Relationships between water use efficiency, carbon isotope discrimination, and turf performance in genotypes of Kentucky bluegrass during drought

Carbon-13 discrimination (Δ) and water use efficiency (WUE) have been largely ignored in turfgrass. Turfgrass performance under drought may be improved by selecting for low Δ, which has been shown to be negatively correlated with WUE. The objectives of this 2-yr greenhouse study were (i) to compare Δ and WUE in 12 genotypes of Kentucky bluegrass (Poa pratensis L.) with evapotranspiration (ET) and rooting for assessing turf performance under drought, and (ii) to determine the relative importance of ET in drought resistance by comparing diverse ET genotypes (6 low and 6 high) during drydown. Clipping dry weight to ET ratio (WUE) was evaluated with sand-filled weighing lysimeters (20-cm diam. by 65-cm depth) during two 4-d periods (Days 1 to 4 and 5 to 8) in 2002 and 2003. Carbon (13C/12C) analysis of leaf clippings was used to derive Δ values. Time domain reflectometry (TDR) was used to measure changes in volumetric soil moisture content (VSMC) at the 8.25-, 17.5-, 35.0-, and 52.5-cm depths to indicate moisture uptake and rooting activity. Drought resistance was visually assessed as wilt and leaf-firing symptoms. Low- and high-ET genotypes were similar in wilt and leaf firing, suggesting low ET may not be relevant in drought survival. Deep rooting at the 52.5-cm depth and higher ET as soils dried were associated with less leaf firing. The WUE at Days 5 to 8 was comparable in some years with rooting. The Δ was not always reliable in assessing WUE, but turf performance under drought was correlated with Δ. Low Δ values were associated with less wilt (r = 0.59, P ≤ 0.05) and leaf firing (r = 0.58, P ≤ 0.05), suggesting that Δ may be a useful selection criterion for superior performance under limiting soil moisture