Populus species are among the fastest-growing forest tree species and are distributed throughout temperate regions of the northern hemisphere. Because of their fast growth and the ease of propagation, they have been considered ideal species for pulpwoods and lumber production. Particularly hybrid poplars have been studied for use in short rotation and biomass production.
Hybrid poplar, Populus alba L. x P. glandulosa Uyeki, has been extensively planted in Korea since it was artificially bred by the Institute of Forest Genetics of Korea in 1956. The superior traits such as drought resistance and faster growth than its parents made the hybrid a promising candidate for lumber production in short rotation in mountainous areas. However, its practical use has been restricted to making chopsticks, match splints, and boxes rather than pulpwoods due to its short fiber length. Another valuable feature of the hybrid is its biomass production capacity. Recently, there has been great interest in biomass production using fast-growing tree species. P. alba x P. glandulosa can be one of the candidates because it has good coppicing and sucker-producing ability.
Populus species, especially hybrids, are highly amenable to tissue culture, cell culture, and genetic manipulation techniques, which makes them a model tree species for in vitro manipulation.
Many of the advantages of vegetative propagation for poplars have been known for a long time. Using conventional methods of root suckers, grafting, and stem cuttings, poplars are reproduced clonally in large numbers for practical cultivation in many countries.
Silviculturally, stem cuttings are used for propagating clones not only in the nursery but also in the field, where unrooted cuttings are preferred over rooted cutting as an inexpensive and easy to handle planting stock. The poor rooting ability of poplars such as aspens is a major obstacle to propagating genetically improved stock. Some poplars, which show variable or poor rooting from cuttings can be improved by crossing with a related species that shows better rooting. Therefore, in recent years new ways for solving these problems have been attempted by the use of in vitro techniques. Also, extensive management methods using natural regeneration by root suckers, with a rotation period of 40 to 60 years, are possible on soils with good water and nutrient supply. Poplars are usually susceptible to diseases, insects, and other types of biotic and abiotic damage. Therefore, resistance to pests, especially to diseases, is a prerequisite to clonal plantings of poplars.
Anthocyanins are major secondary metabolites that have been studied in Populus species. Banning of synthetic red dyes in food products has stimulated interest in the development of pigments from natural sources. Although their primary role in the plant is related to coloration, roles in resistance to pathogens and pharmacological effects for man have also been reported. Since cell culture can be established easily in this hybrid, it can serve as a model system for the production of useful secondary metabolites in tree species.
Anthocyanins are a class of flavonoids that are responsible for the pink, red, violet, and blue color of the flowers. These compounds are water-soluble and accumulate in vacuoles. The red pigment production method may be applied independently of natural weather or soil conditions. Cell and tissue culture can be used to solve this problem. When compared with conventional sources and cell cultures as sources of secondary metabolites, cell cultures can be seen to possess a number of advantages, among them the ability to culture very large amounts of material in one locality if necessary, with no uncertainty in supply, and freedom from environmental factors such as drought and pestilence.
The anthocyanidins (aglycones) differ in the hydroxylation and methylation pattern of the B ring while having a common basic structure of rings A and C. The substitution pattern of the six most common anthocyanidins derived from this structure is specified. The most common O-methyl ethers are peonidin, petunidin, and malvidin, derived from cyanidin or delphinidin. In general, anthocyanidins are present in conjugated forms, bound to sugars. The resulting glycosides become water-soluble and are stored in the vascular sap of mature cells. Glycosidic variation leads to about 200 sugar derivatives of anthocyanidins.
Many cultures were found to be capable of anthocyanin production. In general, it can be assumed that plant cells cultivated in vitro retain the capacity to produce the same anthocyanins as in vivo. Anthocyanins are formed in callus and cell cultures of a number of plants such as strawberry, grapevine, Aralia (), buckwheat, Daucus carota () etc.
Hybrid poplar calli were obtained from explants of young stems on MS agar medium containing 0.5 mg/12,4-D, 0.1 mg/1 BA, and 3% sucrose. After 4 weeks of initial culture, red-colored spotted calli were obtained, and these were transferred to a light regime. The anthocyanin pigments have cyanidin 3-glucoside and 3,5-diglucoside, pelargonidin 3-glucoside, and 3,5-digluciside.
To verify the optimum culture conditions for anthocyanin production, cells were tested by changing the various salt concentrations, growth regulators, and sucrose concentration. Good cell growth was obtained in the light on modified MS medium containing half the concentration of nitrate and double concentration of basic phosphate of MS basal medium, 1 mg/1 IAA, and 7% sucrose. The highest yield of anthocyanin was obtained with 12.5% nitrate, 400% basic phosphate concentration of MS basal medium, 1 mg/1 IAA, and 7% sucrose.
After several subcultures, friable fast-growing and red-spotted callus was isolated by visual selection. By continuous cell aggregate selection and single-cell cloning, friable growth, and high anthocyanin-yielding callus was selected.
Because of their suitability for tissue and cell cultures, hybrid poplars have been extensively studied for biotechnological approaches, including production of useful secondary metabolites by cell culture.
Selections from the book: “Medicinal and Aromatic Plants IX” (1996).