Water status and 13C isotope discrimination in two conventional pastures of Uruguay.

Grasses and legumes in a mixed pasture compete for water, but their competitive ability depends not only on their different absorption capacity, but also on their capacity to regulate water loss by decreasing stomatal conductance. This is reflected in a smaller carbon isotope discrimination (Δ). In this work we studied the changes in water content (WC) and Δ in a pure white clover pasture and in a mixed sward of white clover, lotus and tall fescue, for a winter-spring cycle in an experiment located in southern Uruguay. From winter to spring, WC decreased for fescue and white clover in both pastures, but Δ declined in all species. In spring there was a linear relationship between Δ and WC for white clover and fescue, showing that Δ was a good indicator of water status. We also found relationships between Δ and several climatic variables, showing that they were indicators of changes in water availability or water demand. Due to the grass competition for water, the Δ value of white clover was lower in the mixed sward, and yield also decreased. Therefore, Δ can be a useful tool to assess the impact of water deficit on the productivity and botanical composition of mixed grasslands, because this index is a proxy the water status of plants.