Study On The Mechnism Of Potassium Chloride Supressing Stalk Rot Of Maize

Stalk rot is one of the most widespread and destructive disease that reduces maize yield and quality. As a kind of soil-borne fungal disease, maize stalk rot is difficult to control by conventional strategies such as the use of synthetic fungicides and fungicidal seed treatment. Many studies confirm that application of potassium is an effective method to reduce the stalk rot incidence of maize. However, little attention has been paid to the mechanisms of suppression of corn stalk rot by potassium. A 12-year long-term field experiment with different rates of potassium chloride (KC1) application was used to investigate the suppression effect of KC1 on stalk rot, the effect of soil extracts on the growth of Fusarium graminearum, the most common stalk rot fungi in this area, and the changes of microflora in rhizosphere with plate counting method. A separate field trial with different rates of potassium and chloride application was conducted to valuate the suppression effects of potassium and chloride from KC1 and study the effects of potassium on the distribution of soluble carbohydrate in plant at the growth stages, and the phenolic metabolism in stalk pith tissue of the second internode above ground after inoculation affected by potassium. Hydroponic experiment was conducted to measure the influence of potassium supply on the root exudates using high performance liquid chromatography, and the effects of sugar and phenolic compounds on the growth of F. graminearum was analyzed with plate culture method. Pot trial under greenhouse condition was carried out to examine the alteration of ultrastructure of seedling root tissue 4 days after inoculation with transmission electron microscope. This paper discussed the possible mechanism of potassium chloride suppressing stalk rot of maize from the aspects of microecology of rhizosphere, and plant physiology and biochemistry affected by potassium chloride. The main results are as following:1. In K-deficiency soil, potassium applied as KC1 and KNO₃ decreased the incidence of maize stalk rot significantly, but chloride applied as CaCl₂ had no obvious effect on reducing the incidence of maize stalk rot compared to the control. Resistant cultivar Jidan180 had a higher potassium uptake capacity than susceptible cultivar Jidan327. The significant negative correlations were found between the stalk rot incidence and the K content in root, stem and ear-leaf of maize at the growth stages.2. KC1 addition to the PDA medium could not directly suppress F. graminearum development. Soil extracts from soil with long-term KC1 treatments suppressed F. graminearum development more significantly, compared with that from the treatment of no KC1 supply. Long-term application of KC1 increased the populations of rhizosphere fungi and actinomyces at the early growth stages. However, there was no significant difference existed in rhizosphere bacteria population among the treatments. The sensitive infection stages of pathogen to maize were consisted with the stages when the significant difference in rhizosphere fungi and actinomyces populations. And at seedling stage, Trichoderma was the fungi dominating in the rhizosphere of maize with long-term KC1 application. Therefore, the change in rhizosphere microflora (especially fungi) perhaps was one of the mechanisms of suppressing maize stalk rot by KC1 application.3. Potassium supply decreased the total amount of sugar exudates and stimulated the release oforganic acids and phenolic acids under nitrate as the main nitrogen source. Potassium significantly increased the ferulic acid content and decreased the chlorogenic acid content. The amount of ferulic acid exudates by root of resistance cultivar was higher than that of susceptible cultivar, while a reverse trend was found in chlorogenic acid. In vitro, the growth of Fusarium graminearum was stimulated by sugar and inhibited by ferulic acid and chlorogenic acid, and the suppression effect of ferulic acid was more effective than chlorogenic acid. The decrease of sugar content and the increase of the amount of ferulic acid in seedling root exudates with potassium supply maybe help reduce the infection probability by F. graminearum. The cell wall of maize root tip with potassium supply became thicker than that without K application. Once the cell of maize root tip with potassium supply was infected, it would secreted material for degrading fungi hypha, and inhibited the rapid development of pathology in host. Mitochondria accumulation which marked the abnormal of respiration was found in the cell of seedling root with K deficiency.4. Potassium stimulated carbohydrate translocation to root and stem, and increased the sugar content after the tasselling stage. Sugar contents in root at the filling stage and those in stem at the dough stage were positively correlated with the resistance to stalk rot. The total soluble sugar content in root or stem could be used as a better indicator of resistance to maize stalk rot compared to the sucrose content and the reducing sugar content. Enhancement of sugar metabolism by potassium was responsible for the resistance to maize stalk rot.5. The initial level of lignin in the susceptible cultivar was higher than that in the resistant cultivar, while the content of induced lignin by inoculation in susceptible cultivar was lower than that in resistant cultivar. Therefore, it was not the initial level of lignin but the induced level of lignin could indicate the resistance to stalk rot. Infection of F. graminearum increased the accumulation of induce lignin and triggered rapid increase in activities of phenylalanine ammonia-lyase (PAL), tyrosine ammonia-lyase (TAL), peroxidase (POD) and cinnamyl alcohol dehydrogenase (CAD) in the stalk pith tissue of maize with potassium application, compared with the K-deficiency plant. These results implied that after inoculation, potassium could stimulate the rapid expression of defence genes, increase the activities of defense enzymes, promote the synthesis of inducing soluble phenolic compounds and then enhance the resistance of maize plants against stalk rot.6. The background level of phenolics and the induced phenolics by inoculation in the susceptible cultivar was higher than that in the resistant cultivar. Potassium supply increased the rapid accumulation of induced phenolics and increased the activity of polyphenoloxidase (PPO), stimulated the translation of phenolics compounds to quinone Regardless of inoculation, the concentration of chlorogenic acid increased with the increase of potassium application rates, which was the predominant phenolic compound in maize stalk pith tissue, and chlorogenic acid proved to inhibit the growth of F. graminearum in vitro. The results suggested that chlorogenic acid may play a role in inhibiting the development of F. graminearum in the stalk pith tissue of maizeIn conclusion, the possible mechanisms of potassium chloride suppressing stalk rot of maize could be summarized as follows: 1) KC1 impeded directly the infection of pathology by making the cell wallthicker, and decreased indirectly the probability of infection of F. graminearum by changing the root exudates and microflora in rhizosphere. 2) KCl maintained plant physiology vigor higher at the later growth stage by accelerating the transportation of carbohydrate and increasing sugar available to the root and stem tissue after tasselling stage. 3) KCl enhanced the resistance of maize against stalk rot through stimulating the rapid expression of defence genes, furthermore increasing the activities of defense enzymes, and then promoting the synthesis of inducing soluble phenolic compounds after infection by pathology. All of those may contribute to the decrease in incidence of maize stalk rot.