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Germination temperatures also vary considerably with seed source. Germination temperatures between 24 and 30°C were best for ponderosa pine seed sources east of the Rocky Mountains, whereas temperatures of 35°C or higher were optimal for Pacific Northwest sources (Callaham, 1964). Optimal germination temperatures often are different for seeds obtained from different plants of the same species.

Seeds of many species germinate equally well over a rather wide temperature range. For example, seeds of lodgepole pine germinated at about the same rate at 20 as at 30°C (Critchfield, 1957), and germination of jack pine seeds did not vary appreciably at 15, 21, or 27°C under continuous light (Ackerman and Farrar, 1965). Kaufmann and Eckard (1977) found that total emergence of seedlings of Englemann spruce and lodgepole pine was similar at 16 and 25°C, but it was reduced at 35°C. However, at 16°C, spruce seedlings emerged 2 days sooner than pine seedlings. Other species have a rather narrow temperature range that may vary somewhat with preconditioning of seeds. For example, Norway maple seeds germinated best at temperatures between 5 and 10°C. Box elder seeds had 67% germination as temperature alternated between 10 and 25°C, but only 12% when temperatures were alternated between 20 and 25°C (Roe, 1941).

Although seeds of many species will germinate at a constant temperature, seed germination of most species requires or is increased by diurnal temperature fluctuations (Hatano and Asakawa, 1964). If seeds of Man-churian ash were first exposed to moist, low-temperature treatment and then placed at a constant temperature of 25°C, only a few germinated, and at a constant temperature of 8°C seed germination was greatly delayed. In comparison, alternating the temperature between 8°C for 20 hr and 25°C for 4 hr each day greatly accelerated germination. Germination of Japanese red pine seeds also was accelerated by diurnal thermoperiodicity. Maximum germination of Phellodendron wilsonii seeds was possible only with alternating exposure to temperatures of 35°C (8 hr in light) and 10°C (in darkness). This effect was demonstrated for stored seeds, fresh seeds, and even dry seeds (Lin et ai, 1994). A regime of alternating temperatures between 20 and 30°C was adopted by the International Rules for Seed Testing to test the germination of seeds of many species of woody plants.

Radiation

Whereas most seeds of temperate-zone species germinate as well in the dark as in the light, those of some species require light for germination. Seeds of these species will germinate at very low illuminance, those of spruce requiring only 0.08 lux; birch, 1 lux; and pine, 5 lux. Seeds of a few species require up to 100 lux for germination (Jones, 1961). Germination of seeds of some species (e.g., Fraser fir) is stimulated by light even though there is no absolute requirement (Henry and Blazich, 1990). The degree of stimulation of germination by light is influenced by several environmental factors, with stratification (moist prechilling) and temperature being very important. Seeds of a number of tropical species require light for germination. These include seeds of Cecropia obtusifolia, C. peltata, Trema micrantha, T. orientalis, and Piper auritum (Whitmore, 1983; Orozco-Segovia et al., 1993). In Ghana, the seeds of 96 species germinated readily only in full light, and seeds of 25 other species germinated in the shade (Hall and Swaine, 1980). The light requirement for germination may vary over time. Seeds of some species may become more sensitive, and those of other species less sensitive, over time (Vazquez-Yanes and Orozco-Segovia, 1994).

Seedlings that develop from large seeds usually tolerate heavy shading better than seedlings emerging from small seeds. An increase in seed size from 0.03 to 30 mg among a variety of species was associated with increased seedling survival in heavy shade from 10-15 days to 30-40 days (Leishman and Westoby, 1994). The larger seeds provided a greater initial energy reserve, which may be advantageous in habitats in which canopy gaps are regularly created. Seedlings from large seeds showed greater early height growth. This may be an advantage for regeneration in habitats with a steep light gradient, as for seedlings emerging from seeds that germinate below litter.

Day Length For seeds of the majority of light-sensitive species of woody plants, the most rapid and greatest total germination occurs in daily light periods of 8 to 12 hr. Interrupting the dark period with a short light flash or increasing the temperature usually has the same effect as extending the duration of exposure to light. In eastern hemlock, 8- or 12-hr days produced maximum seed germination, with no added response by increasing day length to 14 or 20 hr (Olson et al., 1959). Eucalyptus seeds germinated well in 8-hr days and those of birch in 20-hr days. Seeds of Douglas fir, however, germinated in continuous light or 16-hr days, but not in 8-hr days (Jones, 1961).

Wavelength Germination of seeds of a number of species of herbaceous and woody plants is sensitive to the wavelength of light. Examples of woody angiosperms showing such sensitivity are hairy birch, Manchurian ash, and American elm. Sensitive gymnosperms include species of Abies, Picea, and Pinus (e.g., Japanese black pine, eastern white pine, longleaf pine, and Virginia pine) (Hatano and Asakawa, 1964; Roller, 1972).

