Water shortage and low phosphorus (P) availability limit yields in soybean. Roots plays important roles in water-limited and P-deficient environment, but the underlying mechanisms are largely unknown. In this study we determined the responses of four soybean varieties (Huandsedadou (HD), Bailudou (BLD), Jindou 21 (J21) and Zhonghuang 30 (ZH)) to three P levels [applied 0 (P0), 60 (P60) and 120 (P120) mg P kg–1 dry soil to the upper 0.4 m of the soil profile] and two water treatment [well-watered (WW) and water-stressed (WS)] with special reference to root morphology and architecture, we compared yield and its components, root morphology and root architecture to find out which variety and/or what kind of root architecture had high grain yield under P and drought stress. The results showed that water stress and low P, respectively, significantly reduced grain yield by 60 % and 40 %, daily water use by 66 % and 31 %, P accumulation by 40 % and 80 %, and N accumulation by 39 % and 65 %. The cultivar ZH with the lowest daily water use had the highest grain yield at P60 and P120 under drought. Increased root length was positively associated with N and P accumulation in both the WW and WS treatments, but not with grain yield under water and P deficits. However, in the WS treatment, high adventitious and lateral root densities were associated with high N and P uptake per unit root length which in turn was significantly and positively associated with grain yield. Our results suggest that (1) genetic variation of grain yield, daily water use, P and N accumulation and root morphology architecture were observed among the soybean cultivars and ZH had the best yield performance under P and water limited conditions; (2) water has a major influence on nutrient uptake and grain yield, additional P supply can modestly increase yields under drought in some soybean genotypes; (3) while conserved water use plays an important role in grain yield under drought, root traits also contribute to high nutrient uptake efficiency and benefit yield under drought.

Crop plants grow, and then, they allocate resources to different structures, including seeds and fruits, which represent yield in most crops. We define the yield stability of a genotype as its ability to reduce the effects of temporal variation in resources and conditions on yield production, and we argue that yield stability can be understood in terms of two processes: (1) crop survival and growth (biomass production): the ability of the crop plants to survive and produce biomass under the range of conditions to which it is exposed and (2) the pattern of allocation of this biomass to yield across this range of conditions. Plant breeders and crop physiologists have focused on (1), but much less attention has been paid to (2). We hypothesize that (2) is primarily the result of reproductive allometry: the quantitative relationship between vegetative and reproductive biomass. Ecological theory and the allometric models we present predict a tradeoff between (a) the ability of a genotype to produce yield over a wide variety of conditions and (b) its ability to produce very high yields under optimal or near-optimal conditions. We reanalyze the data from two recent studies, and the results are consistent with this hypothesis. Yield stability in crops corresponds to bet-hedging in evolutionary ecological theory. It is the most appropriate strategy for smallholder farmers in developing countries, a group that comprises most of the farmers in the world. Researchers and crop breeders need to rethink their objectives if they want to develop optimal varieties for these farmers.

Differences in phosphorus (P) utilization efficiency (PUtE) and/or yield are closely linked to differences in root functional traits under low soil P availability. However, our understanding of how soil P availability mediates the intraspecific variation in root functional traits for breeding high-P efficiency genotypes to increase PUtE and yield remains limited. We investigated plant growth parameters and pivotal root functional traits associated with P acquisition in 20 alfalfa genotypes with contrasting P efficiencies and supplied with low, medium, or high levels of P. We observed that tradeoffs occurred in root functional traits among alfalfa genotypes under low-P stress. High P efficiency genotypes displayed higher shoot biomass and PUtE by relying on thicker and more robust roots, elevated concentrations of carboxylate exudates, and enhanced colonization by arbuscular mycorrhizal fungi. In contrast, low-P efficiency genotypes exhibited a relatively high root-to-shoot ratio and primarily depended on higher tissue P concentrations but relatively slender roots along with comparatively high rhizosphere pH. Consequently, high PUtE and productivity under low-P conditions among alfalfa can be identified by screening for a phenotype with thick roots, increased exudate concentrations, and mycorrhizal colonization, opening up the potential for breeding for P-efficient lines.

Crop breeding has been successful in increasing crop grain yield (GY; reproductive biomass) largely through reduced vegetative size, increased reproductive effort (RE = reproductive biomass/total biomass) and increased water-use efficiency (WUE) in grain production. Flowering time is an important life history trait that signifies the switch from vegetative to reproductive growth. The relationship between GY and time from sowing to flowering (Tsf) is unclear. We fit the relationships between GY and RE vs. Tsf to the logistic model using data from 18 spring wheat genotypes grown under simulated rainfed conditions. Tsf accounted for water use before and after flowering, root length density, total leaf area, and the time from flowering to harvest. Early flowering meant decreased water use before flowering and increased water use afterward. Soil water remaining at harvest was positively correlated with yield. Early flowering genotypes have a higher WUE of grain production, but there was no significant difference in the WUE of total biomass production. The relationship between grain yield and Tsf is described as a unimodal curve, as is the relationship between RE and Tsf. Higher yields and a higher RE have been achieved through earlier flowering, and both RE and Tsf reached their optimal values for maximizing GY. Crop breeding is unlikely to achieve further increases in GY through this route in the future. The results suggest that breeding does not improve biomass’s water-use efficiency, but causes changes in biomass allocation strategy, and this could be a new direction for genetically improving grain yield.