Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 158

Salicaceae . The Salicaceae and the Flacourtiaceae and 29
other families are grouped under the Malpighiales of the
angiosperms and are native to and widely distributed
throughout the northern hemisphere. Poplars can be found
in hot and cold climates. There are about 35 species of
Populus in the northern hemisphere, and about eight species
of Populus are native to North America (Fig. 7.1 ). Its wide
distribution is evident from the fact that the wood of poplar
was widely used, rather than canvas, by Renaissance
painters in western Europe, with perhaps the most famous
example of art created on a poplar panel being the Mona Lisa
(Durant 1953). Poplars are dioecius, which means poplars do
not have male and female reproductive parts on the same
tree; rather, separate male and female trees are required to
reproduce through wind pollination. Because of this charac-
teristic, they can easily form natural hybrids.
Poplars typically are found along waterways for it is in
such settings that the many pollinated cottony seeds can fall
to moist soil conducive to successful germination. This life
cycle may be an advantage for the survival of poplars across
the Great Plains of the United States, where frequent prairie
fires from lightning strikes occur. The most widely
distributed tree in North America is the quaking aspen, or
Populus tremuloides (Robinson 1958). They also are
the most widely studied. Other important species found
worldwide include P. acuminata , alba , angustifolia ,
balsamifera , deltoides, fremontii , heterophylla , nigra ,
texana , trichocarpa, and welizeni (Robinson 1958). Poplars
also were present and widely distributed as far back in the
geologic record as the Cretaceous Period, more than
90 MYa.
The most well-known and broadly-used poplars, both for
commercial and phytoremediation applications, are not the
native specimens but hybrids. A hybrid is the result of the
cross pollination of two different species of the same genus.
Such cross pollination, called hybridization, was first
recorded in 1761 by the German botanist Jakob Gottlieb
K

parent plants of the same genus is to have the resultant
hybrid seedling contain the best genetic traits of each of
the parents, called heterosis.
The development of a hybrid poplar tree for phyto-
remediation purposes, for example, is similar to the long-
practiced method of plant domestication. The best examples
of plant domestication come from the development of cereal
grain crops, such as wheat or rice. The best forms of these
wild plants and others, such as sunflower, were intentionally
selected by Native North Americans over many thousands of
years to produce larger and larger seeds. In doing so, they
also ended up with a plant of annual ecology rather than the
perennial history it originated from. Detriments of this
change are discussed in Chap. 16.
The first hybrid poplar was created in 1912 as a cross
between the eastern cottonwood ( Populus deltoides ) from
the Midwest to eastern United States and native western
black cottonwood Populus trichocarpa . The result was
a sterile hybrid poplar, or a cottonless cottonwood. It
contained the rapid secondary and primary growth habits
of each species, respectively. Since that time, many hybrid
poplars have been created. To ease confusion, hybrids are
referred to by the initials of the first letter of each species in
the cross pollination. For example, the hybrid from the cross
pollination between Populus trichocarpa and Populus
deltoides was given the prefix TD; the female parent is listed
first. The prefix of the cross between Populus deltoides and
Populus nigra is, therefore, DN. Hybrids also are denoted by
numbers assigned by their crossers, such as 49-177, where
the first number refers to the cross number and the second
number refers to the seedling number. The numbering con-
vention refers to plants produced by the Washington State
University Poplar Research Program.
Much information is available about the genus Populus,
generated by those who have an economic interest in trees,
such as the short-rotation wood culture (SRWC) industry,
the paper and pulp industry, or state-based experimental
stations interested in the reforestation of strip-mined land
or landfills. This interest is economically driven, as 1 acre of
poplar trees can produce 10 dry tons/year within a 6- to 8-
year rotation, relative to 4 dry tons/year with a 50-year
rotation for Douglas fir. Recently, these parties, the private
sector, and federal government agencies such as the DOE,
have become interested in using biomass derived from
hybrid poplars as an alternative energy source, a practice
common in Europe. Probably one of the first agencies to
offer hybrid poplars to the general public in the United States
was the Northeastern Forest Experiment Station in the
1920s.
The economic interest in poplars also drove the recent
work in describing the entire genome of poplars. In this
sense, poplars can be considered analogous to the impor-
tance of fruit flies to geneticists. As reported in Sterky et al.

oelreuter (1733-1806) when he crossed Nicotiana
paniculata pollen with Nicotiana rustica (Gager 1934).
A hybrid should not be confused with a clone, which is a
vegetative propagation of an individual genotype that
maintains the genetic character of the original plant. A
clone is designated by the suffix cl . A clone also can be
artificially produced from the undifferentiated cells of
plants. This was discovered in the 1950s by Georges
Morel, who agitated undifferentiated plant cells to which
he added growth hormones. These cells then were separated
and grew into individual plants that were genetically identi-
cal to the source of the cells. Cultivars are clones derived
from closely related plants and given the suffix cv . The
production of hybrid poplars follows the process tradition-
ally used to produce desirable traits in other plants, such as
roses. The ultimate goal of any attempt at crossing different

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