The germination response to wavelength is controlled by the phy-tochrome pigment system. Red light promotes germination and far-red light inhibits it. If seeds are exposed to red (650 nm) and far-red light (730 nm), their capacity to germinate depends on the irradiation that is given last, with the influence of red light in promoting germination being nullified if it is followed by far-red light. If, however, far-red light is followed by red light, germination is stimulated (Table 2.3). In forests, the litter may inhibit seed germination because of the low red to far-red ratio of the light transmitted by the litter layer (Vazquez-Yanes et al, 1990). It should also be noted that the red-far red ratio of light beneath a canopy already is low relative to direct sunlight.

The red light requirement for promoting germination often is not rigid, varying with temperature or duration of water uptake by seeds. Toole et al. (1961) noted, for example, that germination of Virginia pine seeds occurred faster in seeds promoted with red light after a 20-day period of water imbibition at 5°C than in seeds given a 1-day period of imbibition. Greater germination of seeds was promoted by red light when they had previously absorbed water at 5°C rather than at 25°C.

Both metabolic activity and mitosis in embryos are stimulated by red light and inhibited by far-red light in light-sensitive seeds. Significant increases in respiration of Scotch pine seeds were induced by red light after imbibition for 24 hr, whereas mitotic activity was stimulated after 36 hr of

Table 2.3 Influence of Alternating Red and Far-Red Irradiation on Germination of Virginia Pine Seeds«

Character of irradiation'' Germination (%)

Dark control 4

R 92

"From Toole et al. (1961). ''R, red; FR, far red.

imbibition. Radicles did not emerge until after more than 48 hr of imbibition (Nyman, 1961).

Phytochrome, a widely distributed light-receptive, protein-pigment complex, is involved in the red-far red phenomena described above. Light acts on phytochrome to change it from an inactive form, with maximum absorption in the red part of the spectrum (660 nm, Pr), to the active form, with maximum absorption in the far-red (730 nm, Pfr) (Fig. 2.7). In mature seeds some Pfr often is present, but during imbibition it changes to the inactive Pr form. Seeds conditioned to germinate through the activity of red light can revert to a nongerminating state by exposure to far-red radiation, which changes Pfr back to Pr (Taylorson and Hendricks, 1976). In the dark, Pfr may gradually revert to or it may be destroyed by denaturation or enzyme-catalyzed conversion to an inactive form (Hart, 1988).

Figure 2.7 Photoconversion of phytochrome by red and far-red light into far-red absorbing (Pfr) and red-absorbing (Pr) forms, respectively. Pfr may gradually revert to Pr in the dark, or it may be destroyed. From Plant Physiology, 2nd Ed., by Frank B. Salisbury and Cleon W. Ross. © 1978 by Wadsworth Publishing Company, Inc. Reprinted by permission of the publisher.

Figure 2.7 Photoconversion of phytochrome by red and far-red light into far-red absorbing (Pfr) and red-absorbing (Pr) forms, respectively. Pfr may gradually revert to Pr in the dark, or it may be destroyed. From Plant Physiology, 2nd Ed., by Frank B. Salisbury and Cleon W. Ross. © 1978 by Wadsworth Publishing Company, Inc. Reprinted by permission of the publisher.

Oxygen

As intense respiration is characteristic of the early phase of seed germination, it is not surprising that oxygen supply affects germination (see Chapter 6 of Kozlowski and Pallardy, 1997). Seeds usually require higher oxygen concentrations for germination than seedlings need for growth. The relatively high oxygen requirements of seeds of some species are the result of their seed coats acting as barriers to diffusion of oxygen into seeds. Removal of coats of red pine seeds or exposure of intact seeds to high oxygen concentrations greatly accelerated the rate of oxygen uptake. Removal of seed coats, followed by exposure of the decoated seeds to high oxygen concentrations, accelerated respiration even more (Kozlowski and Gentile, 1959).

Oxygen plays a role as the primary electron acceptor in respiration. In some species it also may be involved in inactivation of one or more inhibitors. Germination of isolated embryos of European white birch and European silver birch was prevented by aqueous extracts of seeds, but such inhibition could be decreased by exposure to light (Black and Wareing, 1959). It appeared that the intact seed coat prevented germination in the dark by reducing the oxygen supply below a critical level. Nevertheless, embryos without seed coats germinated in low concentrations of oxygen; hence, the embryo appeared to have a high oxygen requirement only when the seed coat was present.

