Optical properties and contribution of cuticle to UV protection in plants: experiments with apple fruit.

To assess the UV-screening capacity of plant surface structures, the optical properties of isolated cuticle and detached peel of apple fruit (Malus domestica Borkh., cv. Antonovka) have been studied. It was found that the cuticle exhibits considerable scattering of UV radiation, negligible absorption between 500-800 nm and attenuates UV radiation: on average, cuticular transmittance of non-reflected light amounts to 35.7 +/- 20.2 and 14.2 +/- 7.1% at 375 and 300 nm, respectively. The principal UV-A absorbers in the cuticle were identified as quercetin glycosides with an in vivo absorption maximum near 375 nm and content ranging from 10 to 70 nmol cm(-2). On the shaded side of apple fruit, both UV-A and UV-B absorption by the peel is, to a large extent, governed by cuticular phenolics, whereas on the sunlit surface, the absorption of the peel in the UV-A range is determined mainly by vacuolar peel flavonoids. It is concluded that a massive build-up of flavonoids in the peel cells located just below the cuticle, resulting in trapping of radiation in a broad spectral range, plays a dominant role in the long-term adaptation of apple fruit to elevated levels of solar radiation.