Diversity in Leaf Blade Anatomy Among Philippine Hoya R. Br. Species

With over 100 species indigenous to the Philippines, Hoya R. Br. is the most species-rich genus of Family Apocynaceae in the archipelago. Most species of this genus are epiphytic, and some species are hemiepiphytic or terrestrial. Epiphytic species must have adaptive features that help them survive water shortages caused by not having their roots anchored to the soil. The anatomical features of the leaf blades of six species native to the Philippines Hoya [H. espaldoniana Kloppenb., Siar, and Cajano; H. halconensis Schltr. ex Kloppenb.; H. lacunosa Blume; H. lucardenasiana Kloppenb., Siar, and Cajano; H. madulidii Kloppenb.; and H. meliflua (Blanco) Merr.] were investigated, and emphasis was placed on the comparison between the epiphytic and non-epiphytic species. Diversity was observed among the six species. Two species (H. lacunosa and H. meliflua) have multiple upper epidermis consisting of two cell layers, contrary to the other species, which have simple epidermis. The three epiphytic species (H. espaldoniana, H. lacunosa, and H. lucardenasiana) have relatively thick cuticle on both upper and lower sides as well as homogeneous mesophyll with few air spaces. This feature helps prevent excessive water loss, thus, partially explaining how these species survive as epiphytes. On the other hand, the non-epiphytic species (H. halconensis, H. madulidii, and H. meliflua) have thinner cuticle layers. It was also observed that the epiphytic species have homogeneous mesophyll while the non-epiphytic species have heterogeneous mesophyll. Anatomical diversity in the Philippine species of Hoya must have played a role in the diversification of the genus in the archipelago. Examining the anatomy of their roots and stems will help in further understanding this.

Keywords: Apocynaceae, Asclepiadoideae, epiphyte, mesophyll, multiple epidermis