Radiation and Water Use Efficiency of Soybean Genotypes that Differ in Wilting Response to Drought.

When most soybean genotypes are wilted due to drought, a few plant introductions have little or no wilting for several days.  The mechanism(s) responsible for delayed wilting is unknown.  It was hypothesized that genotypes having delayed wilting conserve soil moisture under well-watered conditions by decreased stomatal conductance (g_s).  This hypothesis predicts that decreased g_s will also result in lower radiation use efficiency (RUE) and increased water use efficiency (WUE).

To test this hypothesis, field experiments were conducted in 2007 and are being repeated in 2008 at Fayetteville, AR.  RUE, g_s, volumetric soil moisture contents (θ_v), and WUE (carbon isotope discrimination, CID) were measured for 10 different genotypes that differ in wilting characteristics.   RUE was measured in irrigated plots that were planted in 19-cm rows (60 seeds m^-2).  Light interception data were collected biweekly beginning at emergence.  Biomass samples (2m²) were harvested at 12, 25, 43, and 63 days after sowing.  RUE was determined from the increase in biomass between harvests per unit of solar radiation intercepted.  Soil moisture and g_s measurements were made several times during the growing season, and leaf samples were collected 63 days after sowing for CID analysis.

RUE differed among genotypes ranging from 1.23 to 0.92 g MJ^-1 with the extreme values corresponding to the fastest and slowest wilting genotypes, respectively. Above-ground biomass at maturity was closely associated (R²=0.81) with RUE values determined during vegetative growth.  One delayed-wilting genotype had RUE values similar to high-yielding (but fast-wilting) cultivars.  This observation provides evidence that there is more than one mechanism for delayed wilting.  Among the 10 genotypes, there were no differences in g_s or θ_v.  Samples for CID are awaiting analysis and will be reported.