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Genotypic Variation for Three Physiological Traits Affecting Drought Tolerance in Soybean

^a Dep. Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602
^b USDA-ARS, 3127 Ligon St., Raleigh, NC 27695
^c Dep. Plant Agriculture, Univ. of Guelph, Guelph, ON Canada, N1G 2W1

View larger version (19K): [in this window] [in a new window]   Fig. 1. Water stress protocol, using null balance lysimetry. Relative soil water content (RSWC) vs. time is shown for one drought pot and one control pot in a single replication. After reaching their new target weights on each day of the experiment, drought pots were maintained within ± 3% RSWC of the target weight by frequent water additions. Control pots were maintained between 77 and 83% RSWC at all times. The first 24 h of data were used to normalize subsequent days' water use, to account for differences in plant size.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Plateau regression applied to the relationship between the normalized transpiration ratio (NTR) and relative soil water content (RSWC) for two entries (Haskell and G2120) in a single replication. Data for which NTR < 0.1 (horizontal dashed line) were not included in the regression. The critical relative soil water content (RSWCc) is the RSWC where NTR is estimated to begin to decline; above RSWCc, NTR is assumed to be 1.0. The RSWC at which NTR = 0.1 (RSWC10) was estimated by interpolation from the regression. This value was used to calculate the critical fraction of transpirable soil water (FTSWC) from RSWCC as described in the text.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Average dry matter production vs. average water used for 23 soybean lines during vegetative growth in a greenhouse. Each data point is the mean of five or six observations depending on genotype, as shown in Tables 2 and 3. The slope of the regression line through the origin (3.00 g L–1) closely estimates mean water use efficiency (3.04 g L–1).

View larger version (16K): [in this window] [in a new window]   Fig. 4. Linear correlation between water use efficiency (WUE) and minimum epidermal conductance (ge) for 23 soybean genotypes. Data are the same as shown in Tables 2 and 3 (WUE) and Table 4 (ge). WUE and ge data were collected in separate experiments using different greenhouse culture systems. n = 5 or 6 for WUE data, and n = 6 for ge data. Fendou 34 was identified as an outlier (see text).