Shifts in plant water use patterns during increasing VPD across 122 years of breeding in U.S. Midwest spring wheat (Triticum aestivum L.) genotypes

Spring wheat (Triticum aestivum L.) yields in continental cropping systems of the U.S. Midwest have shown a consistent upward trajectory over the past century due to successful breeding efforts and improvements in crop management. However, the looming threat of increasingly extreme temperature trends and the rising evaporative demand (vapor pressure deficit, VPD) during the cropping season in that region may require adaptive water use strategies to maintain or further increase wheat yield potential. Therefore, our objective was to investigate whether the observed increase in yield over the last 122 years coincides with a shift in plant water use strategies, i.e., the transpiration rate (TR) sensitivity to rising VPD.

In this study, we selected 15 spring wheat genotypes from Minnesota with a year of release (YOR) spanning from 1898 to 2020 to capture the genetic yield gains achieved during that period by the local breeding program. We tested plant transpiration rate in response to rising VPD ranging from 0.5 to 2.8 kPa in a climate chamber in wet soil and potted conditions. Additionally, we measured several traits that capture plant hydraulic properties, including stomatal conductance, plant hydraulic conductance, leaf area, above- and belowground biomass, and stomatal morphological properties.

Our investigation revealed that at a critical VPD beyond 1.83 ± 0.17 kPa, a significant portion of the tested genotypes expressed a limited increase in TR with increasing VPD, indicating a decline in stomatal conductance. No discernible correlation was observed between parameters characterizing plant water use strategies or the plant hydraulic system and YOR over the whole 122-year window. However, a moving window analysis unveiled that post the green revolution (around 1960 ± 15 years), breeding for yield indirectly favored less hydraulically conductive plants with a reduced leaf area and a linearization of the transpiration rate response to increasing VPD, as evidenced by a decreasing difference in slopes beyond a critical VPD. This resulted in a less pronounced reduction in water use due to a restricted TR response to increasing VPD and, thus, a lower sensitivity to rising VPD. 

Our study indicates that hydraulic traits such as the TR sensitivity to VPD might have been under the control of a cryptic selection mechanism by breeders as they increased wheat yield potential in the region, at least from the 1960s onwards. This points to the promising possibility of using such traits to improve yields under drier climates.