Genetic Variability and Control of Nodal Root Angle in Sorghum

Sorghum [Sorghum bicolor (L.) Moench] is a major dryland cereal crop in environments with low and unpredictable rainfall. Root angle at the seedling stage can potentially influence root architecture and hence adaptation to drought. The aims of this study were to investigate the extent of genetic variation in nodal root angle of sorghum and to quantify the general combining ability (GCA) and specific combining ability (SCA) for the trait. A diverse range of inbred lines and hybrids was grown in custom-made containers in a glasshouse. Plants were harvested when six leaves had fully expanded and encompassing angle of first flush of nodal roots (relative to vertical), leaf area, and root and shoot dry weight were determined for each plant. Nodal root angle ranged from 15 to 50 degrees for inbred lines and 14 to 43 degrees for hybrids and had a moderately high heritability (47%) in inbred lines. However, significant interaction between male and female parents in hybrids indicated significant SCA. Variation in nodal root angle was independent of variation in plant size (shoot and root weight and leaf area). Variability in plant size among hybrids was associated with the female parent (p < 0.001) and resulted in significant GCA. Screening and selection for nodal root angle should not be affected by potential differences in plant size between inbred lines and hybrids, although the significant SCA would limit predictability of root angle in hybrids based on the root angle of the parental inbred lines.