Relationship between carbon isotope discrimination, water use efficiency, and evapotranspiration in Kentucky bluegrass

Carbon isotope discrimination (Delta) is negatively correlated with water use efficiency (WUE) in several C-3 grass species and was proposed as a selection criterion for improving WUE. Our objectives were to determine the relationship between Delta and WUE in cultivars of Kentucky bluegrass (Pea pratensis L., KBG) maintained as turf, and to determine the relationship between cultivar Delta and evapotranspiration (ET rate measured under non-limiting soil moisture conditions, We view Delta as a potential index of ET and an alternative to weighing lysimeters in selecting C-3 turfgrass for reduced ET. WUE of 11 cultivars was evaluated in 25-cm-diam., fritted clay filled lysimeters, over two 4-d harvest periods, Clipping dry weight accumulated above a 50-mm base mowing height for each 4-d period was used as the productivity component in calculating WUE. Gravimetric water loss determined by the water balanced method was used as the transpirational component, Cultivars differed significantly (P less than or equal to 0.05) in WUE, Delta, and ET, depending on the harvest period, Also, the reliability of Delta in predicting WUE and ET was dependent on the harvest period, During the first harvest period, Delta was inversely related with WUE (r = -0.55h**, significant at P less than or equal to 0.01) and cultivar ET varied significantly with Delta (r = 0.64*, significant at P less than or equal to 0.05), During the second harvest period, no significant relationship (P > 0.05) developed between Delta and WUE (r = -0.21), or between Delta and cultivar ET (r = 0.32), Observations for the first and second harvest periods varied considerably in canopy temperature. with temperature ranging from 20.5 to 22.0 +/- 1.3 degrees C, and 27.3 to 36.0 +/- 3.4 degrees C, respectively, Theory predicts under the assumption of equal leaf temperature, Delta and WUE are correlated. It was likely that leaf temperature differences during the second harvest contributed significantly to the breakdown of the relationship between Delta and WUE. This assumption was likely met during the first harvest period only, therefore Delta and WUE (and ET) were related. Selecting for low Delta appears to have potential for identifying cool season turfgrass for reduced ET. However, further research is needed to determine the reliability of Delta in predicting ET under non-limiting and water limiting conditions.