However, there is lack of data on the natural levels of endogenous plant compounds in both transgenic and conventional pest-protected plants and on how these levels vary with the plant's environment (see section 2.4.1, section 3.2.4, and section 3.4.1).
In addition, any changes in the use of traditional pesticides may be considered in parallel to these pleiotropic effects, as the benefits of decreased use of toxic pesticides may offset the risks of increased toxicity due to genetic modification.
Secondary plant chemicals (chemical compounds that are not required for normal growth and development of the plant) can be toxic to humans and other mammals (Senti and Rizek 1974), and the concentrations of these chemicals can be changed, either purposely or inadvertently, by conventional or transgenic genetic modifications. For example, potatoes, a major source of starch and good-quality protein, contain toxic glycoalkaloids to which humans appear to be more sensitive than other mammals. The modes of action seem to be cholinesterase inhibition in the nervous system and disruption of cell membranes in other organ systems (Friedman and McDonald 1997). This is similar to the mode of action of organophosphate and carbamate insecticides. Many factors, both genetic and environmental, affect glycoalkaloid concentrations in the potato and distribution to different plant parts.
A number of confirmed cases of human poisoning have been caused by potatoes with high glycoalkaloid concentrations (Friedman and McDonald 1997). The conventionally-bred Lenape variety of potato is an example of why great caution must be exercised in the development of new varieties of food plants that contain human toxins. The Lenape variety was developed by crossing Solanum tuberosum and S. chacoense to incorporate resistance to certain pests based on leptines (Sturckow and Low 1961). This variety was released for commercial use in 1967 (Akeley et al. 1968), but was soon withdrawn from the market (Sinden and Webb 1972). The new variety was indeed resistant to pests and had other desirable characteristics, but there were reports of illness, caused by ingesting tubers with high glycoalkaloid content (Zitnak and Johnson 1970). In Sweden, a popular commercial potato variety, Magnum Bonum, was withdrawn from the market for similar reasons (Hellenas et al. 1995).
Another problem of potential importance, the appearance of toxins that were not present in the parental lines, also has been demonstrated in potato. Somatic hybrids between Solanum brevidens and S. tuberosum contained the steroidal alkaloid demissine, not found in either parental line. Laurila et al. (1996) advanced the plausible hypothesis that the hydroge-