Biodiversity of arbuscular mycorrhizal fungi (AMF) in Guangdong province and their roles in plant phosphorus acquisition

Abstract

It is well documented that the mutualistic associations between arbuscular mycorrhizal fungi (AMF) (Zygomycota: Glomales) and most of the terrestrial plants enhance the uptake of mineral nutrients in plants, especially the less mobile elements in soil such as P and Zn, frequently resulting in improved growth and survival. AMF are ubiquitous in terrestrial ecosystems and variability in symbiotic effectiveness among species or isolates exist. Biodiversity studies of AMF provide baseline information for the characterization of the role the AMF symbioses play in ecosystem functioning. Soils in Guangdong province are generally acidic, causing accumulation of toxic ions (H+, Mn2+, and Al3+) and deficiencies in major nutrients, such as Ca2+, Mg2+ and PO43- to plants. The incorporation of mycorrhizal technology in agriculture and forestation may increase mineral availability and reduce mineral toxicity to plants grown under acid soil stress. This study devotes efforts in three major aspects of mycorrhizal research. Firstly, the biodiversity and taxonomy of AMF in Guangdong province were investigated. Secondly, the symbiotic efficiency, in terms of biomass and P content of host plant, was tested using the pure AMF isolates obtained. Thirdly, the effects of AMF inoculation (Glomus intraradices and G manihotis) on phosphate acquisition in lettuce with two different P sources were studied. Soil samples have been collected from 35 sites in Guangdong province, 46 AMF morphotypes have been trapped from 23 of the soil samples. Pure spore cultures of 9 of the isolated morphotypes have been obtained and classified according to their morphological characteristics. These isolates have also been registered at the European Bank of Glomales (the BEG database) and an internationally recognized code (the BEG number) was assigned for each of them. Our results showed that Glomaceae was the most abundant family in the pure culture attempts. The symbiotic efficiency of eight of the local isolates was investigated using lettuce as the host plant. The results illustrated that lettuce inoculated with AMF isolates coded BEG150, BEG164, BEG165, BEG166, BEG169 yielded significantly (p<0.05) higher biomass than BEG149, BEG151 and BEG 171. These results agreed with others reports that AMF have varied efficiency in enhancing plant growth. Therefore, the selection of suitable AMF species for specific habitat and particular crop is an important area for exploitation. Our results demonstrated that in low level of available phosphate in soil, mycorrhizal inoculated lettuce produced higher biomass than the un-inoculated plants. This was shown at acidic condition (pH5) with low dosage of P(0.3mgP/kg sand) applied in both the rock phosphate (RP) and soluble phosphate (SP) treatments, and at alkaline condition (pH8) with high dosage (l25mgP/kg sand) of RP. In alkaline condition, although RP was applied at high dosage, the actual available phosphate was low because solubilization of RP into available phosphate is very low in alkaline condition, but relatively higher in acidic condition. The mycorrhizal effect at low available phosphate could attribute to the increase in absorption surface, higher affinity and lower threshold concentration to phosphate, and production of soluble phosphatases of the AMF hyphae. Hence, the supply of P to the host plant was enhanced. This experiment elucidated the substantial symbiotic efficiency at low available phosphate environment. When the available phosphate level was high, there was no significant difference among inoculated and un-inoculated plants. This was shown at acidic pH with high dosage of SP and RP, and at alkaline pH with high dosage of SP. The results indicated that lettuce was not dependent on mycorrhizal inoculation when soil phosphate level was not limiting.