Abstract:

Chinese fir (Cunninghamia lanceolata) is a dominant, native commercial tree species in South China known for rapid growth and high quality timber. Replanting this species in pure stands has resulted in extensive areas that are poorly established and have low productivity. This serious problem has attracted nationwide attention. Recent research suggests that low productivity is caused by allelopathy. According to some laboratory studies, phenolics such as vanillin extracted from Chinese fir roots, fresh leaves, and litter inhibit seed germination and growth of seedlings. When soil phenolics have a concentration higher than the threshold of toxicity they will inhibit Chinese fir growth. However, it has been unclear how the phenolics affect the Chinese fir. The purpose of this study is to determine the concentration at which vanillin develops an allelopathic effect on nutrient absorbency in Chinese fir. The effects of different concentrations of vanillin on nutrient absorbency in Chinese fir seedlings were examined using a culture solution. The mother solution (MS) was made by dissolving nutrients (49.5g NH4NO3, 57g KNO3, 13.2g CaCl2·2H2O, 11.1g MgSO4·7H2O, 5.1g KH2PO4 and 1.179g Na2 EDTA) in 1500　ml distilled water. The mother solution of 1000 mmol·L-1 vanillin was made by dissolving 15.2g vanillin in 100 ml distilled water which was then diluted to six different levels of culture solution: 10, 1, 0.1, 0.01, 0.001, and 0.000　1 mmol·L-1, each with nutrient solutions of 0.5 MS. Distilled water was used as the control. The solution was adjusted to pH 5.1 with 100 mmol·L-1 HCl or NaOH. One-year-old Chinese fir seedlings were grown in separate plastic containers and ventilated every day using an electromotor. Each treatment was replicated three times. After seven days, concentrations of NO-3, NH+4, SO42-and HPO42- ions in the culture solutions were analyzed. Results showed that vanillin at higher concentrations inhibited uptake of these ions by China fir seedlings. The concentrations of residual NO-3 ion in the culture solution with vanillin at 10 mmol·L－1 and 1 mmol·L－1 were 5　704% and 1　561% higher than that in the control, respectively, indicating that vanillin could significantly inhibit uptake of NO-3 ion (p<0.01). At a vanillin level of 10 mmol·L－1, NO-3 ion in the culture solution was even greater than that at the beginning of the treatment. It appears that in the presence of vanillin the roots of Chinese fir seedlings produced a net output of NO-3 ion. The higher the concentrationof vanillin in the culture solution the more the nutrient ions remained in culture solution. This suggests that vanillin reduces the absorptive capacity of Chinesefir roots. The concentrations of residual NH+4 ion in the culture solution with vanillin at concentrations of 10 mmol·L－1 and 1 mmol·L－1 were 29.9% and 11.6% higher, respectively, than in the control. That is, the uptake of NH+4 by Chinese fir roots was lower in culture solution with vanillin than in the control, suggesting that vanillin significantly (p<0.05) inhibited the uptake of NH+4 ion. At concentrations of 10 mmol·L－1 and 1 mmol·L－1 vanillin, the residual SO42- in solution were 557.4% and 1 026.2%, respectively, greater than that in the control, indicating that vanillin significantly (p<0.01) reduced the uptake of SO42- ion.Using the culture solution with a vanillin concentration of 10 mmol·L－1,the residual HPO42- ion was 310.5% greater than that in the control. At the vanillin concentration of 0.01 mmol·L－1 the residual HPO42- ion was 32.5% lower than in the control. This suggests that 10 mmol·L－1 vanillin could significantly (p<0.01) inhibit uptake of HPO42- by Chinese fir seedlings, and that 0.01 mmol·L－1 vanillin could significantly accelerate uptake of HPO2-4 ion. The activity of the root system of Chinese fir seedlings was inhibited by vanillin. culture-solutions containing vanillin at 10, 1, and 0.1 mmol·L－1 activity of the root system was reduced 78.8%, 51.6% and 33.1%, respectively, compared with the control, indicating that vanillin significantly inhibited activity of the root system of Chinese fir seedlings. Lower activity of the root system decreased absorptive ability resulting in lower seedling growth. We conclude that vanillin at specific concentrations in the soil might produce allelopathic effects on Chinese fir seedlings by decreasing activity of the root system, resulting in inhibition of uptake of NO-3, NH+4, SO42- and HPO42- ions.