The effect of epicuticular wax (EW) load on the cuticular transpiration rate (Tc of sorghum [Sorghum bicolor (L.) Moench] leaves was studied. Leaves from bloom, bloomless, and sparse bloom isogenic lines and various bloom‐type hybrids were collected from field plots after anthesis in 1976, 1977, and 1981. Epicuticular wax was extracted with chloroform and quantified using a colorimetric method. The Tc of detached leaves were measured with a humidity sensor in a closed cuvette or calculated from the mass of water lost per unit time under standardized conditions. The Tc increased as EW decreased over the range of 0.1 to 0.03 g m−2 when data from all genotypes were pooled for analysis. However, among the normal (bloom) phenotypes there was no clear association between rates of water loss and EW. These data suggest that EW greater than about 0.067 g m−2 provide an effective barrier to water loss through cuticles of sorghum leaves under most conditions. Although the two methods of measuring Tc produced the same qualitative results, Tc rates calculated from the mass of water transpired under more “realistic” controlled conditions were higher and probably more directly related to rates of water loss under field conditions. Because of its speed and technical simplicity, the water loss method appears superior when large numbers of samples must be evaluated.

The effects of genotype and environment on the epicuticular wax (EW) load on sorghum [Sorghum bicolor (L.) Moench.] leaves were studied in a series of field experiments between 1976 and 1981. Entries in the tests were normal, bloom‐type sorghums and represented a range of lines and hybrids. Leaf samples were collected near anthesis, EW was extracted with chloroform, and quantities of EW were estimated by a colorimetric method. In a 4‐year test at Temple, Tex., significant variability in average EW loads occurred among genotypes (0.97 to 1.63 mg dm−2) and among years (0.91 to 1.51 mg dm−2). EW loads increased under hot, dry conditions, but wax loads were not closely related to seasonal rainfall. General and specific combining abilities were determined in a study of 30 F1 hybrids from crosses between six male and five female parents in five environments. General combining abilities of both males and females were significant (P < 0.001), but specific combining ability effects were not significant. Both the broad and narrow‐sense heritabilities were estimated at 36.0%. Because the parents were not chosen in a random manner, these estimates only pertain to our specific germplasm and cannot be considered general. The portion of the genetic variance contributed by females was 73% while the males contributed 27%. The best parental lines for increasing EW loads in hybrids were RTx430 among the males and ATx623 among the females. RTx7078, RTx7000, and A4R generally decreased the EW load of their hybrids. Genetic stability for EW load across environments also varied. The value of low genetic stability of adaptive responses such as EW loads for crops grown in drought‐prone environments is discussed.

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