Preliminary study of allelopathy of root exudates of Chinese fir

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is a major commercial tree species in South China producing about one fourth of the Nation's total high quality commercial timber. Because of the growing need for timber, pure Chinese fir plantations have been extensively replanted on the same site in successive rotations. The resulting soil degradation has resulted in poor establishment and decline in productivity which have become a major barrier to the sustainable management of Chinese fir plantations. Many documented studies suggest that allelopathy within Chinese fir plantations is likely one of the causes of soil degradation. According to some laboratory studies, extracts of Chinese fir roots, fresh leaves, and litter including phenolics such as vanillic and ferulic acids, e.g., p-hydroxybenzoic, o-coumaric, m-coumaric and p-coumaric acids, inhibit Chinese fir seed germination. However, the allelopathy of root exudates from Chinese fir plantations has never been reported. Live lateral roots from Chinese fir were collected from both pure- and mixed-species plantations. Exudates were extracted and concentrated with a revolving evaporator to four different levels of 0.5, 1, 2, and 4 times original exudate concentration. Distilled water was used as control. Twenty viable, disinfected seeds were placed on filter paper in petri dishes. Each dish was treated with 1ml of exudate, irrigated once a day, and incubated at 25℃. Each treatment had three replicates. At the end of the 20-day incubation period, fresh weight, radicle length, and stem length of germinants were recorded. Chinese fir root exudates at the concentration of 4x were shown to significantly (p 0.05) inhibit the growth of Chinese fir germinants. The fresh weight of germinants treated with 4x root exudates of Chinese fir from the pure Chinese fir plantation of the first rotation, second rotation, and mixed-species plantation was reduced by 36.9%, 31.6% and 10.4%, respectively. Radicle length was reduced by 60.5%, 48.3% and 18.3%, respectively. And stem length treated with 4x root exudates of Chinese fir from the three plantations was reduced by 21.4%, 16.7% and 13.1%, respectively. Differences in the effects of 4x root exudates on the growth of Chinese fir germinants from the first rotation and the second rotation of pure Chinese fir plantations were insignificant (p 0.05). Differences in effects of 4x root exudates on the growth of Chinese fir germinants from pure and mixed plantation were significant (p 0.05). The effects of 4x root exudates of Chinese fir on the growth of Chinese fir germinants decreased in order from Chinese fir plantations of first rotation, second rotation, and mixed species. As allelopathic effects of 4x root exudates of Chinese fir from the mixed-species plantation were significantly lower than those from the pure plantation, developing mixed-species plantations might be an effective approach to resolving the problem of low productivity of replanted pure plantations.