Root sprouting ability and growth dynamics of the rootsuckers of Emmenopterys henryi, a rare and endangered plant endemic to China

Abstract

Emmenopterys henryi of the family Rubiaceae is an endangered species endemic to China and under China State protection (Category II). The population of wild E. henryi has declined rapidly due to habitat destruction, overexploitation, and poor natural regeneration. The E. henryi root sprouts are stimulated by external disturbances, such as injuries; moreover, this phenomenon might be an important means of regeneration and may mitigate its extinction. The E. henryi tree roots were treated in situ and subjected to four injury levels (control, scraping, peeling, and cutting), two exposure levels (exposed and buried), and three seasons (spring, summer, and fall). To study the effects of injury, tree ages, root diameters, and soil depth on the hormone contents in the root of E. henryi, we observed the growth dynamics and quantitative variance of the root sprouts. Our results showed that the callus areas of the injured roots vary significantly between different treatments, and the order of the ratios of callus areas was scraping>peeling>cutting; for different seasons, the order of the ratios of callus area was spring>summer>fall. Exposure stimulated root sprout formation, which decreased as soil depth increased. As the root diameter increased, the number of root sprouts formed in the physically damaged roots linearly increased with injuries but declined in the undamaged roots. As the E. henryi tree grew, the ability of sprouting of the root increased. Cutting significantly stimulated more sprouts than other injury treatments. However, most sprouts grew together with a high density on the callus, which resulted in high mortality. Initially, scraping and peeling stimulated fewer sprouts than cutting, but the stimulation time of the former two levels was longer and eventually resulted in more sprouts. Moreover, differences in root sprout ability among E. henryi trees are attributed to different hormones in the root. A high ratio of cytokinin (CK) to auxin (IAA) ratio means a strong stump stool sprout of E. henryi. Studies showed that artificial damage treatment to enhance the root sprouting ability is based on the increase in CK/IAA ratio in the E. henryi root in its original habitat. Based on the effects of different damage treatments on the amount and growth of rootsucker of E. henryi, treatment measures for scraping or peeling should be performed to stimulate the sprouting of E. henryi root to provide adequate sprouting seedlings and regenerate the population.