| At present, with the development of watermelon industrialization, specialization and regularization, continuous cropping obstacle problem of watermelon is more severe. The grafting cultural is the major treatment to reduce continuous cropping obstacle in recent years, and the allelopathic effects of grafted watermelon were not reported as yet. In this paper, the allelopathic effects of grafted watermelon were systemically studied using hydroponics. We identified allelochemicals in root exudates of the watermelon seedlings, the gourd seedlings and the grafted seedlings by GC-MS, and the root exudates was collected with Amberlite XAD-4 resin and the allelochemicals in stem-leaf were extracted with ultrasonic oscillation., and at one time tested on watermelon seeds germination. The main results were presented as follows:1.There were obvious differences in the root exudates and the categories and contents of the stem-leaf allelochemicals among the three kinds of seedlings. Forty-one chemical substances were found, in which there were 16 esters,4 amines,3 ketones,2 alcohols and 2 benzenes and 1 species of nitrile, ether and oxime respectively. In comparison with own-rooted seedlings, the contents of some amines increased while the contents of esters and ketones decreased in grafted seedlings.2. The root exudates and the categories and contents of the stem-leaf allelochemicals in the own-rooted seedlings also had obvious differences, including 22 root exudates,19 stem allelochemicals and 13 allelochemicals. The results indicated the effects of the root exudates and the allelochemicals on continuous cropping obstacle were different.3. The root exudates and the root and stem-leaf aqueous extract affected seeds germination rate and germination index varied with different concentrations, and both of them showed low concentration had promotion while high concentration had inhibition. At the concentration of 2.5μL/L, they had stronger stimulating effect than the own-rooted seedlings'. At 5μL/L, the root aqueous extract of the gourd promoted the germination index while inhibited the seeds germination rate. And at 10μL/L and 20μL/L, the own-rooted plant had the strongest inhibition. The impact of the aqueous extract on watermelon seeds germination was greater than the root exudates.4. The different treatments of the root exudates and the root and stem-leaf aqueous extract had the consistent effects on the root and stem-leaf natural weight of the watermelon seedlings. They had the positive effect at 2.5μL/L and had the negative effect at 10μL/L and 20μL/L. The inhibiting effect became stronger significantly with the increasing of concentrations. At 2.5μL/L, the root exudates and the aqueous extracts of the own-rooted plant had the weakest promotion while they had the strongest inhibition at 10μL/L and 20μL/L, but just the gourd and the grafted plants'had the promotion effects at 5μL/L. The stem-leaf aqueous extract of the own-rooted seedlings had the strongest negative effect on the root natural weight, and at the same time the root exudates had the strongest negative effect on the stem-leaf natural weight5. As a whole, the effect of the root exudates and the root and stem-leaf aqueous extract under different conditions on shoot height and root length was that low concentration to promote while high concentration to inhibit. At the concentration of 2.5μL/L, the root exudates and the root and stem-leaf aqueous extract of the own-rooted seedlings had the weakest promotion effect,but they had the strongest inhibition effect at 10μL/L and 20μL/L. And only the gourd plants and watermelon grafting promoted at 5μL/L6. The different treatments of the root exudates and the root and stem-leaf aqueous extract had the consistent effects on the root vigor of the watermelon seedlings as well, and the significant differences were observed among part of treatments. At the concentration of 2.5μL/L, the promotion effect of the gourds' was strongest in all, and at 5μL/L just the gourds'promoted. At 10μL/L and 20μL/L, the inhibition effect of the own-rooted plants' was strongest. At the low concentration, the promotion of the root exudates of the own-rooted seedlings was stronger than the stem-leaf, while the inhibition was weaker at the high concentration.
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