Optimum Root Biomass for Maximum Grain Yield in Bread Wheat Under Well-Watered and Drought Field Conditions.

An optimum root system might enhance water and nutrient uptake under drought environments without depleting soil moisture, thus improving grain yield. Two experiments were conducted at the University of California, Riverside, under well-watered and droughted field conditions using five RILs with root biomass ranging from 1.175 to 7.850 g plant-1 plus a check parent cv, ‘Yecora Rojo’. Covariate analysis between grain yield (GY) with number of days from sowing to anthesis (DTA) and to physiological maturity (DTM), grain filling period (GFP), and plant height (PH) were not significant. The main effect of irrigation on DTM, GFP, PH, number of tillers (NT) and spikes (NS) per 50 cm, thousand grain weight (TGW), GY, and shoot biomass (SB) was significant, but not on days to anthesis, number of grains per spike, and harvest index. The main effect of genotype on the traits was highly significant. Drought reduced DTM by 5%, GFP by 10%, NS per 50 cm by 24.6%, and TGW by 11% which resulted in 25% and 27% reduction in GY and SB, respectively. Stress tolerance index (STI) of the genotypes calculated on GY ranged from 0.52 to 0.88. A quadratic relationship was observed between root biomass measured under well-watered glasshouse conditions with GY measured under well-watered (R2 = 0.62) and droughted field conditions (R2 = 0.93). Grain yield under well-watered and droughted field conditions maximized at 5.806 and 4.575 t ha-1 when root biomass was 1.630 and 3.975 g plant-1, respectively. These results signify the importance of a separate breeding program to develop wheat genotypes adapted to drought-prone environments. Over-sized root biomass reduced grain yield under both irrigation regimes.