Gas Exchange and Transpiration Ratio in Sorghum

Decreased water availability and increased food demand worldwide require development of more water‐efficient crops. This study was conducted to examine preflower transpiration ratio (CO2 assimilation rate [A]/transpiration rate [E], A:E) in four inbred sorghum [Sorghum bicolor (L.) Moench] lines and 12 of their F1 hybrids in terms of phenotypic and genetic variation, inheritance (i.e., additive and nonadditive genetic effects), and its relationship to water use efficiency (WUE) (i.e., the ratio of whole‐plant biomass to cumulative transpiration). Lines were selected with contrasting A and A:E in a field trial and then crossed in a full diallel pattern. All hybrids and lines were further analyzed for A, E, A:E, biomass, and WUE in a greenhouse. Variation among hybrids and lines was significant for all traits evaluated. In both environments, the most contrasting lines for A:E were Tx430 and Tx7078. Average A:E was 3.10 mmol CO2 mol−1 H2O for Tx430 and 2.91 for Tx7078. Both Tx430 and Tx7078 had highest A. Genetic effects resulting from general (GCA) and specific combining ability (SCA) for A, E, and A:E were significant for all traits. Tx430 had positive and Tx7078 negative GCA effects on A:E. Tx430 hybrids had the highest A:E values, and Tx7078 hybrids the lowest. The A:E ratio was correlated with total biomass and WUE for all crosses. Data suggest that concomitant selection for high A:E and A at preflowering could result in improved WUE and biomass of mature sorghum.