Embryonic Development and Potential Causes of Reproductive Failure in Tilia taishanensis S. B. Liang

In this study, the rare and endangered tree species Tilia taishanensis S. B. Liang was selected as the research material, and its embryonic developmental characteristics and abortion mechanism were systematically observed using scanning electron microscopy and resin semi-thin sectioning techniques. The results showed that its pollen was bicellular, the anther wall belonged to the dicotyledonous type with a secretory tapetum, and asynchronous meiosis occurred during microsporogenesis. The pistil possessed a pentalocular ovary and anatropous ovules, and the embryo sac exhibited a typical seven-celled, eight-nucleate structure of the Polygonum type. Male gametophyte development initiated 1–2 weeks earlier than female gametophyte development, although both gametophytes matured synchronously in late June, and the embryo and endosperm developed following the Solanad type and nuclear type, respectively. Multiple reproductive barriers existed in this species: the pollen viability was only approximately 10%, and abnormal tapetal degeneration interrupted nutrient supply for microspores; continuous embryo sac degeneration may contribute to a hollow seed rate of 90%; developmental asynchrony between male and female gametophytes further reduced fertilization efficiency. This study explores the embryonic developmental characteristics and abortion mechanism of T. taishanensis, providing cytological evidence for elucidating its endangered causes and germplasm conservation.