Variation Of Plants Mycorrhizal Dependency And Mobilization Of Scarcely-Soluble Phosphate By VA Mycorrhizae

In pot culture and three-compartment rhizobox cultivation, variation between mycorrhizal dependency of plants and the responsible mechanisms were investigated, with maize (Zea mays L.), wheat (Triticum eastivum L.), clover (Trifolium repens L.) and soybean (Glycine max L.) as host plants, Glomus mosseae and Glomus versiforme as arbuscular mycorrhizal fungal species.Results showed that root hair density was higher for wheat plant than for maize plant, resulting in a higher mycorrhizal dependency for maize than for wheat. Inoculation of AM fungi significantly enhanced P uptake rate by root, increased P content, total P uptake and chlorophyll content for maize plants, without significant impact on wheat plants. Contribution rate of AM to total P uptake was higher for maize plant than for wheat plant. Plant growth of maize and wheat responded greater to Glomus mosseae than to Glomus versiforme.At high P application, mycorrhizal dependency of soybean was higher than that of clover. With inoculation of AM fungi, P uptake rate by root was significantly promoted, P content, total P uptake, chlorophyll content and photosynthetic rate were increased for soybean and clover plants. Higher contribution of AM to total P uptake for soybean thanfor clover was observed.Relationship between P efficiency and mycorrhizal dependency was further investigated, with different P efficient wheat genotypes as host plants. Results indicated that plant growth of wheat responded little to AM at low P application, while it was inhibited at high P application. Compared to low P efficient genotype, high P efficient genotype was lower in phloem sugar translocation rate and R/S ratio. AM significantly promoted P efficiency and P uptake rate by root, with lower increase in high P efficient genotype than in low P efficient genotype. Correlation analysis demonstrated that there was a significantly negative relationship between mycorrhizal dependency and P efficiency, P uptake rate by root. Low amount of carbohydrate was translocated to root system in high P efficient genotype, leading to shorter external hyphae length associated with its root system than low P efficient genotype root system. Lower contribution rate ofhyphae to plant P uptake was found in high P efficient genotype than in low P efficient genotype, of root, which is the original reason responsible for different mycorrhizal dependency.According to these results above, it is concluded that P uptake rate by root is the fundamental factor underlying plant mycorrhizal dependency, AM may increase P uptake rate by root. Amount of carbohydrate translocated to root is much different between plants, resulting in different P uptake by hyphae and P uptake rate by root. Plantmycorrhizal dependency is lower at high P application than at low P application.By methods of indirectly labelling with NaH232PO4 and directly labelling in atomic pile, Ca2-P, Cag-P, Calo-P, Fe-P and Al-P were isotopically labelled. Combining with technology of soil solution extracting in situ, the ability to mobilize phosphates by AM fungi was estimated, with clover as host plant and Glomus versiforme as fungal species.Results demonstrated that external hyphae mobilized CarP, Ca8-P, Fe-P and Al-P, while mobilized almost no Calo-P. P uptake from phosphates by mycorrhizal clover was in the order of Ca2-P > Ca8-P, Al-P > Fe-P. Result further showed that soil pH in hyphosphere was decreased, ranging from 0.2 to 0.4 units. P concentration in hyphosphere was significantly higher with application of Ca2-P and Cag-P than with other phosphates or control. It is concluded that AM fungi and plants mobilize the same kinds of scarcely-soluble phosphates, soil pH decrease is one of the mobilizing mechanisms. The difference between P-supplying potential of phosphates relates to their nature.