Platycladus orientalis, Thuja occidentalis, and T. plicata are evergreen conifers and from each species, cultivars have been selected to fill many niches in modern landscapes. Platycladus orientalis, oriental arborvitae, is native to China and Korea and is adaptable to a range of soil types and pH and is moderately resistant to deer browsing (Dirr, 2009). Thuja occidentalis, eastern or american arborvitae, is native to eastern North America and is tolerant of limestone soils, heat, and drought (Dirr, 2009). Thuja plicata, western red cedar or western arborvitae, is native to western North America from Alaska to northern California. Western red cedar forms a statuesque tree in the landscape, tolerates varying soil moisture, and is pH-adaptable (Dirr, 2009). Selections of these three species include varying growth forms, foliage morphology, and foliage color. Winter foliage color of available cultivars includes green, yellow, yellow–orange, bronze, brown, plum, and the range thereof; however, cultivars exhibiting green foliage during winter are the fewest in number (Dirr, 2009).
Foliage of most selections of the related japanese-cedar is yellow, brown, or bronze during winter. However, tetraploid forms of japanese-cedar have been reported to remain green during the photoinhibitory conditions of winter compared with diploid forms (Niwa and Sasaki, 2003). Previous research identified a technique to develop tetraploids (2n = 4x = 44) of japanese-cedar with the goal of developing new cultivars that remain green during winter (Contreras et al., 2010). The objectives of the current research were to evaluate the effectiveness of this technique and determine optimal treatment duration for doubling chromosome numbers in oriental arborvitae, american arborvitae, and western red cedar.
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