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.

Michael Gebretsadik Gebre,Hugh James Earl

doi:10.3389/fpls.2021.581127

Abstract

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.

Highlights

Soybean [Glycine max (L.) Merr.] is the number one field crop grown in Ontario, Canada, with a cultivated area of more than 1.3 million ha and a value of over $1.9 billion in 2018 (OMAFRA, 2020)
Drought stress experiments conducted in frequently watered small pots, may not present soil water and rooting profile variation by depth similar to what is encountered in a field environment. To alleviate those limitations associated with small pots and permit meaningful controlled environment studies of soybean responses to drought stress, and to capture root dry matter (DM) distribution and volumetric soil water content (VSWC) profiles by depth, we developed and characterized a 1-m rooting profile method (Gebre and Earl, 2020)
Averaged across the two fertilizer placement treatments, the drought stress (50% soil water holding capacity (SWHC)) treatment significantly reduced SY by 40% (p < 0.0001), PN

Summary

Introduction

Soybean [Glycine max (L.) Merr.] is the number one field crop grown in Ontario, Canada, with a cultivated area of more than 1.3 million ha and a value of over $1.9 billion in 2018 (OMAFRA, 2020). Soybean is grown mostly under rainfed conditions, so soil water limitations occurring during critical stages of crop development significantly reduce Ontario’s soybean yield potential (Hufstetler et al, 2007; Visser, 2014). Recent research results show that soil water deficits constitute a significant limitation to Ontario’s soybean yield in most growing seasons. Demonstrated losses in field experiments ranged from 8 to 24% and supplemental irrigation during the reproductive stage was found to enhance soybean yield by 10–25%, mostly from an increase in number of pods (Earl, 2012; Visser, 2014). Even in unusually wet years, soybean yields in Ontario are reduced by transient soil water deficits, and in drier years, yield losses may exceed 25% Numerous controlled environment and field studies have reported yield reductions ranging from 24 to 80% in soybean subjected to different levels of drought stress (e.g., Frederick et al, 2001; Sadeghipour and Abbasi, 2012; He et al, 2016, 2017; Wei et al, 2018; Giordani et al, 2019; Gebre, 2020)

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