Combining abilities and quantitative inheritance of sorgoleone exudation in Sorghum bicolor

Sorghum (Sorghum bicolor) has the ability to inhibit the conversion of ammonium to nitrate (biological nitrification inhibition [BNI]) in the rhizosphere, which in turn prevents the loss of bioavailable nitrogen. Sorgoleone is a lipidic compound secreted by sorghum root hairs and is responsible for roughly 60% of BNI activity in sorghum. Previous studies revealed variation in sorgoleone secretion among different accessions and cultivars. However, little information is available regarding the genetic inheritance of sorgoleone secretion in sorghum. To increase sorgoleone through breeding, an understanding of the inheritance of this trait is required. In this study, 21 seed parents and 21 pollinator parents from the Texas AgriLife Research sorghum breeding program were crossed in an incomplete factorial design to generate 158 hybrids, and sorgoleone secretion from both hybrids and inbreds was quantified. There was significant variation in sorgoleone secretion across hybrids and inbred lines, and small but significant mid-parent heterosis was observed in the hybrids. A linear mixed model analysis to calculate general and specific combining abilities for inbred parents and hybrids detected significant genetic effects for the male, the female, and male × female interactions (p < 0.001). Broad-sense heritability was high ($H^2$ = 0.87), while narrow-sense heritability for the seed parents and pollinator parents was moderate ($H_f^2$= 0.35 and $H_m^2$ = 0.39, respectively). These results indicate that sorgoleone exudation is primarily driven by additive genetic effects, but dominance effects are important for optimum production. These findings indicate that selection for increased sorgoleone root exudation among elite, adapted grain sorghum hybrids and inbred should be effective.