Interspecific relationship between Armeniaca hongpingensis T. T. Yu & C. L. Li and related species based on morphological characters

Armeniaca hongpingensis T. T. Yu & C. L. Li is a critically endangered species distributed only in Hongping Town, Shennongjia, Hubei Province, China. According to the Flora of China, A. hongpingensis and A. mume var. bungo Makino may be natural hybrids of A. mume Lam. and A. vulgaris Siebold. To date, however, no morphological study on the relationship between A. hongpingensis, A. vulgaris, A. mume, and A. mume var. bungo has been conducted. Through field investigations, 21 quantitative and 15 qualitative leaf, flower, fruit, kernel, and branch characters were obtained, followed by cluster analysis. Results showed that both PAM and UPGMA cluster analysis clearly divided the specimens into four species, indicating obvious distinguishing characters among them. Based on principal component analysis, the absolute weight values of annual branch color and hair-covering status, hair-covering status of leaf and fruit, leaf shape index (leaf shape, leaf tip, leaf base), and surface characters of kernel in the first three principal components were all above 0.9, indicating that these traits play important roles in the classification of the four Armeniaca species. Based on electron microscopy, the pollen grains were identified as single grain (monads), isopolar, radially symmetrical, and tri-colporate. The pollen surfaces of A. vulgaris, A. mume, and A. mume var. bungo were striate-ornamented, while the pollen of A. hongpingensis was discontinuously striped and showed inconspicuous perforation. The epidermal cells of the leaves of the four plants all showed striate keratinization. Compared to the other species, the stripes on the cell surface of A. hongpingensis were often curved, overlapped, and without fixed direction. According to cluster analysis and micromorphological characters, A. hongpingensis should be treated as an independent species from A. vulgaris and A. mume.