Seasonal changes of furanocoumarin concentrations in leaves ofHeracleum lanatum

Concentrations of three dermatitis-inducing furanocoumarins— xanthotoxin, bergapten and psoralen—were measured in whole leaves ofHeracleum lanatum and in extracts of the leaf surface over an entire vegetative season. The concentrations of surface furanocoumarins, localized by extraction involving brief dipping in almost-boiling water followed by HPLC quantitative analysis, increased until the middle of May and decreased until maturity. The concentration on autumn leaves (new growth) was 20–100 times as high as the ones in May, or those of similar size in April. Furanocoumarin concentrations in the whole leaf at different stages of leaf development varied, being the highest April 25, then decreasing sharply with rapid leaf enlargement. Again, in the small autumn leaves the coumarin concentration was two to three times that in April. Seasonal changes in surface furanocoumarins may be important in that these compounds are postulated to form the first defense barrier of the plant.