Photosynthesis and Nutrient-use Efficiency of Barley in Response to Low Arbuscular Mycorrhizal Colonization and Addition of Phosphorus

The effects of arbuscular mycorrhizal infection by Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe on growth and photosynthesis of barley (Hordeum vulgare L. cv. Manitou) were investigated in sand culture at five levels of calcium phosphate (50, 100, 200, 400 and 800 mg P kg(-1) ). Mycorrhizal infection was low and varied with P supply, declining from 3.3% at 50 mg P kg(-1) to 1.5% at the highest P concentration. In general, there were small differences in biomass between mycorrhizal (+AM) and non-mycorrhizal (-AM) barley but a significant reduction in dry mass of senesced leaves occurred in the +AM plants. Leaf P concentrations increased with P application, but did not differ between + AM and -AM plants. Although there were no differences in dry mass between + AM and -AM plants at 50 mg P kg(-1) , it was at this lowest P supply that +AM plants had higher rates of photosynthesis and greater P-use and N-use efficiencies. The mycorrhizal enhancement of maximum photosynthetic rate at the lowest P level was associated with a higher stomatal conductance, but was not related to increased leaf P or to changes in photon yield or the ratio of variable (FV) to maximum (FM) chlorophyll fluorescence.