Parthenium argentatum

Parthenium argentatum, commonly known as the guayule (ɡwaɪˈuːliː or waɪˈuːleɪ, as in Spanish), is a perennial woody shrub in the family Asteraceae that is native to the rangeland area of the Chihuahuan Desert; including the southwestern United States and northern Mexico. It was first documented by J.M. Bigelow in 1852 through the Mexican Boundary Survey and was first described by Asa Gray. Natural rubber, ethanol, non-toxic adhesives, and other specialty chemicals can be extracted from guayule. An alternative source of latex that is hypoallergenic, unlike the normal Hevea rubber, can also be extracted. While Castilla elastica was the most widely used rubber source of Mesoamericans in pre-Columbian times, guayule was also used, though less frequently. The name "guayule" derives from the Nahuatl word ulli/olli, "rubber". Guayule grows in rocky, limestone desert areas in full sun. The plant's outer branches and leaves are covered in fine silvery hairs called trichomes, and yellow-white flowers grow from stems at the top of the plant. The densely haired leaves are covered with white wax to help prevent drying. The plant has an extensive root system, which lends to its drought resistance. One taproot extends down, while lateral roots extend from the taproot to the side. In some plants, lateral roots are longer than the taproot while in other plants, the opposite is true. U.S. indigenous populations of guayule occur in the Trans Pecos region of southwestern Texas. It can also be found in the low desert regions of Arizona, New Mexico, and some parts of Southern California and the Mexican states of Zacatecas, Coahuila, Chihuahua, San Luis Potosí, Nuevo León, and Tamaulipas and is able to be cultivated in similar climates around the world. Guayule breeding programs have been facilitated in order to domesticate, commercialize, and develop higher yielding cultivars. Selection of high-yielding guayule is complicated by its breeding system, which is primarily apomixis (asexual cloning via gametes).