Typical small-pot culture systems are not ideal for controlled environment phenotyping for drought tolerance, especially for root-related traits. We grew soybean plants in a greenhouse in 1-m rooting columns filled with amended field soil to test the effects of drought stress on water use, root growth, shoot growth, and yield components. There were three watering treatments, beginning at first flower: watered daily to 100% of the maximum soil water holding capacity (control), 75% (mild drought stress), or 50% (drought stress). We also tested whether applying fertilizer throughout the 1-m soil depth instead of only in the top 30 cm would modify root distribution by depth in the soil profile and thereby affect responses to drought stress. Distributing the fertilizer over the entire 1-m soil depth altered the root biomass distribution and volumetric soil water content profile at first flower, but these effects did not persist to maturity and thus did not enhance drought tolerance. Compared to the control (100%) watering treatment, the 50% watering treatment significantly reduced seed yield by 40%, pod number by 42%, seeds per pod by 3%, shoot dry matter by 48%, root dry matter by 53%, and water use by 52%. Effects of the 75% watering treatment were intermittent between the 50 and 100%. The 50% treatment significantly increased root-to-shoot dry matter ratio by 23%, harvest index by 17%, and water-use efficiency by 7%. Seed size was not affected by either fertilizer or watering treatments. More than 65% of the total root dry matter was distributed in the upper 20 cm of the profile in all watering treatments. However, the two drought stress treatments, especially the mild drought stress, had a greater proportion of root dry matter located in the deeper soil layers. The overall coefficient of variation for seed yield was low at 5.3%, suggesting good repeatability of the treatments. Drought stress imposed in this culture system affected yield components similarly to what is observed in the field, with pod number being the component most strongly affected. This system should be useful for identifying variation among soybean lines for a wide variety of traits related to drought tolerance.

The pattern of soil water availability in frequently watered small pots is different from field environments. In small pots, volumetric soil water content (VSWC) is relatively high throughout the rooting zone due to a lack of suction to remove water from large and midsize capillaries. This necessitates the use of growing media with large pore space to avoid anaerobic conditions and so prohibits the use of field soil (FS) in small pots. We hypothesized that in 1-m rooting columns, the 0.01-MPa gravitational potential difference between top and bottom may permit the use of lightly-amended FS as a growing medium and provide for realistic VSWC and rooting profiles by depth. This study aimed to investigate the effects of amending a typical sand-based potting mix with different proportions of FS on soybean growth [dry matter (DM) accumulation], water use, VSWC and rooting profiles by depth under control and water stress conditions, in 1-m rooting columns (polyvinyl chloride tubes having an inside diameter of 10 cm and length of 1 m). We tested three growth media (0, 50, and 67% FS mixes), watered daily to either 100% of the maximum soil water holding capacity (SWHC; control) or 75% SWHC (stress). VSWC was calculated from time-domain reflectometry measurements. Compared to all growth media, the 67% FS mix resulted in the highest DM accumulation, water use, water use efficiency (WUE), and also produced realistic VSWC and rooting profiles by depth similar to those reported in the literature under field conditions. Compared to the control, the water stress treatment reduced shoot DM by 24%, root DM by 13%, whole-plant DM by 22%, and water use by 25%, but increased root-to-shoot DM ratio by 18% and WUE by 6%. Of the three growth media tested, the 67% FS mix was the most suitable growth medium for controlled environment phenotyping studies of root functional traits affecting drought tolerance in soybean. This study provides novel phenotyping tools to select for root function and yield formation traits that could increase soybean yield under soil water deficit conditions.

Drought stress significantly limits soybean [Glycine max (L.) Merr.] yields in Ontario, Canada. Many studies of genetic variation for drought tolerance compare commercial lines with exotic, unadapted germplasm. We hypothesized that even current commercial cultivars adapted to Ontario would differ significantly for traits related to drought tolerance. In a greenhouse experiment, we grew fifteen soybean cultivars in field soil amended with sand in 1-m rooting columns, which allowed for simulation of field-like soil water profiles and rooting depths. Two watering treatments were imposed from the first flower until maturity by daily restoration of soil water to either 100% (control), or 50% (drought stress) of the maximum soil water holding capacity. Throughout the experiment, we measured volumetric soil water content at different depths in the soil profile, but found no evidence at any developmental stage that the cultivars differed for their ability to extract soil water from different depths. Drought stress reduced seed yield by 51% on average. Similar to the effects of drought in the field, pod number was the yield component most affected, with effects on seeds per pod and single-seed weight being comparatively minor. There were significant cultivar × treatment interactions for seed yield, pod number, shoot dry matter, and water use. 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 (seed yield ratio, SYR). Regression and principal component analyses revealed that drought-tolerant (high-SYR) cultivars were consistently those that maintained relatively high values for water use, biomass accumulation and pod number under drought stress; high water use efficiency under drought stress was also associated with a high SYR. 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 helps define the physiological basis of soybean cultivar differences in drought tolerance, and provides direction for soybean breeders to select traits that could improve yield under drought stress.