Quantitative estimation of water uptake by mycorrhizal extraradical hyphae in citrus under drought stress

Mycorrhizal hyphae have the functioning on water absorption from soil, while the information regarding hyphal water absorption rate is not fully known. In this study, 37-μm nylon meshes were positioned in a pot bottom to allow mycorrhizal extraradical hyphae, but not roots, to pass through the mesh. The whole pot was placed in a beaker, where distilled water was supplied, reaching a 0.5-cm air gap between the pot bottom and water surface of the beaker. Citrus rootstock, trifoliate orange (Poncirus trifoliata) seedlings of pots were inoculated with Funneliformis mosseae and Paraglomus occultum and also exposed to well-watered (WW) and drought stress (DS). The 71-days soil DS significantly inhibited root mycorrhizal colonization and hyphal length in soil and mesh, regardless of mycorrhizal fungal species. The hyphal water absorption rate was 0.607-1.973 mg H2O/h/mm for F. mosseae and 0.126-0.963 mg H2O/h/mm for P. occultum, respectively. The DS treatment significantly elevated hyphal water absorption rate by 2.3-6.6 times. The increase of leaf water potential by mycorrhization was higher under DS than under WW. Our results provide a quantitative estimation of water absorption rate by mycorrhizal extraradical hyphae and also suggest more important water contribution of hyphae to the host plant under DS than under WW.