Quantitative trait locus mapping of the transpiration ratio related to preflowering drought tolerance in sorghum (Sorghum bicolor).

To meet future food needs, grain production must increase despite reduced water availability, so waterproductivity must rise. One way to do this is to raise the ratio of biomass produced to water transpired, which is controlled by the ratio of CO2 assimilation (A) to transpiration (E) (i.e. the transpiration ratio, A : E divided by vapour pressure deficit) or anything affecting stomatal movement.. We describe the genetic variation and basis of A, E and A : E among 70 recombinant inbred lines (RILs) of sorghum (Sorghum bicolor (L.) Moench), using greenhouse experiments. Experiment 1 used 40% and 80% of field capacity (FC) as water regimes; Experiment 2 used 80% FC. Genotype had a significant effect on A, E and A : E. In Experiment 1, mean values for A : E were 1.2-4.4 mmol CO2 mol-1 H2O kPa-1 and 1.6-3.1 mmol CO2 mol-1 H2O kPa-1 under 40% and 80% FC, respectively. In Experiment 2, values were 5.6-9.8 mmol CO2 mol-1 H2O kPa-1. Pooled data for A : E and A : E VPD-1 from Experiment 1 indicate that A : E fell quickly at temperatures >32.3°C. A : E distributions were skewed. Mean heritabilities for A : E were 0.9 (40% FC) and 0.8 (80% FC). Three significant quantitative trait loci (QTLs) associated with A:E, two on SBI-09 and one on SBI-10, accounted for 17-21% of the phenotypic variation. Subsequent experiments identified 38 QTLs controlling variation in height, flowering, biomass, leaf area, greenness and stomatal density. Colocalisation of A : E QTLs with agronomic traits indicated that these QTLs can be used for improving sorghum performance through marker assisted selection (MAS) under preflowering drought stress.