Environmental Engineering Reference
In-Depth Information
years ago, switchgrass and other members of these prairie remnants rapidly moved north, colonizing
a wide range of habitats under increasingly longer day length, but shorter growing seasons. High
levels of genetic diversity within these remnant populations allowed natural selection to proceed for
traits necessary to survive in northern latitudes, including early flowering and cold tolerance.
As a result of natural selection, switchgrass ecotypes are photoperiod sensitive, requiring short
days to induce flowering (Benedict 1941). Photoperiod requirements are based on the latitude-
of-origin of individual ecotypes. Flowering is induced by decreases in day length following the
summer solstice. In North America, exporting northern ecotypes south exposes them to a shorter-
than-normal day length during summer months, which causes early flowering, often drastically
reducing their ability to utilize the full growing season because of early senescence (Vogel 2004).
The opposite occurs when southern ecotypes are exported north. They remain vegetative for a
longer period of time, with a longer photosynthetically active period, often producing more forage
than northern ecotypes (Newell 1968a). When grown in the central Great Plains, switchgrasses
from the Dakotas (northern ecotypes) flower and mature early and are short in stature whereas those
from Texas and Oklahoma (southern ecotypes) flower late and are tall (Cornelius and Johnson 1941;
McMillan 1959). This patterned response to photoperiod also occurs for switchgrass cultivars and
ecotypes grown in Europe where it is not native (Elbersen et al. 2003).
The photoperiod response also appears to be associated with winter survival and cold tolerance.
Southern ecotypes exported too far north will not survive winters because they stay vegetative too
late in the fall, lack the ability to store sufficient carbohydrates during winter, and lack sufficient
cold/freezing tolerance (Casler et al. 2004, 2007a). As a general rule, switchgrass germplasm should
not be exported more than one USDA Plant Hardiness Zone (Cathey 1990) north or south of its area
of origin because of these adaptation issues. At this time, the genetic regulation of these latitude-
associated traits is unknown.
Switchgrass is a cross-pollinated species with a gametophytic self-compatibility system that is
similar to the S-Z incompatibility system found in other Poaceae (Martinez-Reyna and Vogel 2002).
Pollen is dispersed by wind. Self-compatibility, as measured by seed set from bagged panicles, is
typically less than 1% (Talbert et al. 1983; Martinez-Reyna and Vogel 2002). A postfertilization
incompatibility system also exists that inhibits intermatings among octaploid and tetraploid plants
(Martinez-Reyna and Vogel 2002). The postfertilization incompatibility system between ploidy
levels in switchgrass appears to be similar to the endosperm balance number system found in other
species. The postfertilization incompatibility system is probably responsible for the relatively low
frequency of hexaploid plants in native prairies. The tetraploid and octaploid plants in native prairies
may exist as separate and distinct populations.
As switchgrass migrated north from the glacial refugia, wind pollination and migratory animals
that carried seeds, as undigested feed or as hitchhikers in fur or feathers, were likely responsible
for considerable genetic mixing along populational boundaries and, perhaps, over large geographic
regions. Cross-pollination balanced the effects of natural selection, causing some genetic
homogenization across sites and large amounts of genetic variation within sites (Casler et al. 2007b).
Even though the tallgrass prairie ecosystem is highly fragmented with only about 1% intact, a
vast array of genetic variability has been preserved both within and among prairie remnant sites.
Analyses of RAPD markers for plants collected from prairie remnants from the Dakotas to New
York suggest that these isolated prairie remnants continue to act as one large remnant population
that contains many subpopulations each capable of representing much of the variability present
within the population as a whole (Casler et al. 2007b). Although the destruction of the tallgrass
prairie and associated ecosystems was nearly complete 100 years ago, self-incompatibility and
polyploidy have served to preserve genetic variability within this species.
Because of the effects of photoperiod of flowering and the large temperature differential across
the north-south gradient of natural switchgrass populations, latitude is the most important factor
determining adaptation of switchgrass ecotypes and cultivars. Differentiation along east-west
gradients of switchgrass populations tend to be less obvious than along north-south gradients, most
