Influence of Arbuscular Mycorrhizal Fungi on Root Allocation and Morphology in Two Medicago Species

Premise of research. Root allocation and morphology can be influenced by nutritional symbiosis with arbuscular mycorrhizal (AM) fungi. In the presence of AM fungi, plants could alter root allocation and morphology in ways that increase plant reliance on mycorrhizal-mediated nutrient acquisition services. When colonized by AM fungi, plants may reduce root allocation and produce roots with reduced absorptive surface area (i.e., lower specific root length [SRL], root length per unit of mass) and greater habitat area for AM fungal colonization (i.e., lower root tissue density [RTD], root mass per unit of volume).</p> Methodology. We assessed root plasticity in response to AM fungi in two mycorrhizal-dependent species, Medicago lupulina and M. truncatula. Plants were grown with or without AM fungi at both low and high phosphorus levels. The supplemental phosphorus treatment allowed comparison of AM fungal-inoculated plants with uninoculated plants of a similar size.</p> Pivotal results. Growth of M. lupulina and M. truncatula was stimulated by AM fungi at low phosphorus, but there was no effect of AM fungi on growth at high phosphorus. The root allocation, SRL, and RTD of inoculated plants did not differ from that of uninoculated plants of a similar size in a majority of comparisons. When statistically significant differences were observed, the effects were not consistent with the predictions. For example, the RTD of M. truncatula was higher for AM fungal-inoculated plants than for uninoculated controls of a similar size.</p> Conclusions. The results of our experiments with two Medicago species suggest that plants do not necessarily modify their root systems to reduce absorptive surface area and increase habitat space for fungal colonization in the presence of AM fungi. Therefore, reduced root allocation and the development of roots with lower absorptive surface area may not be necessary for some plant species to obtain mutualistic benefits from nutritional symbiosis with AM fungi.</p>