Histological localization of tannins at different developmental stages of vegetative and reproductive organs in Medinilla magnifica (Melastomataceae)

• Tannin cells in vegetative organs are distributed near the vascular tissues.
• The leaves have tanniniferous palisade mesophyll.
• Tissues of the carpel are densely infiltrated by the tannins until fruit set.
• Tannin density decreases through development and maturation of flowers and fruits.
• Adaptations for defense and seed dispersal may be related to the tannin distribution.

Medinilla magnifica Lindl. (Melastomataceae), a Philippine-native shrub, has been found to produce diverse polyphenolic compounds, including novel forms of tannins with bioactive properties. Despite the great deal of explorations that were made on the tannin constituents of the plant, localization of the compounds in its organs has not been described. Taking this into consideration, we examined the anatomy of M. magnifica with particular focus on the distribution patterns of tannins in its vegetative and reproductive organs. Analyses were performed by scanning electron and light microscopy where tannin cells were made prominent by histochemical stains and iron salt solution. It was revealed that tannin cells have relatively defined patterns of distribution in the plant organs, with specificity of accumulation around or near the vascular tissues. The leaf amassed tannins in the palisade mesophyll and in the ground tissues surrounding the veins with greater concentration at the apical region. Tannins were less abundant in the stems and adventitious roots but were still concentrated near the vascular tissues. The reproductive structures displayed a decreasing tannin concentration through maturation but with maintained density in tissues surrounding the carpel until fruit-set. The site-specific distribution of tannin cells could indicate their protective role to vital structures against pathogens and herbivores, whereas the decrease of tannins in the tissues of developing flowers and fruits could be an apparent adaptation to make the fruits more palatable to potential seed dispersers.