Wheat breeding for drought tolerance

Due to considerable influence of environment on yield, breeding for drought tolerance could benefit from focusing on selection of more heritable physiological traits, such as carbon isotope discrimination (as measured by delta, ∆) for indirectly assessing water use efficiency (WUE) of wheat (Triticum aestivum L.). Spring and winter wheat cultivars were assayed for ∆, and these values were used to determine the relationships with performance in over 13 environments in the U.S. Pacific Northwest. The correlation coefficients of ∆ values between the wheat cultivars grown in different environments ranged from 0.11 to 0.73 for both spring and winter wheat. There was significant genotypic variation for ∆ in soft spring and hard winter wheat but not in hard spring and soft winter wheat. The ∆ values were poor indicators of yield for this set of wheat cultivars in most environments, although low values (better WUE) were sometimes correlated with yield. A population of 165 hard spring wheat recombinant inbred lines derived from a cross between two hard spring wheat varieties that differed in ∆ was also screened in low-rainfall dryland and irrigated environments. High yields in this recombinant inbred population were weakly correlated with high ∆ values or low WUE in most environments.

Canopy temperature (CT) is considered a reliable proxy for stomatal conductance. Low CT values of plant canopies under water-limited conditions are associated with high transpiration indicating plants’ drought tolerance. Many U.S. Pacific Northwest (PNW) adapted wheat (Triticum aestivum L.) cultivars lack stress-adaptive traits resulting in poor performance in drought environments. This study aims to identify the stress-adaptive traits by evaluating the CT in spring wheat populations across different soil moisture conditions in the PNW. An infrared thermometer was used to estimate the CT in two families of recombinant inbred lines, ‘Alpowa’ × ‘Express’ (AE population) and ‘Hollis’ × ‘Drysdale’ (HD population), in rainfed and irrigated environments of the dryland PNW in 2011 to 2013. Higher reductions in grain yield up to 170%, spike length up to 25%, and spikelets spike−1 up to 19% were observed in a rainfed environment compared to the reductions in an irrigated environment. A significant variation in CT was observed in both AE and HD populations. With 1 °C increase in CT at the anthesis stage, grain yield was lowered up to 38 g m−2. Low CT was associated with high grain yield and agronomic traits in both wheat populations (r =  − 0.18 to − 0.55, P ≤ 0.05). The highest association between CT and grain yield was observed at anthesis (r =  − 0.47) and milking (r =  − 0.38) stages (P ≤ 0.001). Our results show that screening for low CT during terminal wheat growth stage is an effective strategy for improving the selection of new drought-tolerant wheat varieties in the PNW.