Influence of Vesicular–arbuscular Mycorrhiza on the Hydraulic Conductivity of Roots of Two Citrus Rootstocks

S ummary Carrizo citrange (CC) and sour orange (SO) seedlings were grown in a low phosphorus (P) sandy soil and either inoculated with Glomus intraradices Schenck & Smith or fertilized with soluble P. Mycorrhizal seedlings had nutritionally sufficient levels of leaf P, non‐mycorrhizal plants of similar size were P‐deficient. The root–shoot ratio of both rootstocks was reduced by mycorrhizal colonization, but root hydraulic conductivity per unit root length of mycorrhizal CC and SO was more than twice that of non‐mycorrhizal seedlings under well‐watered conditions. Mycorrhizal plants also had significantly higher transpiration rates when standardized on a root length basis, and greater transpiration appeared to be related to the increased conductivity of roots. Flow of water to roots via hyphae alone could not account for the greater water uptake by mycorrhizal roots. Apparently, mycorrhizal enchancement of P nutrition was primarily responsible for the greater conductivity of roots since no differences were found between root hydraulic conductivity of mycorrhizal and non‐mycorrhizal CC of equal P status under well‐watered conditions.