Genotypic differences in root traits to design drought-avoiding soybean ideotypes

Genotypic differences in root traits to design drought-avoiding soybean ideotypes
https://doi.org/10.1051/ocl/2022021
The global soybean production in 2019 reached 334 million tons of which 80% were achieved in Brazil, USA and Argentina only (FAOstat, 2021). In the European Union, soybean production has increased progressively over the last 5 years, reaching 2,8 million tons on 933,000 ha in 2021 (European Commission, 2021). However, this production is still not sufficient to satisfy the increasing needs of the European market for food and feed requiring importation of soybean from the American continent. Indeed, out of 17 million tons of crude proteins imported annually in the European Union, 13 million tons come from soybean, corresponding to 30 million tons of equivalent grains (European Commission, 2018). In France, the strategic plan of the oil–protein sector aims at increasing the soybean acreage with an objective of 300,000 ha by 2030 (versus 186,000 ha in 2020). An increasing interest in this crop derives from the quality of its grain (41% proteins) as well as from the agronomic and environmental benefits of this crop (diversification of crop rotation, lower need for pesticides, low greenhouse gas emissions, biological nitrogen fixation, etc.) (Jouffret et al., 2015).
The global soybean production in 2019 reached 334 million tons of which 80% were achieved in Brazil, USA and Argentina only (FAOstat, 2021). In the European Union, soybean production has increased progressively over the last 5 years, reaching 2,8 million tons on 933,000 ha in 2021 (European Commission, 2021). However, this production is still not sufficient to satisfy the increasing needs of the European market for food and feed requiring importation of soybean from the American continent. Indeed, out of 17 million tons of crude proteins imported annually in the European Union, 13 million tons come from soybean, corresponding to 30 million tons of equivalent grains (European Commission, 2018). In France, the strategic plan of the oil–protein sector aims at increasing the soybean acreage with an objective of 300,000 ha by 2030 (versus 186,000 ha in 2020). An increasing interest in this crop derives from the quality of its grain (41% proteins) as well as from the agronomic and environmental benefits of this crop (diversification of crop rotation, lower need for pesticides, low greenhouse gas emissions, biological nitrogen fixation, etc.) (Jouffret et al., 2015).
In Europe, soybean is mainly grown in southern and continental parts, where increasing drought and heat waves are expected in the near future (Dai, 2013; Rojas et al., 2019). Soybean yield and its stability are constrained by drought, which is the most important limiting abiotic stress causing yield losses (up to 40%), particularly when it occurs during both the vegetative and the reproductive stages (Specht et al., 1999). However, the most critical period of water stress in this crop occurs from flowering stage (Meckel et al., 1984). Several agronomic practices and adaptive strategies could be planned to counteract crop losses due to water stress and to promote the soybean acreage (Maury et al., 2015). One of these strategies is the choice of cultivars adapted to water-limited environments through drought escape, dehydration tolerance and drought avoidance (Turner et al., 2001). Drought escape is an agronomic practice corresponding to the introduction of early-maturing cultivars or shifting sowing dates earlier. Dehydration tolerance is the ability of plant cells to continue the metabolic process at low leaf water status by various physiological adaptations such as osmotic adjustment. On the other hand, drought avoidance (or dehydration postponement) occurs when plants are able to keep a favourable water status under drought either by limiting water loss from leaves through reduced stomatal conductance, by reducing leaf absorption of radiation or by enhancing the root water uptake through a deeper rooting system. Deepening rooting system could be a major adaptation trait to climate change for both increasing soybean yield and decreasing annual yield variability (Battisti and Sentelhas, 2017).
dentifying ideotypes for root structural traits (morphology and architecture) involved in drought avoidance is useful in guiding the development of soybean cultivars to enhance soil exploration and thus water acquisition under water-limited conditions. Many efforts in plant breeding have been made to improve drought avoidance and resource acquisition. Several studies highlighted the interaction between structural and functional root traits. Crops characterized by a large root diameter could have an increased ability to penetrate the hard soil (Bengough et al., 2011). Lynch (2013) showed that roots growing vertically at low metabolic cost (steep, cheap and deep) contribute to build an interesting ideotype in maize since root system with rapid exploitation of deep soil could optimize both water and N uptake. However, in the case of phosphorus (immobile resource), soybean cultivars with root phenes, such as shallower root growth angle of basal roots and long root hairs, have been a good choice to enhance P acquisition in low-P soils (Lynch, 2011; Lynch and Brown, 2001; Richardson et al., 2011; Wang et al., 2010). In addition, a relationship was observed between root architecture and arbuscular mycorrhizal fungi colonisation in soybean. The deep root genotype had greater colonisation by these fungi at low P compared to the shallow root genotype (Wang et al., 2011). Previous studies reported some specific soybean root traits involved in water uptake particularly under drought (Boote, 2011; Valliyodan et al., 2017). On the other hand, functional traits, such as N₂ fixation, are related to root nodulation ability (Prudent et al., 2016) that could occur in interaction with structural root traits (morphology and architecture) and may thus be involved in drought avoidance (Pantalone et al., 1996a, 1996b).
Soybean has a simple rooting system named allorhizic (Fig. 1), consisting of a taproot and lateral roots (Ao et al., 2010; Fenta et al., 2011, 2014). The taproot may reach a depth of 200 cm and most lateral roots emerge from the upper 10 to 15 cm of the taproot (Lersten and Carlson, 2004). More than 50% of the roots are localised in the first 20 cm of soil layer (Hoogenboom et al., 1987). Moreover, 95–97% of the total root weight and 85–70% of the total root surface area have been found in the upper 23 cm of soil (Benjamin and Nielsen, 2006; Mitchell and Russell, 2010).
The soybean root consists of a simple architecture and morphology, which is similar to that of oilseed rape (Louvieaux et al., 2020). However, there is paucity of information on the soybean rooting system compared to other crop species such as cereals (e.g., maize) or other leguminous species (e.g., common bean). Indeed, maize is characterized by a complex rooting system composed of different root types viz. primary, crown, brace and seminal roots. Moreover, since lateral roots in maize are more metabolically demanding per gram of tissue than axial roots, a balance is required between the need of soil exploration and the metabolic need (Lynch, 2013).
During the vegetative phase, soybean rooting can reach 30 and 40 cm depth at 23 (second unrolled trifoliate leaf: V2) and 30 (third unrolled trifoliate leaf: V3) days after sowing, respectively (Fenta et al., 2014; Torrion et al., 2012). At the beginning of flowering stage (R1), rooting depth may vary from 50 to 70 cm (Böhm et al., 1977; Manavalan et al., 2010; Matsuo et al., 2013). The maximum rooting depth in soybean differs between cultivars and could reach from 70 to 200 cm (Lersten and Carlson, 2004). An in-depth growth of rooting system could end between R1 to R3 (Kaspar et al., 1978), or could continue until full seed stage (R6) (Torrion et al., 2012). On cultivars with indeterminate growth habit, the taproot extension follows a linear model with a progression of 1,3 cm per day (Torrion et al., 2012). Soybean root system could continue to grow after the beginning of pod filling but with no further increase in root weight from the late bloom to mid-pod fill growth stages (Mitchell and Russell, 2010). However, the root surface area increased between these two latter stages under both dryland and irrigated conditions (Benjamin and Nielsen, 2006). Therefore, there are contradictory information in the literature about soybean root growth in relation to cultivars and environmental conditions.