Trichomes and naphthoquinones protect Streptocarpus dunnii Hook.f. against environmental stresses

Streptocarpus dunnii Hook.f. (Gesneriaceae) grows wild in South Africa in rocky terrain and at high altitude. It possesses unique patterns of growth, is acaulescent, unifoliate and monocarpic and produces unique naphthoquinones. Morphological examinations of Streptocarpus species have tended to concentrate on germination, cotyledon anatomy and development of the primary leaf (phyllomorph) while mature phyllomorphs are rarely examined. This study sought to examine the morphology of the mature phyllomorph and to more accurately determine the type and site of naphthoquinone(s)excretion. Plants were developed from invitro plantlets originating from authenticated seeds and allowed to grow in a peat-based potting compost for a year, under ambient room conditions. Mature lamina samples were examined morphologically by light, stereo and scanning electron microscopy. Results showed anisocytic stomata only on the abaxial epidermis, with numerous non-glandular and glandular trichomes on both epidermises, especially at the proximal end of the phyllomorph. Energy-dispersiveX-ray analysis indicated both trichome types to be rich in potassium. Glandular trichomes were more abundant on the abaxial surface, especially at non-glandular trichome bases over veins and adjacent to stomata in lamina areas between veins. These trichomes were shown for the first time to be the source of the naphthoquinones in S.dunnii. Chromatographic and FT-IR analyses confirmed the glandular secretions as being mainly (±)-dunnione. Results suggest that mature phyllomorphs with densely packed epidermal trichomes and a restricted distribution of stomata are adaptations by this plant to dehydration stress, while dunnione-type naphthoquinones may function in the amelioration of oxidative stress whether of biotic or abiotic origin.