As mentioned, soaking seeds for a few hours hastens germination, but prolonged soaking induces injury and loss in viability of many seeds, presumably because of the reduced concentration and availability of dissolved oxygen in comparison with that of air. Unless aseptic conditions are maintained, long soaking periods also may favor activities of harmful microorganisms. However, seeds of bottomland species, such as tupelo gum and bald cypress, have low oxygen requirements and can endure prolonged inundation without loss of viability. Hosner (1957) did not find any appreciable effect on germination of six bottomland species from soaking seeds for as long as 32 days.

Seedbeds

Because of wide differences among seedbeds in physical characteristics, temperature, and availability of water and mineral nutrients, establishment of plants varies greatly in different seedbeds (Winget and Kozlowski, 1965a). Mineral soil usually is a good seedbed because of its high infiltration capacity, adequate aeration, and capacity to establish close hydraulic contact between soil particles and seeds. Because of its high water-holding capacity, sphagnum moss often is a suitable seedbed for germination, but it may subsequently smother small seedlings. Decayed wood also is an excel lent natural seedbed for seeds of some plants, probably because of its capacity for retention of water (Place, 1955).

Litter may or may not be a good seedbed depending on the plant species, amount and type of litter, and prevailing environmental conditions. Both seed germination and growth of young seedlings are influenced by litter. By altering the microenvironment of the topsoil, litter intercepts light and rain and affects the surface structure of the seedbed, hence regulating transfer of heat and water between the soil and aboveground atmosphere. Litter influences plant community structure directly (e.g., by effects on seed germination and seedling establishment) and indirectly by influencing availability of light, water, and mineral nutrients (Facelli and Pickett, 1991b).

Facelli and Pickett (1991a) predicted that the indirect effects of litter on the structure of old-field plant communities were important when (1) the tree population was not limited by dispersal or physical stress, (2) interspecific competition was more important than other biotic interactions in regulating establishment and growth of trees, (3) establishment of herbaceous plants was affected more by litter than was that of trees, and (4) plasticity of herbaceous plants was too slow to rapidly compensate for the lower density by increasing the size of individual plants.

In deserts, litter is considered beneficial because it increases the availability of soil water by inhibiting evaporation from the soil. In many forests, however, litter is a less suitable seedbed than mineral soil because it warms slowly, inhibits penetration by roots, prevents seeds from establishing hydraulic contact with the mineral soil, dries rapidly, and shades small seedlings.

The litter mat often prevents seeds and seedling roots from reaching the soil. In California, the roots of blue oak seedlings often failed to reach the soil when seeds germinated on top of a thick layer of litter (Borchert et al., 1989). Tao et al. (1987) showed a strong inhibitory effect of litter on seed germination and establishment of both gymnosperms (Picea jezoensis, Larix dahurica) and angiosperms (Populus davidiana, Betula costata, B. dahurica, B. platyphylla) in Korean pine forests. Tilia amurensis, with very large and heavy seeds, was an exception. Its strong radicle readily penetrated the litter layer into the moist substrate. Matted litter consisting of leaf litter held together by fungal mycelium has a greater inhibitory effect on seedling establishment than loose litter.

Litter often retards seedling establishment by decreasing availability of resources. For example, litter may prevent germination of seeds that respond positively to light (Sydes and Grime, 1981a,b). Litter also may decrease availability of water to young seedlings. Shortly after seeds germinated in litter the rootlets of yellow birch seedlings tended to grow horizontally over the leaf mat (Winget and Kozlowski, 1965a). The major rootlets often were completely exposed to air, and only the secondary rootlets penetrated the leaf mat. The seedling stems often were prostrate or nearly so, with only a few millimeters of their stem tips oriented vertically. Occasionally the levering action of a rootlet against the surface of the leaf mat overturned seedlings, hence exposing the roots, followed by desiccation of these seedlings.

The effects of plant litter on germination and emergence of various species of plants may be quite different. Litter reduced and delayed final emergence of yellow birch seedlings, and the emerging seedlings had reduced root-shoot ratios and longer hypocotyls than control seedlings. By comparison, litter did not affect emergence of staghorn sumac seedlings but altered biomass allocation (Peterson and Facelli, 1992). The biomasses of stems, leaves, and roots of staghorn sumac seedlings were progressively reduced by increasing amounts of litter, and by leaf litter relative to needle litter. The different responses of these species to the presence of litter were attributed to differences in seed size (birch mean seed size, 1.0 mg; staghorn sumac seed size, 8.5 mg) and germination cues (birch seed requires light for germination; staghorn sumac seed requires chemical or heat scarification).

