RELATIONSHIP BETWEEN CARBON-ISOTOPE DISCRIMINATION, WATER-USE EFFICIENCY, AND EVAPOTRANSPIRATION IN KENTUCKY BLUEGRASS

Carbon isotope discrimination (Delta) is negatively correlated with wa ter use efficiency (WUE) in several C-3 grass species and was proposed as a selection criterion for improving WUE. Our objectives were to de termine the relationship between Delta and WUE in cultivars of Kentuck y bluegrass (Pea pratensis L., KBG) maintained as turf, and to determi ne the relationship between cultivar Delta and evapotranspiration (ET rate measured under non-limiting soil moisture conditions, We view Del ta as a potential index of ET and an alternative to weighing lysimeter s in selecting C-3 turfgrass for reduced ET. WUE of 11 cultivars was e valuated in 25-cm-diam., fritted clay filled lysimeters, over two 4-d harvest periods, Clipping dry weight accumulated above a 50-mm base mo wing height for each 4-d period was used as the productivity component in calculating WUE. Gravimetric water loss determined by the water ba lanced method was used as the transpirational component, Cultivars dif fered 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 = -0.55 h*, significant at P less than or equal to 0.01) and cultivar ET vari ed significantly with Delta (r = 0.64, significant at P less than or equal to 0.05), During the second harvest period, no significant relat ionship (P > 0.05) developed between Delta and WUE (r = -0.21), or bet ween 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 assump tion of equal leaf temperature, Delta and WUE are correlated. It was l ikely that leaf temperature differences during the second harvest cont ributed significantly to the breakdown of the relationship between Del ta and WUE. This assumption was likely met during the first harvest pe riod only, therefore Delta and WUE (and ET) were related. Selecting fo r low Delta appears to have potential for identifying cool season turf grass for reduced ET. However, further research is needed to determine the reliability of Delta in predicting ET under non-limiting and wate r limiting conditions.