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Crop improvement of grain sorghum in Australia: a physiological assessment of the genetic variation in the staygreen character in sorghum

Borrell, A.K. Dr; Ph: (07) 4661 2944; Fax: (07) 4661 5257 ; borrela@dpi.qld.gov.au

Research organisations: Farming Systems Institute, Queensland Department of Primary Industries, MS 508, via Warwick Qld 4370; CSIRO Tropical Agriculture, 306 Carmody Road, St Lucia Qld 4067; Department of Agriculture, University of Queensland, St Lucia, Qld 4072

Objectives:

1. To assess the extent of genetic variation in staygreen and understand the physiological basis of such variation;

2. To determine the value of staygreen in a wide range of target environments using simulation modelling;

3. To enhance the selection of staygreen through genetic analysis and phenotypic assessment of this trait for molecular marker development.

Methodology: Field studies were undertaken in north-eastern Australia to determine the effects of three water regimes and nine hybrids on the components of green leaf area at maturity. Nine hybrids varying in the A35 and RQL12 sources of stay-green were grown under a fully irrigated control, post-flowering water deficit, and terminal water deficit. This project is closely linked to four other projects within the Grains Research and Development Corporation program titled 'The crop improvement of grain sorghum in Australia': grain sorghum germplasm enhancement, genetic potential to improve utilisation of water and nitrogen resources in grain sorghum production, marker assisted selection in sorghum, and assessing target environments to improve efficiency of sorghum breeding. It is also linked with the Australian Centre for International Agricultural Research (ACIAR) project 'Overcoming production constraints to sorghum in rainfed environments in India and Australia'. This proposal has evolved from the outcomes of an earlier GRDC project (DAQ230 NR) bearing the same name (A physiological assessment of the genetic variation in the staygreen character in sorghum). The preliminary research from this earlier project suggests that incorporation of drought resistance via staygreen into commercial hybrids will be highly beneficial to the sorghum industry in dry years, without imposing a yield cost in wetter years, indicating that more detailed research should be undertaken in this promising area. Drought is a major constraint to rainfed sorghum production in northern Australia. One of the biggest challenges for sorghum improvement programs is to develop plants which have an advantage in water limited environments. This has led to a search for traits which confer drought resistance eg staygreen, osmotic adjustment and transpiration efficiency. In particular, genotypic variation in such traits has been sought. Rapid premature plant death generally occurs when water is limiting during the grain filling period in sorghum (Stout and Simpson 1978; Rosenow and Clark 1981). During post anthesis drought, genotypes possessing the staygreen trait maintain more photosynthetically active leaves compared with genotypes not possessing this trait (Rosenow et al. 1983). The expression of staygreen has also been reported in a number of other cereals including maize (Tollenaar and Daynard 1978; Gentinetta et al. 1986), rice (Wada and Wada 1991) and oats (Helsel and Frey 1978). The physiological basis of genetic variation in the staygreen trait is not well understood. Sorghum genotypes with this trait continue to fill their grain normally under drought conditions (Rosenow and Clark 1981) and display increased resistance to charcoal rot (Rosenow, 1984) and lodging resistance (Henzell et al. 1984; Woodfin et al. 1988). Staygreen genotypes have also been reported to contain more cytokinins (McBee 1984) and basal stem sugars (Duncan 1984) compared with senescent genotypes. Further study is proposed in three key areas. The first is physiology. On the basis of results from DAQ230 NR, further experiments will be undertaken on transpiration, transpiration efficiency or carbohydrate partitioning to clarify the physiological basis of the processes controlling staygreen. This will include collaboration with scientists at University of Queensland (Dr Mortlock) to examine the association between staygreen and other physiological mechanisms such as transpiration efficiency and osmotic adjustment. The second is simulation modelling. Data from these physiological studies will be incorporated into the sorghum component of the APSIM model and used to assess the value of staygreen in a range of target environments. The third is the selection of staygreen. Using data and the proposed experiments to determine heritability and combining ability. These analyses will assist in the selection of parents in the Queensland Department of Primary Industries sorghum breeding program. In collaboration with CSIRO, rate of leaf senescence will be measured in recombinant inbred lines and QL39/QL41 progeny varying in staygreen. This will assist in the development of molecular markers for this trait.

