Genetic Variability for Gas Exchange Rates and Transpiration Ratio Related to Pre-Flower Drought Tolerance in Sorghum.

There is an increasing need to improve crop water-use efficiency (WUE) (i.e., the ratio of whole-plant biomass to cumulative transpiration) due to decreased water availability and increased food and energy demands throughout the world.  In previous studies, the preflowering transpiration ratio A:E [CO₂ assimilation rate (A) divided by transpiration rate (E)] of sorghum leaves was  correlated with WUE. The present greenhouse study was conducted to examine preflower A:E  in a recombinant  inbred line population of sorghum (Sorghum bicolor L. Moench) lines.  A total of 71 lines and two parents (Tx 430 and Tx 7078) were evaluated for in terms of A:E’s relationship to WUE. Parents had been selected with contrasting A:E  based on previous studies.  The experimental design was a Randomized Complete Block with replications, genotype and water regime (40 and 80 per cent of field capacity) as experimental factors. Genotype, water regime and their interaction had a significant effect on A, E and A:E.  Regression of A on E showed a greater genetic variability among the inbred lines than among parents for transpiration ratio (A:E).  Average A:E was 3.07 mmol CO₂ mol^-1 for Tx430 and  2.80 for Tx 7078. Heritability estimates for A:E , A and E were 0.33, 0.14 and 0.08 respectively. These results provide further evidence that there is genetic variability among genotypes for gas exchange rates at pre-flowering in sorghum with heritability values that suggest scope for improved WUE and productivity.