Genetic control of gas exchange processes affecting water use efficiency in grain sorghum

Frequently it is noted that genotypes within a crop species do not differ significantly in water use efficiency (WUE). However, recent evidence suggested genetic variation for WUE in wheat (Triticum aestivum L.), indicating a possibility of exercising selection for the improvement in this character. Therefore, the present study was undertaken to investigate:

1. The genetic variation for the gas exchange components (viz., photosynthesis and stomatal conductance) affecting WUE in grain sorghum (Sorghum bicolor L. Moench);
2. To determine the genetic control of these gas exchange processes; and
3. To study the relationships between the growth behavior of sorghum hybrids with their photosynthetic characteristics.

For this purpose, I have evaluated 30 hybrids, produced by crossing six parental lines in a complete diallel fashion, under field conditions during 1985 and 1986. In addition, three A-lines were crossed with three R-lines to produce nine more hybrids to investigate the effect of male sterile cytoplasm. All the entries were subjected to varying degrees of water stress by imposing irrigation treatments. Data were collected on gas exchange processes and plant growth parameters on all the genotypes and irrigation treatments throughout the growing season in both years.

There was 60% variation among the genotypes for photosynthesis (A), stomatal conductance (g), and A:g ratio, both across the season and in any given sampling time-irrigation level. With moderate water stress, A was reduced more than g, indicating that the initial site of stress sensitivity was at the chloroplast level. As stress intensified, g and A changed in parallel. Genetic analysis of A, g and A:g ratios showed the importance of specific combining ability (SCA) effects in the inheritance pattern of the gas exchange processes. It was also noticed that reciprocal effects, with a substantial contribution of maternal cytoplasmic effects, were important in the inheritance of A. The genotypes also differed significantly in their growth behavior, with a range of 30-40% for leaf area per plant (LA), growth rate (GR), and total dry weight (TDW). There was no consistent relationship between A and each of the growth parameters for all the genotypes. However, some genotypes were identified in both the high and low categories of mean A in which A was strongly associated with LA, GR, and TDW.

The results suggested that there is substantial genetic variation for gas exchange processes affecting WUE and that non-additive genetic effects are important in governing their inheritance. Therefore, any breeding method which exploits hybrid vigor should be considered in increasing WUE in grain sorghum. Proper choice of lines used as seed parents should also be used to exploit the maternal effects and the cytoplasm-nuclear interactions in the inheritance of A. The relationship between A and growth parameters among certain genotypes, can be used to increase biomass production.