Using multi-environment sugar beet variety trials to screen for drought tolerance

Drought is a major limitation to sugar beet yield in many rainfed areas. Varieties with increased drought tolerance would improve productivity, but breeding specifically for drought tolerance is time-consuming and expensive. Here we explore a complementary approach based on evaluation of data collected from multi-environment sugar beet variety trials (METs). The approach was to assign a drought stress index (DSI) to each trial, and then plot the regression of relative yield performance of each variety against a range of DSI. Varieties were classed according to their intercept (yield potential under low-stress conditions) and their slope (which indicates relative drought tolerance or susceptibility). The database consisted of 121 trials of 137 varieties conducted on 38 sites from 1989 to 1999. Three different methods to calculate DSI were compared; the preferred method was based on the cumulative daily ratio of actual to potential evapotranspiration because it was the best predictor for measured sugar yield losses in 25 years of rainfed/irrigated trials at Broom's Barn. The actual evapotranspiration for each trial was derived from a crop growth model using site-specific soil and weather inputs. Line comparison regression analysis revealed 20 varieties that showed significant positive or negative slopes, including seven varieties with average yield potential, but better than average yields under dry conditions. Conversely, there were 10 varieties that had good yield potential but showed poor performance under drought. ANOVA of trials testing some of these varieties within a single year showed that the ‘crossover’ behaviour was statistically significant and consistent across years. Results confirm the diversity for drought tolerance within sugar beet germplasm. This approach of assessing MET data using the DSI as an environmental descriptor can be used by breeders to evaluate drought tolerance and yield stability in current breeding material.