The effect of waterlogging and drought conditions on the symbiosis between arbuscular-mycorrhizal fungi (AM) and Lotus glaber plants in a saline-sodic soil were studied to investigate the importance of symbiosis to permit plants tolerate and grow under stress conditions imposed by the environment. Lotus glaber Mill., is the unique naturalised legume on lowland grasslands in the Flooding Pampa of Buenos Aires Province, Argentina. During the rainy season (autumn, winter and part of the spring) flooding is a common climate phenomenon in these lowlands. The duration of flooding is variable, ranging from a few days to more than seven months depending on topography, soil and season (Escudero and Mendoza, 2005). Additionally during the summer, drought is also frequent. The soils of these lowlands are characterised by high pH, salinity and sodicity, and in summer, an alternating sequence of dry and wet pulses originated by rainfall and high evaporation rates. These situations impose plants to a variety of stress conditions.

The soil was air dried and sieved through 2mm-mesh screen. A total of twenty 2.0 L closed bottom pots were filled with 1.350 g of soil. Five Lotus glaber Mill., plants were grown in a Typic Natraquoll saline-sodic soil (pH 9.4; electrical conductivity 5.07 dS/cm, exchangeable sodium 60%) in a greenhouse for 75 days and then subjected to waterlogging or drought for an additional period of 35 days. After the initial period of growth in soil at near field capacity (36% w/w), 5 of the pots were harvested. Five of the remaining 15 pots were flooded with demineralised water to one cm above the soil surface (waterlogged treatment), 5 pots were kept near -0.19 MPa of soil water potential (drought treatment), and 5 control treatment pots were kept near field capacity, -0.05 MPa (control treatment). Plants in the three treatments (control, waterlogged and drought pots) were grown for an additional 35 days. That is, 75 days near field capacity followed by 35 days near field capacity, waterlogging and drought. The effect of waterlogging and of drought on AM-plant symbiosis were evaluated by measuring the morphology of mycorrhizal colonisation and Rhizobium nodules in L. glaber roots before and after waterlogging or drought treatments.

Soil waterlogging and drought decreased total root length, root length colonised (AM root length), arbuscular colonisation (AC root length) and number of entry points per unit of colonised root length. Waterlogging also increased vesicle colonisation (VC root length) and Rhizobium nodules in roots, but drought treatment did not affect vesicle colonisation and nodule formation. Spore density in soil decreased by waterlogging but it had no changes with respect to drought treatment.

The results indicate that L. glaber can grow, become nodulated by Rhizobium and colonised by mycorrhizas under waterlogged or drought conditions. Susceptibility index (SI) suggests that soil waterlogging has a greater depressive effect on mycorrhizal colonisation morphology than soil drought.