The spectral properties of Hypericum perforatum leaves: the implications for its photoactivated defences

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Abstract
Unlike most other plant-derived phototoxins that are activated by ultraviolet light, hypericin from Hypericum species causes photoactivated damage by absorbing visible light (550–610 nm, maximum at 585 nm). Clear glands from Hypericum perforatum L. transmitted 66%, veins, 6% and the mesophyll, 1% of the light at 585 nm. When the total area of the various structures was taken into account, leaves transmitted approximately 2% of the light in the photoactive range of hypericin. Other studies have shown that this intensity of light is sufficient to cause light-induced mortality in insects fed on artificial diets containing hypericin. Having a phototoxin that is activated by the same wavelengths of light that are reflected and transmitted by plants may prevent insects from avoiding phototoxicity by simply hiding under or rolling leaves. However, a survey of herbarium specimens indicated that clear glands were not an obligate component of the photoactivated defences of Hypericum species.