Changes Of Root Exudates In The Wheat-watermelon Intercropping System And Its Mechanism Of Resistance To Watermelon Fusarium Wilt

The continuous cropping of watermelon is widely increasing and the problems related with continuous cropping are becoming more and more serious.Continuous cropping has become the main factor restricting sustainable watermelon production.Appropriate intercropping can effectively solve the problem of continuous cropping and is beneficial for sustainable watermelon production.However,the mechanism underline this is still not clear.This study was designed to analyze the effects of root exudates of wheat and watermelon on watermelon seed germination,seedlings growth and the growth of Fusariu oxysporum f.sp.niveum?FON?;the effects of wheat-watermelon intercropping and watermelon mono-cropping on growth,Fusarium wilt occurrence and soil microorganism of watermelon after FON inoculation;the differences of root exudates between wheat and watermelon and the its effect on FON growth;the relationship between the changes of root exudates of wheat-watermelon intercropping and watermelon mono-cropping,and the occurrence of Fusarium wilt of watermelon;the relationship between root SA synthesis and expression of disease-related defense-responsive genes,and the occurrence of Fusarium wilt of watermelon in wheat-watermelon intercropping and watermelon mono-cropping.This study can provide theoretical basis for wheat-watermelon intercropping to enhance the resistance against Fusarium wilt of watermelon.Main results of this study were as follow:1.At concentration ranging from 10 mL plant^-11 to 0.5 mL plant^-1,the lower concentration of wheat root exudates stimulated the growth of watermelon seedling;root exudates of 5 mL plant^-11 had the strongest stimulatory effects.The growth of FON was significantly inhibited by different concentration of 3 wheat cultivars root exudates.EM18 showed greater inhibitory effect compared with EM352 and ZM9023.Watermelon root exudates at concentration ranging from 0.01g mL^-11 to 0.1g mL^-1significantly promoted the germination,sporulation and growth of FON.On contrary,the germination,sporulation,and growth of FON was apparently inhibited by the addition of wheat root exudates.2.Compared with watermelon mono-cropping,wheat-watermelon intercropping decreased the occurrence of Fusarium wilt of watermelon after FON inoculation.The root vigor,chlorophyll content,photosynthetic rate,intercellular CO₂ concentration,stomatal conductance of watermelon leaves,and per fruit weight of watermelon in wheat and watermelon roots non-separation and mesh separation treatments were significantly higher compared with monoculture watermelon,respectively.The number of soil bacteria and total microbes in watermelon and wheat roots non-separation and mesh separation treatments were increased.Moreover,the number of soil fungi and FON in watermelon's rhizosphere were decreased.At different watermelon's growth stages,soil invertase,protease,urease,dehydrogenase,peroxidase and phosphatase activities in watermelon and wheat roots non-separation and mesh separation treatments were significantly higher than those of monoculture watermelon,respectively.The incidence of watermelon Fusarium wilt was reduced in the wheat intercropping system.3.The types and quantities of root exudates of wheat and watermelon were different.Among phenolic acids,the highest concentration of coumeric acid was identified in root exudates of wheat.Coumaric acid is strong inhibiter that inhibited the growth of FON compared with other acids.In watermelon's root exudates,P-hydroxybenzoic acid,phthalic acid,syringic acid,ferulic acid,SA,and cinnamic acid were detected.Among these acids,phthalic acid,SA,and total phenolic acid in watermelon's exudates were3-fold higher compared with wheat's root exudates.Coumaric acid,phthalic acid,syringic acid,and SA were detected in the root exudates of wheat.The concentration of coumaric acid was highest?50%?among the phenolic acids identified in the root exudates of wheat.Moreover,the additional test of exogenous specific phenolic acids application showed increased germination,sporulation,and growth of FON,and these promoted effects were increased with the increase in concentration of p-hydroxybenzoic acid,ferulic acid,and cinnamic acid.We also observed that p-hydroxybenzoic acid,ferulic acid,and cinnamic acid have a dose-dependent stimulatory effect.However,coumaric acid inhibited spore germination,sporulation and growth of FON,which shows that coumaric acid acts in a dose-dependent manner.Four kinds of organic acids namely oxalic acid,malic acid,citric acid and fumaric acid were detected in the root exudates of watermelon and wheat.Whereas,oxalic acid,malic acid,citric acid,succinic acid,and fumaric acid were found in the root exudates of watermelon.The contents of different organic acids were higher in the root exudates of watermelon compared with wheat.The total organic acid content in the root exudates of watermelon was 14-fold higher compared with root exudates of wheat.4.Compared with watermelon mono-cropping,wheat-watermelon intercropping decreased the number of FON in the rhizosphere of watermelon after FON inoculation.Wheat-intercropped watermelon plants also showed less disease incidence compared with mono-cropped watermelon plants.The disease incidence was 11.11%and 20%lower in the intercropping system compared with mono-cropping on days 15 and 25 after FON inoculation,respectively.A study was carried out to check the role of root exudates in wheat intercropping to alleviate Fusarium wilt of watermelon.The results showed that most of the phenolic acids and organic acids present in the root exudates of watermelon in the mono-cropping system were significantly increased after FON infection.Similarly,phenolic acids and organic acids were also found to be higher in FON-inoculated plants on day 25 compared with day 15.However,the quantity of phenolic acids and organic acids secreted from the roots of watermelon was substantially reduced under wheat intercropping systems.Wheat intercropping suppress Fusarium wilt of watermelon by changing the composition of phenolic acid and organic acid secretion from the roots of watermelon.5.Compared with watermelon mono-cropping,wheat intercropping enhanced the resistance of watermelon to FON by inducing SA biosynthesis and up-regulating the expression of disease-and-defense responsive genes in watermelon roots after FON inoculation.The effect of wheat intercropping on endogenous SA of watermelon roots,and the role of endogenous SA to improve the resistance of watermelon Fusarium wilt was assessed.The findings of this study showed that the free and conjugated SA content of watermelon were increased after FON inoculation.Moreover,the quantity of free and conjugated SA in wheat-intercropped watermelon was higher compared with watermelon grown in mono-cropping system.The expression of PAL1,PAL2,PAL12 and ICS genes was significantly increased in wheat-intercropped watermelon after FON inoculation.The enzymatic activities of PAL and BA2H were significantly increased in the roots of watermelon grown in intercropping system after FON inoculation.The results showed that wheat intercropping improved the SA content in watermelon roots at early stages,strongly initiate SA biosynthesis related gene expression and increased the activities of key enzymes.The expression of NPR1,PR1,PR2,WRKY1,WRKY6,WRKY8 genes,and defense in CIPDF2.1 and CIPDF2.4 genes in the roots of watermelon was higher in the intercropping system compared with the mono-cropping system after FON inoculation.The expression of the CAT,SOD,APX,and PPO genes in wheat-intercropped watermelon was significantly increased during the early stages of FON inoculation.Meanwhile,wheat intercropping improved CAT,SOD,APX,PPO,chitinase and?-1,3-glucanase activities,total phenolics and flavonoid contents,and reduced MDA and H₂O₂ contents in the roots of watermelon after FON inoculation.These results indicate that wheat intercropping increases the accumulation of SA in watermelon roots,and SA induces the expression of disease-and-defense responsive genes,through the regulation of anti-oxidative system of watermelon against FON infection.This mechanism shows the improved watermelon resistance to FON by SA production.