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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
* Corresponding author (hjearl{at}uoguelph.ca)
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 (ge) 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 normalized 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 (FTSWC), was determined. The WUE was estimated as the ratio of total plant dry weight to total water used. In the second experiment, ge 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, FTSWC, ge, and also the extent to which NTR recovered on rewatering. Generally, adapted cultivars had greater WUE and lower ge 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 ge across genotypes.
Abbreviations: DAS, days after sowing • FTSW, fraction of transpirable soil water • FTSWC, critical FTSW • ge, minimum epidermal conductance • NTR, normalized transpiration ratio • PPFD, photosynthetic photon flux density • RSWC, relative soil water content • RSWC10, relative soil water content when NTR = 0.1 • RSWCC, critical relative soil water content • TR, transpiration ratio • WD, weight of pot + lid + dry soil • WP, plant fresh weight • WT, target pot weight • WUE, water use efficiency • WW, weight of pot + lid + saturated soil
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