The Study On Allelopathy Under Nutritional Stress Of Macronutrient Element In Watermelon(Citrullus Lanatus)

Continuous cropping obstacle problem of watermelon is the main factor to limit the industrial development. The grafting cultural is the major treatment to reduce continuous cropping obstacle in recent years. The primary obstacle of continuous cropping problems is allelopathy, and therefore, it is very necessary to study the mechanism of watermelon allelopathy. A number of experiments were carried out. And the allelopathy effects of macro-elements stress in watermelon were not reported as yet. The experiment adopted hydroponic culture, biological testing and GC/MS to study allelopathic effects of root exudates and stem-leaf extracts of watermelon under the stresses of different macro-elements(N,P,Ca,Mg) on the watermelon own-rooted seedling.The main results were presented as follows:1. The root exudates, root extracts and stem-leaf extracts of watermelon under the stresses of different macro-elements inhibited the radicel and hypocotyl of seedling to some extent. In the nine treatments, the root exudates also inhibited the germination rate of watermelon seeds. But the root extracts of experiment treatments 1,4,9 and the stem-leaf extracts in treatment 4 promoted the germination rate of watermelon seeds.2. Different nutrients on the contribution of watermelon allelopathy had different values. N in the watermelon root exudates expressed the greatest inhibition on seedling radicel and hypocotyl growth process, and P and Mg contributed to the minimum. Mg in the watermelon root extract expressed the greatest inhibition on seedling radicel and hypocotyl growth process, Ca contributed to the minimum. Ca in the watermelon stem-leaf extract expressed the greatest inhibition on seedling radicel and hypocotyl growth process, N and Mg contributed to the minimum.3. The correlation analysis was done between the allelopathy effect indices resulted from nutritional stress and allelopathy RI values of watermelon radicel caused by the root exudates, root extracts and stem-leaf extracts in nine treatments. The results shown that the allelopathic effects of watermelon root exudates, root and stem-leaf extracts in different receptors were significantly correlated with the nutritional conditions. Watermelon root exudates and root, stem-leaf extracts co-presented significant correlations.4.In the nine different nutritional stress combinations,the root exudate combination N (15 mmol·L-1)+P (1 mmol·L-1)+Ca (4 mmol·L-1)+Mg (2 mmol·L-1) expressed the weakest inhibition on seedling radicel and hypocotyl growth process, while, the combinations N (7.5 mmol·L-1)+P (1 mmol·L-1)+Ca (4 mmol·L-1)+ Mg (2mmol·L-1) and N (7.5 mmol·L-1)+P (0.5 mmol·L-1)+Ca (2mmol·L-1)+ Mg(2 mmol·L-1) of root extracts and the combinations N(15 mmol·L-1)+P(1 mmol·L-1) +Ca (2mmol·L-1)+Mg (2 mmol·L-1) and N (7.5 mmol·L-1)+P (1 mmol·L-1)+ Ca (2 mmol·L-1)+Mg (2 mmol·L-1) expressed the same inhibiting effect.5. Thirty-one chemical substances were found by GC/MS in the root exudates, nineteen in stem-leaf extracts, nine in root extracts. The results indicated the effects of the root exudates and the allelochemicals on continuous cropping obstacle were different. There were obvious differences in the root exudates and the categories and contents of the stem-leaf allelochemicals among the nine treatments. The amount of nutrients stimulated the watermelon, then the watermelon released different allelochemicals to the environmental to response to environmental changes.