The role of mycorrhizal fungi and non-mycorrhizal micro-organisms in iron nutrition of citrus

Abstract

Two citrus rootstocks differing in mycorrhizal dependence and lime tolerance, rough lemon ( Citrus jambhiri Lush.) and trifoliate orange [ Poncirus trifoliata (L.) Raf.], were grown in either an acidic (pH 6.3) potting mix or an alkaline-calcareous (pH 8.1) potting mix. Both potting mixes were autoclaved, fertilized and either inoculated with non-mycorrhizal micro-organisms only or inoculated with mycorrhizal fungi and non-mycorrhizal micro-organisms. Growth of both rootstocks was not increased significantly in either potting mix by VAM fungi. Colonization of the roots by mycorrhizal fungi was more pronounced in the more mycorrhiza-dependent lime-tolerant rough lemon and was lower in both rootstocks in the alkaline-calcareous potting mix than in the acidic potting mix. Root morphology was altered in the acidic potting mix by inoculation with VAM fungi such that root systems were more branched. The inclusion of mycorrhizal fungi in the inoculum increased shoot Fe concentrations alone in comparison to inoculation with non-mycorrhizal micro-organisms suggesting a role for mycorrhiza in host Fe nutrition in acidic conditions. Inoculation with mycorrhizal fungi in the alkaline-calcareous potting mix had no effect on shoot Fe concentrations. In this potting mix mycorrhizal fungi greatly increased the supply of P to host plant shoots. The data indicate that VAM fungi may increase the supply of Fe to host plant shoots in an acidic environment, but not in an alkaline situation. The data do not indicate unequivocally whether this effect in the acid potting mix is due to the direct involvement of the endophyte in Fe uptake and supply to the host or an indirect effect of a change in root growth habit.