Shading mediates the response of mycorrhizal maize (Zea mays L.) seedlings under varying levels of phosphorus

Abstract Arbuscular mycorrhizal fungi (AMF) form a beneficial symbiosis with most plant roots for the provision of phosphorus in exchange for plant carbohydrates, and the carbon-phosphorus relationship between AMF and host plant plays a key role in understanding the AM symbiosis. In agroecosystems, shading significantly affects the crop growth, however, it remains unclear how shading mediates the mycorrhizal response to phosphorus (P) supply. For this, a greenhouse experiment was conducted, and maize seedlings were subjected to shading and non-shading by means of leaves coverage under three P fertilization treatments, i.e., P15: 15 mg kg−1, P40: 40 mg kg−1, and P100: 100 mg kg−1. We used two AMF inocula for maize inoculation, i.e., Rhizophagus intraradices (R. intraradices) and Funneliformis mosseae (F. mosseae). We found that shading significantly decreased the soluble sugar concentration in phloem of mycorrhizal maize but not in the non-mycorrhizal ones. Root biomass and the root-shoot ratio of mycorrhizal maize decreased significantly under the shading treatment at P15, P40, and P100. The gene expression of ZmPht1;6 significantly decreased as the P supply level increased. Generally, shading and AM fungal species had no significant effect on the gene expression of ZmPht1;6. Mycorrhizal P response (MPR) decreased gradually as the P supply level increased. Sugar concentration in the phloem saps of inoculated maize was negatively correlated with the expression of ZmPht1;6 genes. Shading significantly increased the gene expression level of ZmPht1;6 inoculated with R. intraradices but had no effects with F. mosseae inoculation. However, the shoot and root MPR inoculated with F. mosseae were significantly higher than that inoculated with R. intraradices. Our findings demonstrate that shading mediates the carbon allocation of mycorrhizal maize seedlings, and the response is associated with P uptake systems, suggesting that tight linkages between aboveground carbon supply and P resource could play a role in maintaining mycorrhizal functioning in the agroecosystems.