Environmental Engineering Reference
In-Depth Information
22.2 morPholoGy and taxonomy
Switchgrass grows 0.5-3 m tall and most plants are caespitose in appearance, with an occasional
rhizomatous plant. Caespitose plants have short rhizomes and can form a sod over time. The
inflorescence is a diffuse panicle 15-55 cm long with spikelets toward the end of long branches
(Hitchcock 1951; Gould 1975). Spikelets disarticulate below the glumes and are two-flowered with
the upper floret perfect and the lower floret either empty or staminate. Spikelets are 3-5 mm long
and florets are glabrous and awnless. The lemma of the fertile floret is smooth and shiny. Leaves
have rounded sheaths and firm flat blades that can vary from 10 to 60 cm in length. The number
of leaves per culm will vary depending on genotype and environment (Redfearn et al. 1997). The
ligule is a fringed membrane 1.5-3.5 mm long and consists mostly of hairs. Switchgrass reproduces
by seeds, tillers, and rhizomes. It has the Pancoid type of seedling root (Newman and Moser 1988;
Tischler and Voigt 1993). Roots of established plants may reach depths of 3 m (Weaver 1954).
Seed consists of the indurate and smooth lemma and palea, which hold tightly to the caryopsis. The
margins of the lemma are enrolled over the margin of the palea. Glumes are almost entirely removed
by combining and cleaning. On the average there are approximately 850 seeds/g (Wheeler and Hill
1957). Seed weight differences exist within and among cultivars. As an example, genotypic variation
in seed mass within the cultivar Sunburst has been reported with a range of 450-850 seeds/g (Vogel
2002). Switchgrass is easily threshed, cleaned, and planted with commercial planting equipment. A
switchgrass seed industry has existed for over 50 years and numerous private companies and public
crop improvement associations are involved in seed production, distribution, and marketing.
Switchgrass has two distinct ecotypes—upland and lowland (Brunken and Estes 1975). As the
names suggest, the lowland ecotype was originally found on flood plains and riparian zones subject
to occasional flooding and/or waterlogging. The upland ecotype was originally found in upland
areas that were not subject to flooding and often prone to drought. Plants of the upland and lowland
ecotypes are morphologically and genetically distinct from each other. Generally, lowland plants
have a later heading date, taller plant height, larger and thicker stems, fewer stems per plant, more
upright leaf blades, and a more bluish cast than upland plants. Upland and lowland plants can be
easily crossed with each other (Martinez-Reyna et al. 2001) and intermediate types exist in nature,
suggesting that upland x lowland crosses have occurred in natural ecosystems, despite the large dif-
ference in heading date between upland and lowland ecotypes.
22.3 GenetIcs
Switchgrass has a basic chromosome number of
x
= 9 (Gould 1975). A wide range of chromosome
numbers has been reported in the literature including somatic counts of 18, 36, 54, 72, 90, and 108
chromosomes (Nielsen 1944; Barnett and Carver 1967). Switchgrass has small chromosomes that are
difficult to count. Recent studies aided by the use of flow cytometry indicate that most switchgrass
cultivars are either tetraploid (2
n
= 4
x
= 36) or octaploid (2
n
= 8
x
= 72) (Hopkins et al. 1996; Lu
et al. 1998). The tetraploids and octaploids average 3.1 and 6.1 pg 2C
−1
DNA (Lu et al. 1998). The
2C (“C” stands for “constant”) value is the DNA content of a diploid somatic nucleus expressed in
pg (picogram or 10
−12
g) and can be converted to daltons or nucleotide pairs using the formulas:
1 nucleotide pair = 660 Da; 1 pg = 0.965 × 10
9
nucleotide pairs (Bennett and Smith 1976). To date,
all lowland plants appear to be tetraploids on the basis of chromosome counts of mitosis in root tips
or flow cytometry analyses, although most upland plants are tetraploids or octaploids. Tetraploid
and octaploid plants occur sympatrically in over half of the remnant prairies that were evaluated by
Hultquist et al. (1997). They did not report hexaploid plants in remnant prairies. Several researchers
have attempted to relate ploidy levels to morphological traits and geographical distribution, but the
results were inconclusive (Nielsen 1944, 1947; McMillan and Weiler 1959; Barnett and Carver 1967).
There are numerous reports of aneuploidy in switchgrass, particularly at the higher ploidy levels
(Costich et al. 2010), but some disagreement as to the frequency and severity of this phenomenon.
