March is the flowering time for the very famous Yellow Pui in Hong Kong. In fact, the very well-known Tree Cottons are in bloom with showy red flowers at the same time. However, not sure if you have seen any orange or yellow Tree Cotton in Hong Kong (i.e. Lung Cheung Road near Tin Ma Court and Kam Tin Road near Ha Ko Po Tsuen). Are they different tree species?
In fact, there is only 1 species of Tree Cotton, no matter it has red or yellow flowers, and the scientific name is Bombax ceiba. According to Yang, et al. (2015), the sequences similarity of DNA fragments between red and yellow flower Cotton Trees are as high as 99%. So in conclusion, no matter the color, there is only 1 species of Cotton Tree.
If there is only 1 species of Tree Cotton, why there are different colored flowers? Although there is no concrete answer at the moment, some possible explanations were provided. Researchers tried to associate it with gene mutation caused by environmental factors. Anthocyanin, is a plant pigment that forms the color of petals and its chemical structure can be changed to demonstrate different colors. In addition, anthocyanin is used for blocking UV, thus, researchers believe that Cotton Trees growing under different environments, such as dry-hot valleys versus rain-forests, which received long term influence of strong UV or other environmental factors, their off-springs might affect by natural mutation and then further selected naturally under the principle of “Natural Selection”. As a result, cotton trees from different origins might be eventually carrying different colors of flowers.
Nevertheless, anthocyanin is red, blue or purple in color. To demonstrate yellow color, anthocyanin might work with other pigments, like anthoxanthin, which requires further study in this topic.
Lastly, Tree cotton will fruit after flowering. Flowering and fruiting indeed are the life cycle of plants and different tree species perform seed dispersal by wind. Hence, the seeds of tree cotton would attach on a patch of cotton and spread through the wind. Because of the potential allergic effect for some patients, cotton produced by Tree Cotton has been an alternative for true cotton due to the impurities, dusts and molds contained in true cotton. A specialist doctor pointed out that the fruiting time of tree cotton usually take place during the flowering time for various tree species (e.g. Yellow Poinciana, African Tulip Tree and Yellow Pui). However, as the pollens are non-visible, people would assume the allergies were cause by Tree Cotton. After all, Tree Cotton is an important urban tree asset so it is hoped that we can appreciate the gratuitous contribution of it, rather than complain it.
Reference:
1楊建軍，田向楠，伍建榕，鄭豔玲，田斌及馬煥成。2015. 紅花木棉與黃花木棉核糖體DNA分子系統發育研究。中南林業科技大學學報35(4)，135-138
2. 何的煒醫生，2017. 木棉絮 真的與鼻敏感有關係嗎? https://www.drterrieho.com/single-post/木棉絮真的與鼻敏感有關係嗎https://www.drterrieho.com/single-post/木棉絮真的與鼻敏感有關係嗎
