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A wide spectrum of simple and overlapping variations is now documented in plants (Sen and Sharma, 1990; Connoly et al, 1994; Stewart and Porter, 1995; Demeke et al, 1996; Sonnante et al, 1997; James and Ashburner, 1997). In general, all observed variations are broadly grouped into two categories: epigenetic and genetic. Genetic variations in plants are strictly heritable i.e. truthfully passed on from one generation to another through seeds and do not change under conditions of cultivation. They occur invariably due to alterations in the genetic material and may affect both phenotypic and chemical characteristics of a medicinal plant. Epigenetic variations on the other hand, are mostly induced by the environment in which the plants grow and are also partially affected by developmental events. Epigenetic changes in medicinal plants in general include morphological and chemical as well as physiological variations. Therefore a great deal of information- morphological, biochemical, physiological and genetic is necessary before the observed pattern of variation may be interpreted. It is also true that beneath these intraspecific variations, there exists a fixed unchangeable genetic spectrum of characteristics that makes up the species.

Definitive evidence for genetic variations became available only in late 1960s (Ayala and Kiger, 1980). This was provided by artificial selection experiments in cultivated crops such as rice, wheat etc. and laboratory model organisms like Drosophila. This kind of variation may be ultimately related to variation in the sequence of DNA base pairs, that constitutes the genetic code. Genetic variation may arise in individuals by gene and chromosome mutations which can be spread through the population by recombination. The occurrence of two or more indigenous forms in the same population/area and adaptability of these forms to different environment being maintained by natural selection thus became clear. In few such species showing balanced polymorphisms, the heterozygote is favoured compared to either of the homozygotes, and this maintains a high degree of genetic diversity in the species. In fact plants show a rather high degree of heterozygocity. Although reliable estimates of average heterozygocity are now available for more than 100 species of plants and animals, they do not include medicinal plants (Narain, 2000).

References for genetic variation among medicinal plants are scanty although analysis of such variations holds great promise owing to the location specific attributes of the herbs and the attendant diversity of plant-specific compounds of therapeutic and industrial value. It is also now understood that the loss of genetic variation within a given species (genetic depletion) is usually much more serious and occurs much earlier than the total extinction of the species itself. In the case of Dioscorea zingiberensis, plants collected during 1950s accounted for a maximum of 17% diosgenin whereas during 1980s the content was reduced to such an extent that even 4% was considered as high (He and Sheng, 1997). The only solution proposed to this rapid exhaustion of diversity is selection and cultivation of promising chemotypes. Standing examples in this line are Coleus forskohlli (Hegde and Gangadharappa, 1997), Chamomilla recutita (Bettray and Vomel, 1989), Mentha arvensis (Tandon et al, 1998), Aconitum napellus ssp. Tauricum (Colombo et al, 1989), Valeriana officinalis (Bos et al, 1998), Mentha spicata (Misra et al, 1989), Eucommia ulmoides, Coptis chinensis, Magnolia officinalis, Louicera japonica and Tripterygium wilfordii (Anonymous, 1989; Peng and Xiao, 1993).

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