| Aluminum toxicity is the most important limited factor of crop growth and production in acidic soil. Watermelon is one of the most important economic crops in China, and its cultivation area is increasing in recent years. Aluminum will lead to soil degradation, which will affect the production and quality of watermelon seriously. Plant roots can accumulate and secrete organic acids under aluminum stress. In recent years, the release of organic acids by plant roots has became a hot research in the mechanism of aluminum tolerance in plants, the related researchs provide a better theoretical basis on explaining the mechanism of aluminum tolerance in plants. In the present study, we chose two watermelon varieties of zaochunhongyu ?Al-resistant variety, HY? and zaomiwang ?Al-sensitive variety, ZM? to investigate the alleviating effects of citric acid on watermelon seedlings under aluminum stress (1500?mol·L-1; 600mg·kg-1), using different concentrations of organic acids (200,400,800,1200?mol·L-1; 200,400,600.800mg-kg''). We used water culture method and soil culture method to study the influences of exogenous organic acids ?citric acid, oxalic acid? on watermelon growth, physiological characteristics and rhizosphere soil micro ecology under aluminum stres, in order to find the role and mechanism in growth process to provid a theoretical basis on the mechanism of aluminum tolerance in watermelon. The results of this study were summarized as follows: 1. The results of the growth indexs of tow watermelons showed that aluminum stress had an obvious inhibitory effect on the root and aboveground plant growth of watermelon seedlings, with the root length, plant height, rootfresh weight and plantfresh weight decreased, the growth of ZM was more inhibited than HY. After adding organic acids, the toxicity of aluminum in HY and ZM had been effectively alleviated, the growth inhibition of watermelon had recovered modestly, and the root length, plant height, rootfresh weight and plantfresh weight were significantly increased. The alleviating effects of different organic acids species and concentrations on aluminum toxicity in HY and ZM was different. In general, the alleviating effects of the organic acids concentration of 400-800?mol·L-1 on the growth of watermelon seedlings were batter than that of the lowest concentration(200?mol·L-1) and highest concentration 1200?mol·L-1) under aluminum stress, and the alleviating effects of the two organic acids had a significant downward trend under the treatment of 1200?mol·L-1 concentration, especially oxalic acid. In addition, the alleviating effects of organic acids on the growth of ZM were more obvious than that of HY. and citric acid was better than oxalic acid.2. We studied the impacts of organic acids on physiological characteristics of watermelon seedlings under aluminum stress.The results showed that aluminum stress had a destructive effect on the root absorption function and the leaf cell membrane structure of watermelon seedlings, with the leaf electrolyte leakage, the Pro and MDA contents increased, the root activity decreased, and the activity of antioxidant enzymes changed. After adding organic acids, the root inhibition degree of watermelon seedlings was alleviated, and the root activity had increased significantly, the leaf electrolyte leakage, the Pro and MDA contents had all decreased, the activity of antioxidant enzymes had increased. So, organic acids could effectively alleviate the toxicity of aluminum on watermelon seedlings. But the effects of different organic acids species on aluminum toxicity in Watermelon seedlings were different. The changes of physiological indexes of Watermelon under citric acid treatment were more regular than that of oxalic acid treatment, with citric acid concentration increasing, the change trend increased first and then decreased, all indexes had the largest increase under the concentrations of 800?mol·L-1, and the treatment 1200?mol·L-1 began to have a significant decline. But the effects of oxalic acid concentration of 800-1200?mol·L-1 on the physiology of Watermelon seedlings under aluminum stress were better than other treatments. On the whole, the toxicity of aluminum stress on the physiological of ZM was more serious than that of HY, so the detoxification effects of organic acids for aluminum toxicity were better in ZM than in HY, and among all concentrations, the optimum concentration were 400-800?mol·L-1 of citric acid and 800-1200?mol·L-1 of oxalic acid.3. The results of the chlorophyll content and the chlorophyll fluorescence parameters of watermelon seedlings under aluminum stress showed that aluminum stress could result in the chlorophyll content of watermelon seedlings decreased, and the chlorophyll fluorescence parameters were changed, with Fo and NPQ markedly increased, Fm, Fv/Fo, Fv/Fm, Y ?II?, qP and ETR significantly decreased. Visible, aluminum stress which affected the photosynthesis of watermelon seedlings seriously could inhibit the chlorophyll synthesis of watermelon seedlings, make the PS ? reaction center damaged, reduce the energy that participate in the photochemical reaction and decrease the electron transport activity. After adding organic acids, the chlorophyll content of HY and ZM all increased significantly, and they increased significantly in the treatment of the concentration of 400-800?mol·L-1. Meanwhile, the organic acids could effectively weaken the effects of aluminum toxicity on the photoinhibition of photosynthesis and the destruction of photosynthetic apparatus. The changes of the chlorophyll fluorescence parameters of HY were consisten in both organic acids treatments. Fo and NPQ decreased significantly, Fm, Fv/Fo, Fv/Fm, Y ???, qP and ETR all increased at different degrees. The changes the chlorophyll fluorescence parameters of ZM in citric acid treatments were same to HY, but that in oxalic acid treatments were different. In oxalic acid treatments, the chlorophyll fluorescence parameters Fo of ZM decreased, Fm, Fv/Fo, Fv/Fm and ETR increased. Then, when the concentration of oxalic acid<800?mol·L-1, the chlorophyll fluorescence parameters NPQ, Y ???,qP of ZM increased, but they would begin to significantly decrease in the treatment of 1200?mol·L-1. So, the effects of different kinds of organic acids on the chlorophyll fluorescence parameters of watermelon under aluminum stress were different.4. The results of the Soil microbial quantities and the soil enzyme activities in watermelon Rhizosphere under aluminum stress showed that aluminum stress reduced the quantity of bacteria, ctinomycetes and soil enzyme activities of HY and ZM, but promoted the quantity of fungi. After adding organic acids, the inhibiting effect of aluminum on soil microbial quantities and soil enzyme activites in HY and ZM watermelon rhizosphere had been effectively alleviated, the the quantity of bacteria, fungi and ctinomycetes had increased modestly, and the activity of soil urease, invertase, phosphatase, catelase in watermelon rhtizosphere were also significantly increased. When the concentration of citric acid was 400-600mg-kg-'and the concentration of oxalic acid was 600-800 mg·kg-1, the growth of soil microbial in HY and ZM watermelon Rhizosphere was best, soil enzyme activities were also significantly higher than other treatments, the alleviating effects of citric acid were better than oxalic acid, and the effects on ZM were much better than on HY.In conclusion, aluminum stress could inhibite the growth of watermelon seedlings, decrease the physiological indexs of watermelon seedlings, destroy the photosynthesis of Watermelon seedlings, result in the soil microbial quantities and soil enzyme activites in watermelon rhizosphere decreased.The adaptability of HY to aluminum stress was stronger than ZM, and the toxicity degree of aluminum in HY was lower than in ZM. But the toxicity of aluminum in watermelon seedlings had been effectively alleviated after adding organic acids, the growth and physiological inhibition of watermelon had recovered modestly, the alleviating effects of organic acids on ZM were more obvious than on HY, and citric acid was better than oxalic acid. |
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