Two major genes controlling high osmotic adjustment (OA) in grain sorghum have been identified; one additive (0A2), one recessive (oa1). However, neither the physiological mode-of-action nor the genetic correlation between osmotic adjustment and grain yield in sorghum production environments has yet been obtained. This study seeks to quantify the contribution of osmotic adjustment, under the control of these two genes, to grain yield in the sorghum production environments of Australia. Recently a quantitative characterisation of the types of water deficits that occur in the grain sorghum production environments of northern Australia has been achieved. This understanding has the potential to enable grain sorghum breeders and physiologists to identify, from the range of putative drought resistant traits that exist for grain sorghum, the traits that are worthy of pursuing to achieve improvements in either specific or
broad adaptation to the range of water deficit environments encountered in target population of environments. This information is used in combination with physiological and genetic analyses in this thesis as a framework for investigating the contribution of the two OA genes to grain yield under drought in production environments.
Controlled-environment experiments were used to examine the repeatibility of osmotic adjustment expression in three grain sorghum populations segregating for one or both of the major OA genes. The identification of low repeatibility estimates, both within and across three consecutive screens, identified the need for further experimentation, which focused on manipulation of aerial and soil water deficits within the screening environment in an attempt to identify conditions that maximised genotypic discrimination for osmotic adjustment. Combinations of high vapour pressure deficits and the low levels of plant available water maximised
genotypic discrimination for osmotic adjustment. The improved levels of repeatibility were a result of reduced random environmental variation when the genotypes were exposed to severe water stress, combined with the expression of OA in response to substantial declines in relative water content accompanying the declines in leaf water potential.
The contribution of osmotic adjustment to maintenance of grain yield under water deficit environments was investigated across two seasons of field experimentation. Managed rainout-shelter experiments identified stem remobilisation as a plausible mode-of-action for the contribution of high osmotic adjustment to maintenance of high grain yield under a post-anthesis water deficit. Furthermore, in the post-anthesis water deficit experienced in a sample of the production environments, small but significant genetic correlations were observed between maximum osmotic adjustment and grain yield maintenance across three separate
grain sorghum populations that segregated for level of osmotic adjustment capacity.
The sorghum crop module of the agricultural production system simulator (APSIM) was modified to simulate the mode-of-action of osmotic adjustment effects based on the results of this study and other results reported in the literature. Grain yield benefits up to 10% over the standard genotype were obtained across all environment types (patterns of water deficit) generated from 100 year simulations for seven locations in central and south-east Queensland. The probability of realising grain yield benefits were conditional on the type of water deficits encountered, with mainly post-anthesis water deficits that were mild or water deficits that experienced some relief during grain fill being the environment types in which an affect was most likely to be observed.
This study has identified a plausible mode-of-action by which osmotic adjustment can contribute
to increased grain yield of sorghum under water stress. This effect was observed in physiological analysis of specific lines and as a positive genotypic correlation coefficient between levels of osmotic adjustment and grain yield in three different genetic backgrounds. These effects were then analysed through crop simulation to determine the potential yield benefits in water deficit conditions encountered in the target population of production environments of northern Australia. The average simulated yield advantage over all seasons and sites was modest and ranged from 1.5% to 2.5% depending on the mode-of-action of OA implemented in the model.
This study, applied a physiological-genetic modelling framework targeted at the improvement of drought resistance in grain sorghum, and using this approach has quantified the grain yield gains that are expected through increasing osmotic adjustment capacity, of grain sorghum in Australian production environments.
Further research is necessary before this trait can be recommended as a selection criterion in the Australian sorghum breeding program. The difficulties in obtaining repeatable osmotic adjustment measures in the material studied, together with the relative value of osmotic adjustment to the sorghum breeding programs, which has already made sizeable inroads into drought resistance through traits such as staygreen, warrants discussion prior to further study into osmotic adjustment.