SOYBEAN WUE - A PERSPECTIVE BASED ON QTL ANALYSIS IN FIVE MAPPING POPULATIONS

¹ Dep. of Agronomy & Horticulture, Univ. of Nebraska, Lincoln, NE, 68583-0915, USA

Although many plant researchers think of water, when it is scarce, as being a plant abiotic stressor, water is more properly treated as a resource (like CO₂, soil nitrate, etc.) that must be exploited, even when scarce, for crop production. Indeed, in pest-free environments, the amount of crop biomass (BM) at any given time is essentially a linear function of two entities: the amount of water transpired (T) to date, multiplied by a (regression) coefficient more commonly known as (biomass) water-use efficiency (WUEb). Thus: BM = T x WUEb, where WUEb reflects the incremental change in BM obtained per unit of incremental change in T. Seed yield (SY) is a linear function of two entities: BM multiplied by coefficient known as harvest index (HI), where HI reflects the seed fraction of total BM, such that: SY = BM x HI. By substitution: SY = T x WUEb x HI. However, if we let WUEb x HI = WUEy (i.e., change in seed yield per unit change in T), then: SY = T x WUEy. This equation illustrates that to genetically increase SY we must either enhance T (i.e., transpire more! of the available soil water), or improve the WUE coefficient. Field-based research has demonstrated that both soybean biomass and seed yield respond to variable seasonal water amounts in a highly linear manner, which means that genotypic differences in the WUEy coefficient are estimable. If those genotypes are members of a mapping population, then a quantitative trait locus (QTL) analysis becomes possible for WUEy. Phenotypic WUEy data have now been obtained on five map populations of 250-300 members each. Relatively few QTLs for WUEs have been detected, and most map to genomic positions coincident with QTLs governing mean yield performance (i.e., yield averaged over all environments) and/or map to genes known to govern plant growth habit and maturity. To provide a retrospective integrated measure of crop transpiration efficiency (TE) up to the day a newly emerged soybean leaf was sampled, one mapping population was also subjected to a ¹³C isotopic composition analysis. However, the QTLs detected for ¹³C-proxy for TE also mapped to the growth habit and maturity genes. In each mapping population, there was a high correlation between a soybean genotype's yields in contrasting water-abundant and water-scarce environments. This implies that any genetic improvement in soybean yield performance in water-abundant conditions also results in a concomitant (though lesser in degree) improvement in water-scarce conditions (and, of course, vice-versa). This dual result is most likely to accrue from a (genetically encoded) higher WUEy.