Environmental Engineering Reference
In-Depth Information
reduced relative biomass yields as it is moved east into the historic Eastern Forest biome (Hopkins
et al. 1995a; Casler and Boe 2003).
Gene pools are defined so that germplasm originating within any region can be utilized at other
sites within that region without concern about lack of adaptation or fear of contaminating local
switchgrass populations with exotic genetics (Vogel et al. 2005). The switchgrass gene pool concept
is intended to apply to any use of switchgrass germplasm, including development of germplasm pools
and breeding populations, parental materials for cultivar development, and deployment of ecotypes
and/or natural-track cultivars for use in conservation and restoration projects. The northern gene
pools of the United States likely apply to regions of southern Canada that share similar climatic and
edaphic conditions, as evidenced by adaptation of numerous switchgrass cultivars in eastern Canada
(Madakadze et al. 1998, 1999b).
22.4 PhysIoloGy and GroWth
The germination and growth of switchgrass seedlings are reduced at soil temperatures less than
20°C (Hsu et  al. 1985a, 1985b). Consequently, the recommended seeding dates for switchgrass
correspond to those for maize ( Zea mays L.). Switchgrass seedlings have the panicoid seedling
morphology and seedlings emerge by elongation of the mesocotyl or the subcoleoptile internode,
which pushes the crown node and the coleoptile, which stays short, to the soil surface (Hoshikawa
1969; Newman and Moser 1988; Tischler and Voigt 1993). When the coleoptile reaches the soil
surface, light induces the mesocotyl to stop elongating. Adventitious roots, which are necessary
for seedling and plant survival, arise from the crown node at the base of the coleoptile near the
soil surface. Planting seed deeper than 1 cm can adversely affect field establishment because more
seedling reserves are required for mesocotyl elongation. Dry soil conditions at the soil surface
can prevent seedlings from developing adventitious roots to ensure survival, therefore planting
dates should be targeted for periods when the probability of rain is high and soil temperatures are
sufficiently high for germination (Smart and Moser 1997). Planting too late in the summer will result
in stand failures because seedlings will not have adequate time to become established, transition
from juvenile to adult phase, and develop the root reserves necessary to become perennial.
Within 6 weeks of emergence several tillers may be produced. Growth of switchgrass in the
establishment year depends upon soil moisture, fertility, and competition from weeds and other
plants. Switchgrass does not require vernalization to induce flowering. Under optimum conditions,
switchgrass will produce seed in the establishment year but flowering occurs several weeks later
than in following years. This delay in flowering and seed ripening is likely due to the ineffectiveness
of floral induction before the transition from juvenile to adult phase (Poethig 2003). Growth of
switchgrass during the establishment year is slow relative to many other grasses, largely because
many plant resources are being devoted to development of an extensive root system.
New growth in the spring is initiated from axillary buds on the stem, crown, or rhizomes
(Heidemann and Van Riper 1967; Sims et al. 1971; Beaty et al. 1978). The relative amount of new
growth from each type of bud varies with ecotype and strain. Bunch types apparently produce new
tillers from both crown buds and rhizomes (Heidemann and Van Riper 1967; Sims et al. 1971) but
sod-forming plants produce new tillers primarily from rhizomes (Beaty et  al. 1978). Depending
upon the physiological stage and environmental conditions, new growth may be initiated after
harvest from all three types of buds. Plants with short rhizomes produce bunch-type plants, which
can be pushed above the soil line by roots, whereas sod-forming plants have longer rhizomes (Beaty
et  al. 1978). The growth and development of a switchgrass plant depends upon its genotype and
the location where it is evaluated. The development of switchgrass is location dependent because
flowering depends on photoperiod as discussed previously but also growing-degree-days (GDD)
which measure accumulated heat or photosynthesis energy.
The physiological development of switchgrass as determined using a maturity staging system (Moore
et al. 1991) is highly correlated to day-of-the-year (DOY) and GDD in temperate climates such as the
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