Effect of traditional soybean breeding on water use strategy in arid and semi-arid areas

Highlights

• Traditional soybean breeding has changed the water use strategy from profligate to conservative and improved yield performance and WUE_G.

• Soybean breeding has increased the sensitivity of stomatal conductance to water stress and reduced water uptake ability.

• The high sensitivity of stomatal conductance to water stress is correlated with low root hydraulic conductance to limit water use.

Abstract

Water is the main factor determining yield performance under drought conditions. Traditional soybean breeding has significantly increased grain yield under drought, but its effect on water use strategies and associated traits are not well understood. Field and pot experiments with new cultivars and landrace soybean genotypes were undertaken to identify the effect of soybean breeding on water use strategies and related leaf and root functional traits under different water regimes. The new cultivars, on average, had 33.9 %, 45.1 % and 169 % higher grain yields and 47.9 %, 98.4 % and 244 % higher water use efficiencies for grain yield (WUE_G) but 8.8 %, 38.1 % and 19.6 % lower water use than the landraces in the field and pot experiments 1 and 2, respectively. Stomatal conductance decreased in the new cultivars at higher soil water contents than the landraces during soil drying. The new cultivars, on average, had a 22.9 % lower root fraction, 21.5 % lower leaf fraction, 20.6 % lower stem fraction and 69.8 % higher pod fraction than landraces, which had 21.7 % and 26.8 % lower root hydraulic conductance at the R2 and R6 stages, respectively, than the landraces. Our results show that (1) traditional soybean breeding has changed the water use strategy from profligate to conservative and improved yield performance and WUE_G, (2) the high sensitivity of stomatal conductance to water stress is coordinated with lower root hydraulic conductance to limit water use, and (3) selecting new cultivars with high grain yield but low water use may a new way to improve yield performance under water-limited conditions.

Introduction

Soybean is one of the most important oil crops, providing a significant source of protein, macronutrients, and minerals (Foyer et al., 2016; Manavalan et al., 2009). Moisture stress is one of the key abiotic constraints to soybean yield and its stability (Manavalan et al., 2009). Under rainfed conditions, soybean yields tend to be constrained by the crop’s water use efficiency (Daryanto et al., 2015; Polania et al., 2016). Understanding how to increase soybean yield under limited water availability is essential for developing high-yielding soybean cultivars for rainfed environments (Araus et al., 2002; Condon et al., 2004; Araújo et al., 2015) and a better management also needed to optimize the grain yield under water-limited environmental (Di Mauro et al., 2019). An important criterion for developing high-yielding cultivars for moisture-stressed environments is to understand the underlying mechanism of water use in soybean (Blessing et al., 2018).

Grain yield is positively correlated with water use in legumes such as soybean (Glycine max), peanut (Arachis hypogaea), pigeon pea (Cajanus cajan), mung bean (Vigna radiata) and cowpea (Vigna unguiculata), but not faba bean (Vicia faba) and moth bean (Vigna aconitifolia) (Blessing et al., 2018). In an earlier pot experiment, we found that new soybean cultivars with high grain yields used less water than landrace genotypes (He et al., 2016, 2017), which has been similarly observed in oats (Avena nuda L.) (Wang et al., 2017). Thus, we speculate that soybean breeding could change the crop’s water use strategy from profligate (a competitive strategy that enables the crop to take advantage of freely available water) to conservative through a sensitive short-term adjustment of stomatal conductance (gs) or early stomatal closure, which would reduce or delay terminal drought.

Leaf transpiration is a key trait associated with the water use strategy of aboveground traits (Blessing et al., 2018). Reducing tillering and branching (Kim et al., 2010) and reduced leaf number and leaf size (Borrell et al., 2000) can reduce leaf area to decrease water loss. A reduction in transpiration rate can also reduce water loss. Leaf stomatal conductance associated with leaf transpiration and stomatal closure can reduce plant water loss as the soil dries (Jones and Turner, 1980; Morgan, 1984; González et al., 2008). Stomatal closure under water stress is induced by root-source abscisic acid (ABA) (Zhang and Davies, 1990; Liu et al., 2003; He et al., 2016). Genetic variation for stomatal closure response under water stress has been reported in wheat (Xiong et al., 2006), oats (Wang et al., 2017), and soybean (He et al., 2016). Thus, there is potential to improve the efficiency of water use in soybean by breeding for reduced leaf fractions and increased sensitivity of leaf stomatal conductance to water deficit.

Water uptake ability, which can be measured by root hydraulic conductivity (Steudle and Peterson, 1998; Bramley et al., 2009), is tightly linked to a crop’s water use strategy (Vadez, 2014). Genetic variation for root hydraulic conductivity has been observed in rice (Adachi et al., 2010), lupin (Bramley et al., 2009), and soybean (Rincon et al., 2003). However, the effects of soybean breeding on root hydraulic conductivity and its role in determining water use efficiency have not been well-studied in arid and semi-arid areas. Indeed, the association between variation in leaf stomatal conductance or transpiration rate response to water stress and root hydraulic conductivity is not known.

In northwest China, soil water reserves and rainfall are the two main sources of water during the growing season, as no irrigation systems are available for agriculture. This emphasizes the importance of developing soybean cultivars with improved water use efficiency to enhance adaptation in this region, which will increase yields and stability. This study evaluated five soybean landraces and two cultivars using pot and field-based experiments under rainfed conditions to identify the relationship between water use efficiency and grain yield. The pot studies used two soybean landraces and two cultivars to investigate the traits associated with water use by comparing the stomatal conductance (gs) response to progressive soil drying and root hydraulic conductivity in well-watered and water-stressed treatments. It was hypothesized that traditional soybean breeding has (1) changed water use strategies from profligate to conservative, and (2) reduced leaf fractions and root water uptake ability but increased the sensitivity of stomatal conductance to water deficit through efficient water use mechanisms.