic acids, (6) digestion of stored foods and transport of the soluble products to the embryo, (7) increase in cell division and enlargement, and (8) differentiation of cells into tissues and organs. Mature seeds of many species exhibit some degree of dormancy which may be associated with (1) immaturity of the embryo, (2) impermeability of seed coats, (3) resistance of seed coats to embryo growth, and (4) metabolic blocks within the embryo, or various combinations of these. For a long time it was presumed that embryo dormancy was regulated by balances of hormonal growth inhibitors and promoters. Current evidence indicates that control of seed dormancy is regulated by complex interactions of hormones and other endogenous factors in seeds. Seed dormancy can be broken by stratification of moist seeds at low temperature or high temperature, by afterripening in dry storage, or by mechanical and chemical treatment of seed coats to increase their permeability.

Seeds age progressively and exhibit reduced germination capacity and potential for synthesizing compounds needed for growth. Genetic changes in seeds and their progeny also are associated with seed aging.

In both gymnosperm and angiosperm seedlings, the cotyledons play an important role in growth and development of seedlings. Cotyledons are important in storage of foods and mineral nutrients, in photosynthesis, and in transfer to meristematic regions of substances needed for growth. In hypogeous seedlings, the cotyledons remain below ground and serve primarily as storage organs. In epigeous seedlings, the cotyledons emerge above ground. During their development, the cotyledons progress through storage, transition, photosynthetic, and senescent stages.

Growth and survival of outplanted seedlings are affected by nursery practices. Production of high-quality seedlings requires close attention to all phases of nursery management, including preparation of nursery beds, soil management, planting procedures, use of fertilizers, irrigation, root pruning, inoculation with mycorrhizal fungi, and pest control. Seedling quality also is influenced by time of lifting, duration of cold storage of seedlings, storage temperature and humidity, as well as handling of seedlings during lifting and planting.

Most outplanted conifer seedlings grow more slowly than seedlings of broad-leaved trees. Hence, conifer seedlings often are at a disadvantage in competitive situations. Conifers often grow best when the environmental regime is poor for growth of broad-leaved seedlings.

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