Genotypic Variation for Three Physiological Traits Affecting Drought Tolerance in Soybean E. Vicki Hufstetler, H. Roger Boerma, Thomas E. Carter, Jr., and Hugh J. Earl* ABSTRACT Three physiological traits that may affect performance of soybean [Glycine max (L.) Merr.] when soil water availability is limiting are (i) water use efficiency (WUE), (ii) regulation of whole plant water use in response to soil water content, and (iii) leaf epidermal conductance ( g e ) when stomata are closed. Six soybean plant introductions (PIs), eight breeding lines derived from them, and nine cultivars were compared for variability in these three traits during vegetative growth in two greenhouse studies. In the first experiment, whole plant water use, normalized both to plant size and evaporative demand (the nor- malized transpiration ratio, NTR), was monitored during a 10-d cycle of gradually increasing drought stress and then for an additional 2 d following rewatering. The critical soil water content at which each plant began to reduce its water use (FTSW C ), was determined. The WUE was estimated as the ratio of total plant dry weight to total water used. In the second experiment, g e was determined for these same 23 genotypes by measuring leaf water vapor exchange after a 36-h dark adaptation. Substantial variation was found among genotypes for WUE, FTSW C , g e , and also the extent to which NTR recovered on rewatering. Generally, adapted cultivars had greater WUE and lower g e than did PIs. However, PI 471938 and its progeny N98-7264 were clear exceptions to this trend. An unexpected finding was that WUE was significantly negatively correlated with g e across genotypes. D ROUGHT is the leading cause of soybean yield loss in the southeastern USA (Palmer et al., 1996), and so increasing productivity under water deficit stress is an important goal of soybean breeding efforts in this region. Historically, drought tolerance in soybean has been a rather intractable breeding trait for the USA. In the first few decades of modern soybean breeding (1940s through 1970s), breeders were not able to identify any obvious sources of drought tolerance in the adapted breeding pool. Few efforts were undertaken to identify drought tolerance in exotic germplasm because most physiological measures of drought tolerance were time consuming and thus did not lend themselves well to a search for tolerance in the global germplasm collection. In the 1980s and 1990s, several slow-wilting plant intro- ductions (PIs) were discovered (Carter et al., 1999). It quickly became obvious that the slow-wilting trait had some relation to yield under stress, and a recent quan- titative trait loci (QTL) analysis confirmed that two QTL from PI 471938 of Nepal were associated with both slow wilting and improved yield under stress (Lee et al., 2002). Several genetic sources of the slow-wilting trait are now being used in U.S. soybean breeding (Carter et al., 1999). The genetic and physiological bases for this trait are poorly understood at present, and it is unknown whether slow wilting embodies a single mechanism of drought tolerance or perhaps the integration of several. One mechanism for improving drought tolerance in- volves developing soybean lines with higher water use efficiency (WUE, the quantity of crop dry matter accu- mulated per unit of soil water transpired). Genetic vari- ability for WUE has been found in cultivars or lines of several crop species including peanut (Arachis hypogaea L.; Hubick et al., 1988; Wright et al., 1994), cowpea [Vigna unguiculata (L.) Walp; Ismail and Hall, 1993; Ashok et al., 1999], cotton (Gossypium spp.; Quisenberry and McMichael, 1991; Saranga et al., 1998), sorghum [Sor- ghum bicolor (L.) Moench; Donatelli et al., 1992], bar- ley (Hordeum vulgare L.; Hubick and Farquhar, 1989), wheat (Triticum aestivum; Ehdaie and Waines, 1993; Van Den Boogaard et al., 1997), and soybean (Mian et al., 1996; 1998). Another physiological trait that may affect drought tolerance is the decline in whole plant water use during a soil water deficit event. As a soil water deficit develops, plants undergo a transition between the water-replete phase where whole plant water use is not dependent on the soil water content and a second phase where water use is directly related to the availability of soil water (Sinclair and Ludlow, 1986). This transition is associated with a reduction in the average stomatal conductance (e.g., Earl, 2003) and can occur at different soil water contents in different species. For example, Sinclair and Ludlow (1986) found that black gram (Vigna mungo L.) reduced its whole plant transpiration at higher soil water content than did cowpea as soil water content was depleted. Intraspecific differences in this trait have been studied much less extensively than differences in WUE and only in a very few species. Ray and Sinclair (1997) found significant differences among several maize (Zea mays L.) hybrids in the soil water content [ex- pressed as the fraction of transpirable soil water, (FTSW) (Sinclair et al., 1998)] at which whole plant water use be- gan to decline. E.V. Hufstetler and H.R. Boerma, Dep. Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602; T.E. Carter, Jr., USDA-ARS, 3127 Ligon St., Raleigh, NC 27695; H.J. Earl, Dep. Plant Agriculture, Univ. of Guelph, Guelph, ON Canada, N1G 2W1. This research was funded in part by state and Hatch funds allocated to the Georgia Agricultural Experiment Station. The authors also gratefully acknowledge funding from the United Soybean Board that led to the discovery of five slow- wilting soybean plant introductions and six progeny breeding lines used in this work. Received 13 Apr. 2006. *Corresponding author (hjearl@uoguelph.ca). Published in Crop Sci. 47:25–35 (2007). Crop Physiology & Metabolism doi:10.2135/cropsci2006.04.0243 ª Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DAS, days after sowing; FTSW, fraction of transpirable soil water; FTSW C , critical FTSW; g e , minimum epidermal conduc- tance; NTR, normalized transpiration ratio; PPFD, photosynthetic photon flux density; RSWC, relative soil water content; RSWC 10 , relative soil water content when NTR 5 0.1; RSWC C , critical relative soil water content; TR, transpiration ratio; W D , weight of pot 1 lid 1 dry soil; W P , plant fresh weight; W T , target pot weight; WUE, water use efficiency; W W , weight of pot 1 lid 1 saturated soil. Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved. 25 Published online January 22, 2007