Increased stress tolerance of nodule activity in the Medicago-Rhizobium-Glomus symbiosis under drought

Abstract

A comparison was made between Medicago sativa L. cv. Tierra de Campos plants in tripartite symbiosis with Rhizobium meliloti strain 102F51 and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, irrigated with N- and P-free nutrient solution, and nodulated plants plus soluble added phosphorus.

When 45 days old, both groups of plants were stressed by cyclic withholding of water and rewatering at different soil water potentials of −0.1, −0.3, −0.5 and −0.7MPa. After a subsequent 1 month drought-treatment period, the following parameters were evaluated: leaf water potential, intensity of VAM infection, nodule dry weight, total plant growth, phosphorus content and acetylene reduction activity.

Drought stress significantly reduced the dry weight of total plants and nodules as well as P content of non-VAM plants when compared with the tripartite symbiosis. The intensity of VAM infection was not affected by drought.

During drought, nodule activity was significantly higher in VAM than in non-VAM plants; this higher activity cannot be explained by improved P uptake caused by the fungus since P-concentration in mycorrhizal plants was always lower than in phosphorus fertilized plants.