One method for the development of new hostas is the technique of tetraploid conversion. With the application of chemicals such as colchicine and oryzalin, hybridizers and growers are able to create plants with extra sets of chromosomes. While diploid hostas have two sets of chromosomes, tetraploids possess four sets. Hostas which are tetraploid generally have several advantages over their diploid counterparts including increased substance, more vibrant colors, wider margins, larger flowers with thicker petals, and stiffer leaf petioles.
The chemical known as colchicine is derived from the Autumn Crocus (Colchicum autumnale) and is highly poisonous to humans, so it must be handled carefully in the laboratory. An alternative, less toxic chemical, oryzalin, is commonly used today as a pre-emergent herbicide, but can also be used to induce ploidy in plants.
Some of the first chemically-induced tetraploids came about by accidental exposure to an herbicide at Mobjack Nurseries in Virginia. Some containers of Hosta 'Francee' were sprayed with oryzalin, which resulted in two plants with wider leaf margins and brighter variegation. John Machen named and registered these new hostas in the early 1990s as 'Patriot' and 'Minuteman'.
Intentional exposure of hostas to chemicals yields a wide range of results - stunting, twisting, changes in foliage color and variegation. Sometimes plants die immediately after treatment. On the other hand, occasionally there is no apparent change at all. Hybridizers, however, look for that one plant which exhibits the characteristics of a tetraploid.
Occasionally, the conversion process is shown to be incomplete. Some cell layers of the plant may become tetraploid while others layers remain diploid. Hostas with layers having different ploidy are known as ploidy chimeras.
During the last fifteen years, the practice of intentionally converting hostas to develop new cultivars has become much more common, both in the United States and Europe. A list of these plants is provided below. The name of the converted plant is given first, followed by the diploid counterpart.
1"As for the origin of 'Patriot' and 'Minuteman', apparently a pre-emergence herbicide, oryzalin, used for weed control in Machen's Mobjack Nursery in Virginia, unintentionally drifted onto some 'Francee' plants, causing chromosomal doubling. So 'Patriot' and 'Minuteman' are unintentionally chemically induced tetraploid sports of 'Francee'." Pollock, Warren I. "What's in a Hosta Name? Part LIII." The Hosta Journal, Volume 42, Number 2 (2011):69-70. The same is likely true of 'Grand Tiara', 'Liberty', and 'Night before Christmas' which are additional polyploid sports discovered at Mobjack Nursery.
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