Saturday, 15 July 2006

Study of Interactions Between Thiobacillus Bacteria and Mycorrhizal Fungi on Some Growth Characteristics of Wheat Under Greenhouse Conditions.

Hosein Besharati, Soil and Water Institute of Iran, North Kargar Ave, Tehran, Iran

The mutually beneficial symbiosis between plant roots and a special group of soil fungi called mycorrhizal symbiosis, is considered to be one of the most common type of association between plants and microorganisms. Of the most important group among mycorrhizal fungi, we can name arbuscular mycorrhiza (AM) which are obligate symbiotic fungi on the roots of over 80% of plant families. The microsymbiont utilies organic compounds derived from the host plant while in turn improves the host plants resistance against salt stress, drought as well as resistance against disease producing agents under unfavorable environmental conditions through various mechanisms such as by increasing the ability of host plants for absorption of nutrients, release of heavy metals and toxic ions and by producing growth promoting hormones as well as by establishing positive interactions with nitrogen fixing bacteria and phosphate solubilizing microorganisms. Sulfur is the most abundant, least expensive and the most common acid producing substance useful for improving host plants ability to absorb fixed nutrients and for reclaiming sodic soils, suppling host plants sulfate requirements and fighting against some soil borne disease agents in many of the world,s regions. Conditions for improving and accelerating the rate of utilization and the potential of sulfur, requires the presence of sulfur oxidizing bacteria especially Thiobacillus. Due to its fungicide properties, sulfur may reduce or destroy the potential benefits of arbuscular mycorrhiza on host plants by adversely affecting the fungi. On the other hand it is likely that sulfur through oxidation would produce acid in calcareous soils improving the rhizosphere conditions that lead to a better establishment of mycorrhiza fungi on roots increasing their beneficial effects for host plants. In this experiment a randomized complete black factorial test with four replications was used to evaluate the effect of sulfur applications as well as the action of Thiobacillus and mycorrhizal fungi on the yield, rate of phosphorus uptake and percent colonization on wheat roots in a calcareous soil. Experimental variables included 3 levels of sulfur (0, 500, and 1000 kg S /ha), 3 levels of Thiobacillus inoculum (0, 103, and 104 cells/gr of soil), and 3 levels of arbuscular mycorrhizal fungi (0, and 150 spores of Glomus intraradices and Glomus mosseae per plant). Seven sprouted wheat seeds were planted in each pot (contained 4 kg soil) after applying the treatments, then reduced to 4 plants. The soil moisture in the pots was maintained at 80% of F. C. during the 90-days growing period. The dry weight and phosphorus uptake of plant shoots were measured after harvest, and the roots were stained using the method of Griss and Stribly (1991). Sixty cuttings of plant roots were examined microscopically to measure the average rate of root colonization. The results showed that there were significant effects of sulfur and Thiobacillus bacteria application on phosphorus uptake and shoot dry weight, and percent colonization of wheat roots by mycorrhizal fungi. The application of sulfur at a rate of one ton/ha along with Thiobacillus inoculum of 104 cells/gr soil (S2T2 treatment) was the only treatment that increased the dry weight and phosphorus uptake of shoots by 18% and 13.64% compared with the control respectively. Assigning a value of 100 for increase in the dry weight of shoots over the control in triple superphosphate treatment, the corresponding figure for S2T2 would be 58.49. There were no significant differences in phosphorus uptake and shoot dry weights obtained at various rates of mycorrhizal fungi applications. However they significantly affected wheat colonization and phosphorus concentration at 1% level. The rate of root colonization with mycorrhizal fungi in the control treatment, Glomus mosseae, Glomus intraradices, and triple superphosphate treatment were 29.18%, 46.59%, 47.93% and 23.07% respectively. The two named fungi species significantly increased the rate of root colonization as compaired with the control, however super phosphatetreatment reduced the rate. The application of one ton sulfur/ha along with 104 Thiobacillus cells/gr reduced the rate of root colonization by Glomus mosseae by 11% and this decrease was significant at 1% level, while the effect was not significant in the case of Glomus intraradices. Other rates of sulfur and Thiobacillus treatment did not significantly affect percent colonization by the two fungi.

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