Chemicals

A variety of applied chemicals including certain herbicides (Sasaki et at, 1968; Sasaki and Kozlowski, 1968d; Kozlowski and Sasaki, 1970), fungicides (Kozlowski, 1986a), insecticides (Olofinboba and Kozlowski, 1982), growth retardants, fertilizers, and soil salts sometimes inhibit plant establishment by direct suppression of seed germination, toxicity to young seedlings, or both (Kozlowski and Sasaki, 1970). The phytotoxicity of soil-applied chemicals varies greatly with the specific compound applied, dosage, plant species, environmental conditions, and manner of application. Toxicity often is low if the chemical is applied to the soil surface, intermediate if incorporated in the soil, and greatest if maintained in direct contact in solution or suspension with plant tissues (Kozlowski and Torrie, 1965). The high absolute toxicity of many chemicals often is masked because soil-applied compounds are variously lost by evaporation, leaching, microbial or chemical decomposition, and irreversible adsorption in the soil (Kozlowski et al., 1967a,b).

Some herbicides are resistant to microbial action, and their persistence in the soil may affect tree growth long after weeds have been eliminated. For example, 2,3,6-TBA (2,3,6-trichlorobenzoic acid) may persist in the soil for several years. Direct contact of red pine seeds with 2,3,6-TBA did not suppress germination, but continuous contact of the herbicide with recently emerged seedlings caused abnormal plant development and often induced death of the seedlings (Kozlowski, 1986b).

Whereas some herbicides kill seedlings, others cause abnormal develop mental changes such as curling, shriveling, or fusion of cotyledons, and chlorosis, distortion, and growth inhibition of various foliar appendages (Wu et al., 1971). The primary mechanisms of herbicide toxicity are diverse and involve interference with plant processes as well as direct injury to cells and tissues.

Chemical growth retardants and inhibitors have been useful in producing compact plants and reducing the costs of storing and shipping nursery stock. However, some of these compounds may be toxic to seedlings in the cotyledon stage of development (Kozlowski, 1985d).

Soil salinity affects emergence of seedlings by decreasing the osmotic potential of the soil solution and by being toxic to the embryo and seedling. Salinity variously delayed and inhibited seedling emergence, reduced growth of shoots and roots, and altered the mineral concentration of several young citrus rootstocks (Zekri, 1993). Addition of 50 mol m~3 NaCl to a nutrient solution delayed seedling emergence by 3 to 5 days, except for Troyer citrange. Final emergence was reduced by less than 30% in Carrizo citrange, Troyer citrange, and Swingle citrange, and it was reduced over 65% in Ridge pineapple, Cleopatra mandarin, and Rough lemon. The biomasses of both shoot and root were reduced more than half by a 50 mol m-3 treatment. Clemens et al. (1983) showed considerable variation in response of germination of seeds of several Casuarina species to salinity. For example, Casuarina stricta seed showed a 40% decrease in germination when treated with 20 mAi NaCl; germination of seeds of C. distyla, C. inophloia, C. littoralis, C. luehmannii, and C. torulosa was not affected.

Allelopathy

Seed germination and plant growth sometimes are inhibited by a variety of naturally occurring chemicals (allelochems) produced by plants (Rice, 1984; Hytónen, 1992). Toxic chemicals are released by both roots and shoots of some plants by volatilization, leaching, exudation from roots, and decay of plant tissues. Allelochems include phenolic acids, coumarins, qui-nones, terpenes, essential oils, alkaloids, and organic cyanides (Rice, 1974).

Perhaps the best known allelopathic chemical is juglone (Fig. 2.8), produced by plants in the genus Juglans, which inhibits growth of neighboring plants. Several genera of plants that grow in red pine stands are potentially allelopathic. These include Prunus (Brown, 1967), Aster, and Solidago (Fisher et al., 1979). Water extracts of these plants did not inhibit germination of red pine seeds but adversely affected root and shoot growth of young seedlings (Table 2.4). Zackrisson and Nilsson (1992) presented convincing evidence of allelopathic effects of crowberry on regeneration of Scotch pine in Sweden. High dosages of allelopathic compounds occurred in forest soil when ground ice was present. At other times these toxins were detoxified by microorganisms.

Nilsson (1994) studied allelopathic effects of crowberry and root competition on growth of Scotch pine seedlings. Allelopathic effects were reduced by spreading activated charcoal on the soil to absorb the toxins that were leached from crowberry leaves and litter. Belowground competition was reduced by growing individual Scotch pine seedlings in plastic tubes. These experiments showed that two different growth-inhibiting mechanisms of crowberry were operating and that both belowground competi-

Table 2.4 Effect of Water Extracts of Leaves from Six Species and Distilled Water Control on Dry Weights of Roots and Shoots and Root-Shoot Ratios of Red Pine Seedlings after Seven Weeks of Treatment"

Root Shoot Root-shoot ratio

Weight* Percentage Weight* Percentage Percentage

Effect (mg) of control (mg) of control Ratio* of control

Weight* Percentage Weight* Percentage Percentage

Effect (mg) of control (mg) of control Ratio* of control

Control

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