An analysis of the costs and benefits of the cyanogenic system in Trifolium repens L.

The effect of the cyanogenic glucosides linamarin and lotaustralin and their hydrolyzing enzyme linamarase was studied in a B2 generation segregating for the genes Ac and Li. Plants containing the glucosides are protected against grazing by snails both in the seedling stage and as adult plants. In seedlings, however, there is a direct effect on survival, whereas in adult plants the leaf area of plants containing linamarin/lotaustralin is less reduced under intense grazing. Linamarase has no effect on grazing by snails, possibly as a result of the presence of β-glucosidase activity in the gut of these animals. The genes Ac and Li, or genes tightly linked to them, have other effects as well: plants possessing one dominant Ac allele produce fewer flowers than homozygous ac plants. I compared this difference in flower production to the metabolic cost of producing the cyanogenic glucosides. The energy content of the difference in flower head production far exceeded the metabolic cost of cyanoglucoside production in Acac plants. It is possible that the cost of maintaining a certain level of cyanoglucosides is much more important for the plant than the initial cost of biosynthesis. The importance of the effects of Ac and Li in the maintenance of cyanogenic polymorphism in white clover is discussed.