Study On Autotoxicity In Continuous Cropping Obstacle Of Watermelon Plant

At present, with the development of watermelon industry and regularization of produce, continuous cropping obstacle problem of watermelon is more severe. The primary concern of continuous cropping problems is autotoxicity, and therefore, it is very necessary to study the mechanism of watermelon autotoxicity, to provide highly important theory for resolving continuous cropping problem of watermelon. A number of experiments were carried out. In the first series of experiment, studies were carried out on autotoxicity of root exudates from watermelon plant in a hydroponic system by activated charcoal (AC). The results showed that root exudates of watermelon increased the relative conductivity, which proved that membrane permeability was damaged, the phenols content of roots increased and declined chlorophyll content of watermelon plant, affected the photosynthetic material accumulation, resulting in retardation and inhibition of plant growth. Second set of experiments involved studying the effects of recycled solution in successive cropping with watermelon, the results showed as follows: Through watermelon root exudates, the GPOD activities in roots was low, AOS and MDA content in roots were accumulated, which reflected on root membrane lipid peroxidation, and the abnormal metabolism in watermelon seedlings, which inhibited seedlings growth. Phenylalanine ammonia lyase (PAL) activities of watermelon root were also evidently higher without AC. The higher activities of PAL, resulted in the high rates of phenols synthesis, which were poisonous to seedlings at a certain concentration. Addition of AC, reduced the activities of PAL in roots to a lower degree, and also the amounts of phenols synthesized were reduced. Due to the absorption of AC, autotoxicity of root exudates in recycled solution was alleviated. Autotoxicity of watermelon root exudates was also confirmed.All of the extracted substances from watermelon tissues had inhibitory effects on watermelon and lettuce germination and growth, In general, the inhibitory effects of extracts from the tissues were correlated to their concentrations. At a certain ranges of extracts concentrations, inhibition was increased with the extracts concentration increasing. Under low concentration, the inhibition wasn't shown, indeed, the promotion was exhibited. With the concentration increasing, the inhibition was enhanced. Inhibitory effects of leaf extracts and stem extracts were more evident than that of root extracts. Autotoxicity of watermelon stubbles correlated to the watermelon extracts components. Extracts with different solvents showed different effects on seeds germination and its growth. In this experiment, two species solvents, distilled water and 95% ethanol were used to test seed bioassay, which extracts with distilled water as a solvent exhibited more inhibitory effects than those of 95% ethanol. The substances extracted from root stubbles were classified into acidic, basic, water soluble parts, respectively, it was proved that all classes or parts of the substances could inhibit the seedlings' growth and the inhibitory effects were correlative to extracts concentration. Water soluble parts of the extracts with high concentration evidently restrained germination, contrarily, weren't inhibitory effects on germination. At the test concentrations ranges, lettuce germinations weren't influenced by acidic part and basic part. Accordingly, inhibition of the parts from root extracts were generally water soluble part >acidic part>basic part in order.Root exudates of watermelon plant were collected from no AC treatment solutionplanted watermelon and extracts of AC, which added into watermelon hydroponic system. The results showed that watermelon root exudates inhibited the growth of lettuce radicle and lettuce hypocotyls, and the inhibitory effects were markedly correlative to concentrations. When the root exudates in the watermelon hydroponic system accumulated to a certain concentration, the inhibitory effects were showed. Therefore whenever, watermelon roo...