Hypericin content of Nova Scotia biotypes of Hypericum perforatum L

Photosensitization of livestock in Nova Scotia has often been attributed to St. John's wort (Hypericum perforatum L.), even though the weed generally occurs at low densities. This study's objective was to determine whether levels of hypericin, the photodynamic toxin causing photosensitization, were unusually high in Nova Scotia biotypes and whether levels increased in late spring, when photosensitization most commonly occurs. The mean hypericin content in vegetative growth of 11 Nova Scotia H. perforatum biotypes was 195 mu g g(-1) dry wt, whereas levels in biotypes from British Columbia and Australia were at least two- and threefold higher, respectively. Levels in field-collected plants were lowest in May (60 mu g g(-1) dry wt) and peaked in midsummer (160-280 mu g g(-1) dry wt). In a controlled-environment study, hypericin levels increased linearly with increasing temperatures. Furthermore, hypericin levels were not affected by infection by Colletotrichum gloeosporioides. Thus, the hypericin content of Nova Scotia biotypes is too low to be the cause of photosensitization in livestock.