Molecular Markers Associated with Water Use Efficiency and Leaf Ash in Soybean

Water use efficiency (WUE) is an important trait that has been associated with drought tolerance of crop plants, and leaf ash (LASH) generally related to WUE. A restriction fragment length polymorphism (RFLP) map was constructed from a soybean [Glycine max (L.) Merr.] population of 120 F4-derived lines from a cross of’Young’ × P1416937. The purpose of this research was to identify quantitative trait loci (QTL) associated with WUE and LASH in 36-d-old, greenhouse-grown plants. The experimental design was a randomized complete block with six replications. Significant (P < 0.01) phenotypic differences were detected among the lines for both traits. A total of four and six independent RFLP markers were associated with WUE and LASH and if combined each group of markers would explain 38 and 53% of the variability in the respective traits. One marker locus (cr497-1), on USDA Linkage Group J, explained 13.2% of the variation in WUE indicating the presence of a major QTL. The LASH was negatively correlated with WUE (r = -0.40"*), and two QTL were associated with both WUE and LASH. For each of these QTL, the allele for increased WUE was associated with reduced LASH. M ost of the soybean hectarage in the USA is grown in areas where erratic and low rainfall often reduces yield. This is particularly evident in the southern USA where the average annual loss has been estimated to be over 1.0 Mgha-1 (Carter, 1989). A drought tolerant plant introduction (PI 416937) has been identified recently in the USDA soybean germplasm collection and is characterized by tolerance to wilting during severe drought stress (Sloane et al., 1990). The primary drought tolerance mechanism of PI 416937 is a large fibrous root system which is able to extract soil moisture more efficiently than cultivar Forrest. It is also tolerant to aluminum, which gives it an added advantage of root growth in acid soils (Carter and Rufty, 1993). The drought tolerance mechanism of PI 416937 probably results from the maintenance of water supply rather than reduction in the amount of water used. While this is a good strategy for a short duration or intermittent drought, such a strategy may not be as effective under a longer duration drought, during which soil moisture could be completely depleted resulting in earlier plant death. It would be~ logical to try to combine the soil moisture extracting ability of PI 416937 with improved WUE (total biomass accumulation/total water used). In two greenhouse experiments that included 35 soybean genoM.A.R. Mian, D.A. Ashley, H.R. Boerma, and W.A. Parrott, Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602-7272; M.A. Bailey, Pioneer Hi-Bred Int., Inc., 7300 NW 62nd Ave., P.O. Box 1004, Johnston, IA 50131; and R. Wells and T.E. Carter, Jr., (USDA-ARS), Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 276957631. The research was funded by state and Hatch funds allocated to the Georgia Agric. Exp. Stns. and by a grant from the United Soybean Board. Received 30 Aug. 1995. *Corresponding author ([email protected]. uga.edu). Published in Crop Sci. 36:1252-1257 (1996). types, we found Young as one of the most efficient genotypes in terms of WUE (unpublished data, 19921993). Masle et al. (1992) reported a negative association between WUE and LASH across a range of C3 species. Mayland et al. (1993) found that ash concentration (leaf and stem) of crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes] was negatively associated with WUE. Smith et al. (1982) found a significant negative correlation between WUE and calcium contents in the shoots of ryegrass (Lolium sp.). These responses support the concept (Masle et al., 1992) that there is positive linear relationship between leaf or shoot ash contents and the transpiration ratio (1/WUE). This response would depend on the maintenance of a constant concentration of minerals in the transpiration stream. Environmental conditions could cause variation in the mineral concentration; however, comparison of genotypes under the same environmental conditions should provide repeatable responses. A molecular investigation may provide some insight into the genetic relationships between WUE and LASH. While WUE can be readily measured in pots in which evaporation can be minimized, it is difficult to control or measure soil evaporation in the field. To avoid the difficulty of measuring WUE of field grown plants, Farquhar et al. (1982) and Farquhar and Richards (1984) proposed the evaluation of the discrimination of ~3C by leaves (A) during photosynthesis in C3 plants. The A leaf tissue is correlated with WUE in many crop species (Farquhar and Richards, 1984). Martin et al. (1989) found that 70% of the variation in A between Lycopersicon esculentum Miller (drought-sensitive cultivated tomato with high A) and L. pennellii (Cor.) D’Arcy (drought-tolerant wild tomato with low A) was associated with three RFLP loci that mapped to three different chromosomes. The results from this indirect mapping of QTL for WUE indicate the possibility of direct mapping of QTL conditioning WUE, provided the phenotypic measurements of WUE can be accomplished. Marker assisted selection by means of RFLP or other molecular markers offers an important alternative for selection of hard to measure traits (Tanksley et al., 1989). The identification of QTL allows the analysis and selection of complex quantitative traits (e.g., drought tolerance) as a set of single-gene traits. A RFLP analysis allows the localization of QTL and the determination of the relative magnitude of their effects on traits of interest (Reiter et al., 1991). Indirect selection for drought tolerant QTL via molecular markers may improve selection efficiency. The objective of this study was to identify Abbreviations: WUE, water use efficiency; QTL, quantitative trait loci; LASH, leaf ash; RFLP, restriction fragment length polymorphism; LG, linkage group.