Principal Investigator: Hugh Earl
Research Institution: University of Guelph
Timeline: April 2014 – October 2017
- Develop a controlled environment culture system that permits realistic rooting depths and soil water profile gradients that mimic field conditions.
- Determine the soil water profile conditions that result in perception of drought stress by the plant.
- Determine if soybean varieties adapted to Ontario differ in their ability to effectively extract deep soil water during a simulated natural soil drying cycle.
- Determine if there are variety differences in soil water extraction during water stress that result in differences in susceptibility to yield loss and, if so, determine which yield components are affected.
- The development of a convenient method for characterizing soybean variety differences in root functional traits thought to be essential for drought tolerance could lead to drought tolerant soybean varieties.
- The improved understanding of the physiological basis of yield reduction in soybean, as caused by transient soil water deficits with different timings and intensities, will help breeders identify specific traits to be targeted for future improvement.
While increases in atmospheric CO2 concentrations are likely to have a direct positive effect on soybean productivity, other aspects of climate change (increased air temperatures; possible increased frequency of extreme climate events including drought) are expected to be detrimental to yield potential. Even in the absence of climate change effects, year-to-year fluctuations in growing season precipitation are already a major challenge. Recent Grain Farmers of Ontario-funded research has shown that even in unusually wet years, soybean yields in Ontario are reduced by transient soil water deficits, especially during the latter part of the season; in drier years yield losses can exceed 25%. In order to deal proactively with future yield limitations associated with soil water deficits, improving drought tolerance is a major focus of most soybean variety development programs. Such efforts have been hampered by a lack of knowledge about the specific plant traits that would benefit soybean under realistic water stress scenarios (i.e., the timing, intensity and duration of water stress that actually limits yields under Ontario field conditions).
In this project we developed a controlled environment culture system for soybean that uses 2/3 field soil and 1/3 granitic sand in 1-m rooting columns. This system produces soil water profiles with depth that mimic those encountered in a field environment. We completed two vegetative / early reproductive stage experiments to develop soil water deficit simulation protocols. The best of those protocols was then used in a seed-to-maturity experiment to test effects of two fertility environments and three soil water treatments on soybean shoot and root growth, water use, water use efficiency, yield and yield components. This was extremely successful with very good statistical power (yield CV of 5.3%). Effects of soil water deficits on soybean yield components in this experiment were very similar to what is observed in field trials; pod number was the yield component most affected, with effects on seeds per pod and single-seed weight being comparatively minor.
In the final phase of the project, we used these methods to compare soil water deficit responses and root functional traits in 15 Ontario-adapted commercial soybean varieties. We did not find evidence that the varieties differed for their abilities to extract water from deep in the soil profile, but we did find significant variation for seed yield reductions under drought stress. We identified two drought-sensitive (Saska and OAC Drayton) and three drought-tolerant (OAC Lakeview, OAC Champion, and PRO 2715R) cultivars based on their ratios of seed yield under drought stress to seed yield under control conditions. Drought-tolerant cultivars were consistently those that maintained relatively high values for water use, biomass accumulation and pod number under drought stress. One of the cultivars, OAC Lakeview, displayed a distinct mode of drought tolerance, maintaining a very high fraction of its control pod number under drought stress. This study helped define the physiological basis of existing soybean cultivar differences in drought tolerance, and provides direction for soybean breeders to select traits that could improve yield under drought stress.
Most of the work under this project was conducted as part of the PhD dissertation research of Michael Gebre. It has also led directly to two other projects (MSc thesis work of Austin Bruch and MSc thesis work of Alex Cichello) to further optimize the system for high-throughput phenotyping.
External Funding Partners:
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.
Project Related Publications:
Gebre, M.G., and Earl, H.J. 2020. Effects of Growth Medium and Water Stress on Soybean [Glycine max (L.) Merr.] Growth, Soil Water Extraction and Rooting Profiles by Depth in 1-m Rooting Columns. Frontiers in Plant Science (11).
Gebre, M.G., and Earl, H.J. 2021. Soil water deficit and fertilizer placement effects on root biomass distribution, soil water extraction, water use, yield, and yield components of soybean [Glycine max (L.) Merr.] grown in 1-m rooting columns. Frontiers in Plant Science (12).
Gebre, M.G., Rajcan, I., and Earl, H.J. 2022. Genetic variation for effects of drought stress on yield formation traits among commercial soybean [Glycine max (L.) Merr.] cultivars adapted to Ontario, Canada. Frontiers in Plant Science (13).