Trade-Off Of Carbon-Phosphorus In Mutualism In Response To Shading And Phosphorus Levels And The Mycorrhizal Responsiveness Of Grass Species At Different Succession Stages

In nature, the symbiosis between land plants and arbuscular mycorrhizal fungi is one of the most common, oldest and ecologically important mutualisms in terrestrial ecosystems. The trade-off between carbon and phosphorus is the key component in understanding the stabilization of the mycorrhizal mutualism. In the present study, we used triple isotopic labeling of carbon (14C) and phosphorus (32P and33P) within split root chambers to investigate the trade-off of carbon and phosphorus of host plant Allium vineale L. with varying light inputs and phosphorus supply levels. In addition, pot experiments were used to study the mycorrhizal growth responsiveness among plant species of early succession, late succession and exotic plant species. The main results were as follows:1. Pot experiment was used to study the mycorrhizal responsiveness (MR) of inbred maize (Zea mays L. vs. L224) with different soil P supply levels. The mycorrhizal responsiveness of maize plants was significantly affected by the fungal inoculum (Funneliformis mosseae、 Rhizophagus irregularis) and P supply levels (10,20,30,40,50,100mg kg-1). Maize plants exhibited higher MR at lower P supply when inoculated with R. irregularis, and at intermediate P supply when inoculated with F. mosseae. The expression of the AM-inducible Pi transporter gene ZEAma;Phtl;6was neither significantly affected by soil P supply, nor by fungi species. Root P uptake efficiency (RPUE) of maize was greatly increased by mycorrhizal colonization at all P supply level. In conclusion, the maize inbred line of L224was highly responsive to mycorrhizal inoculation.2. Roots of the host plant of Allium vineale L. were grown in spatially separated root chambers. Half of the roots were inoculated with beneficial fungal species of Claroideoglomus candidum and the other was with non-beneficial AM fungal species of Gigaspora margarita. The above-ground was treated with four different shading treatments (no shading, shading for8,6and4weeks). Carbon allocation and phosphorus uptake by host plants were calculated by measuring the radioactivity of labeled C (14C) in roots and P(32P/33P) in shoots. The biomass of host plant was promoted by the beneficial fungi, and the mycorrhizal growth responsiveness (MGR) declined with prolong of the shading treatments. Whereas the growth of host growth was not significantly affected by either shading or the inoculation of non-beneficial mycorrhizal fungus. Host plants preferentially allocated more carbon toward roots associated with the beneficial AM fungus, and in return the fungus delivered more phosphorus to host plant across all shading treatments. The proportion of preferential carbon allocation declines with the increase of shading duration. The host plant allocated approximate25%of the labelled carbon to roots associated with the beneficial AM fungus under un-shading condition and the value declines to15%under8weeks shading treatment. The proportion of preferential carbon allocation to roots associated with the non-beneficial fungus (Gigaspora margarita) did not different among shading treatments and the value was approximate11%. Our findings demonstrate that host plants preferentially allocated more carbon toward the effective mycorrhizal fungus and the allocation was highly relevant to the above-ground light conditions.3. Using the same split root chambers, the C allocation and P uptake across four different soil P levels (1X,2X,3X and4X,1X was background P level) was investigated in the present experiment. Roots were inoculated with the same or different fungi isolates on both sides. The mycorrhizal growth response (MGR) of Claroideoglomus candidum inoculated plants decreased with the increasing soil P supply levels. Host plants preferentially allocated more carbon toward high P side and in return received more phosphorus regardless of the fungi species when both root sides were inoculated with the same fungus species. The carbon allocation to high P vs low P in Gigaspora margarita inoculated treatment increased from3.27in2012to6.03in2013, and the corresponding ratio in Claroideoglomus candidum inoculated plants increased from1.09in2012to7.05in2013. When roots were inoculated with different fungi isolates on both sides, host plants preferentially allocated more carbon toward Claroideoglomus candidum under P supply levels of1X,2X and4X, and the ratio increased with increasing soil P supply levels. Our results indicate that host plants preferential allocated more carbon toward the effective mycorrhizal fungus and the allocation proportion varied with below-ground soil P supply levels.4. Pot experiments were conducted to investigate the mycorrhizal growth responsiveness (MGR) among plant species of early succession (3species), late succession (4species) and exotic (6species) plants. The MGR was significantly different among plant species of different succession stages. The variation and MGR of early succession plants was smaller compared to late succession and exotic plant species. The late succession plant species were more sensitive to local AM fungi and prone to be influenced by exotic plant species. The biomass of exotic plants was promoted when inoculated with local AM fungi species, and the effect was equivalent to those of early succession plant species. The MGR of Allium vineale L. was almost twice higher than those of other exotic plant species. The average MGR of different plant species was significantly correlated with the coefficient of variation. In conclusion, local AM fungi species improve the adaptability of the native plant species and may affect the invasion of exotic plant species.