Río de la Plata grasslands and Lotus tenuis

LEÓN ROLANDO JUAN CARLOS; STRIKER GUSTAVO GABRIEL; INSAUSTI PEDRO; PERELMAN SUSANA

Botanic Garden,University of Valencia,Valencia, Spain.

Workshop; Lotus as a model legume and a sustainable alternative for marginal land reclamation; 2007

Temperate subhumid grasslands, in the eastern part of South America, cover the vast plains of central-eastern Argentina, Uruguay and southern Brazil. This grassland region can be divided into two subregions: (1) the pampas, in Argentine and (2) the campos of Uruguay and Southern Rio Grande do Sul (Brazil) (Soriano, 1991). In the Pampas, some of the areas have seldom or never been cultivated. Grazing has been the only modifying agent in these areas, which are mostly in low-lying locations of the flooding Pampa, the western portion of the Pampa (Soriano, 1991). In the flooding Pampa, the natural grasslands and pastures constitute the basis of cattle breeding. These grasslands show a great floristic heterogeneity in which herbaceous plant communities are arranged forming intricate landscape mosaics (Burkart et al., 1990). Four major grasslands habitat types have been defined: mesophyte prairies (MP), humid prairies (HP), meadows (M) and halophyte steppes (HS) (Ghersa et al.,2007). In comparison to other grasslands in the world, these ones show a great proportion of exotic species: 10% within perennial grasses, 16% within annual grasses, 19% within perennial dicots and 55% within annual dicots. Native legumes in these communities are very scarce (7 species), the exotic ones are more numerous (14 species) and they are more significant in produced biomass (Ghersa et al., 2007). The species of the Lotus genus are of the most recent introduction. In the north of the region, in 1968, Lotus tenuis (Lotus glaber Mill.) showed different constancy values in each one of the communities: MP=8.6, HP=11%, M=8%, HS=1%. In recent evaluations carried out in 51 sites of two of such communities, considerable increases were registered in the presence of L. tenuis: from 4 to 99% in MP and from 4 to 50% in HS (Ghersa et al., 2007). In the south of the region the presence of this legume was already important 30 years ago: MP=8.5%, HP=34%, M=24% and HS=7% (Ghersa et al., 2007). The wide distribution of Lotus tenuis in the humid prairies and meadows is considered of great relevance for forage production. The reason behind such distribution is the high tolerance of L. tenuis to long-term flooding, one of the major disturbances affecting these plant communities. In experiments carried out in these communities we have advanced in the identification of anatomical, morphological and physiological attributes conferring to L. tenuis tolerance to the lack of oxygen due to the flood (Striker et al., 2005). In this sense, we have found that the flooded plants of L. tenuis increase the porosity notably in the stems (porosity 13%) and roots (porosity 28%), they locate a high proportion of their leaves above the water level (53%) and they can maintain unaffected their stomatal conductance and photosynthesis for more than three weeks (Striker et al., 2005). These responses tend to facilitate the capture and transport of oxygen to roots and maintain carbon fixation and biomass production under flooding conditions. Besides, we have studied the interaction between flooding and trampling, an unavoidable sequel of cattle grazing (Striker et al., 2006). In this sense, we found that the roots of L. tenuis possess a low mechanical resistance (250 KPa) with relationship to the pressure that can generate on them the hoof of a cow (>300 KPa) (Striker et al., 2006), in consequence their plants are much damaged when both disturbances are combined. This aspect should be taken into account for grassland management.