Abstract
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.
Highlights
Soybean [Glycine max (L.) Merr.] is the number one field crop grown in ON, Canada, with a cultivated area of more than 1.3 million ha and a value of over $1.7 billion in 2017 (OMAFRA, 2019)
We predicted that the soil mix with the highest fraction of field soil (FS) in 1-m rooting columns would be the best growth medium resulting in higher dry matter (DM), water use, water use efficiency (WUE), and uniform volumetric soil water content (VSWC) and rooting profile variation with depth than the soil mixes containing less FS, under control and stress conditions
In the same 1-m rooting columns, a growth medium that included 67% FS mix provided reasonable field-like soil water and rooting profiles by depth
Summary
Soybean [Glycine max (L.) Merr.] is the number one field crop grown in ON, Canada, with a cultivated area of more than 1.3 million ha and a value of over $1.7 billion in 2017 (OMAFRA, 2019) It is grown mostly under rainfed conditions, so soil water deficits occurring during critical stages of crop development significantly limit Ontario’s soybean yield in most growing seasons, with demonstrated losses in field experiments ranging from 8 to 24% (Hufstetler et al, 2007; Earl, 2012). The pattern of plant water use in response to soil drying is very well studied in soybean and other crop species. There is genetic variation in soybean for the critical soil water content at which water use begins to decline (Hufstetler et al, 2007), indicating that different genotypes make differing “decisions” about how to respond to reduced water availability
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