Progress: Water deficit reduced total plant leaf area by about 12% in the terminal drought treatment compared with the fully irrigated control. Water deficit was sufficiently severe under the terminal drought to limit cell expansion in all leaves emerging after the 9th leaf. Onset of leaf senescence was also delayed in the fully irrigated control compared with the terminal water deficit. In general, rate of leaf senescence increased with water deficit, although water regime and genotype interacted. Hybrids possessing the A35 and RQL12 sources of stay-green retained more green leaf area at maturity compared with intermediate and senescent hybrids. However, the mechanism of leaf area maintenance varied with the source of stay-green. RQL12 hybrids displayed delayed onset and reduced rate of senescence, yet A35 hybrids displayed only delayed onset. Higher green leaf area at maturity in A35 hybrids was also due to increased total plant leaf area prior to anthesis, and this advantage was maintained to maturity. Visual rating of green leaf retention was highly correlated with measured green leaf area at maturity, and these ratings were more highly correlated with green leaf area when taken just before, rather than just after, physiological maturity. These studies further support the use of the visual rating method to select for stay-green in sorghum breeding programs.

These studies suggest that sorghum hybrids possessing the stay-green trait have a significant yield advantage under water-limited conditions compared with hybrids not possessing this trait. Grain yield was correlated positively with green leaf area at maturity and negatively with rate of leaf senescence. Harvest index was not correlated with rate of leaf senescence, indicating the association between high grain sink/source ratio and senescence under water-limited conditions can be broken. When water was limiting during the latter half of the grain filling period, panicle growth in stay-green hybrids was largely dependent on photo-assimilation in the remaining green leaves. Less retention of green leaves in the intermediate and senescent hybrids resulted in lower grain yields, although this yield decline was offset to some extent in the intermediate hybrid by utilisation of stem reserves for grain filling. The positive correlation observed between rate of leaf senescence and the magnitude of stem reserves mobilised provides some evidence that stay-green hybrids are less reliant on non-structural stem carbohydrates for grain filling compared with senescent hybrids, resulting in stronger stems and less lodging. No differences in grain yield were found among 8 of the 9 hybrids under fully irrigated conditions, suggesting that the stay-green trait did not constrain yield when water was not limiting.

Period: starting date 1996-07; completion date 1999-06

Status: ongoing

Keywords: crop modelling; drought; genetic factors; plant physiology; Sorghum

Publications:

Borrell, A.K. (1993). Understanding staygreen in grain sorghum. Australian Grain Magazine 3(5), 5.

Borrell, A.K. (1995). Improving crop production in water limited environments. Australian Grain Magazine 4(6), 27-28.

Borrell, A.K. (1995) (ed.). Lessons and opportunities from the drought. Proceedings from a symposium organised by the Australian Institute of Agricultural Science, University of Southern Queensland, November 1994. ISBN 0 85856 120 4. AIAS occasional publication no. 84.

Borrell, A.K. (1995). A review of crop science in Australia. Agricultural Science (Journal of the Australian Institute of Agricultural Science and the New Zealand Institute of Agricultural Science 8(6), 29-33.

Borrell, A.K. and Douglas, A.C.L. (1996). Maintaining green leaf area in grain sorghum increases yield in a water-limited environment, pp. 315-322. In (M. Foale and R.G. Henzell, eds) Proceedings of the Third Australian Sorghum Conference, Tamworth, 20 to 22 February 1996. Australian Institute of Agricultural Science Occasional Publication No. 93.

Borrell, A.K. and Douglas, A.C.L. (1997). Maintaining green leaf area in grain sorghum increases nitrogen uptake under post-anthesis drought. International Sorghum and Millets Newsletter 38, 89-92.

Borrell, A.K., Douglas, A.C.L. and Henzell, R.G. (1998). Genotypic variation for rate and onset of leaf senescence in grain sorghum, pp. 379-382. Agronomy - Growing a Greener Future. In (D.L. Michalk and J.E. Pratley eds) Proceedings 9th Australian Agronomy Conference, Charles Sturt University, Wagga Wagga.

Borrell, A.K., Henzell, R.G. and Douglas, A.C.L. (1996). Visual rating of green leaf retention is highly correlated with measured green leaf area in sorghum. In (M. Foale and R.G. Henzell, eds) Proceedings of the Third Australian Sorghum Conference, Tamworth, 20 to 22 February 1996. Australian Institute of Agricultural Science Occasional Publication No. 93.

Henzell, R.G., Hammer, G.L., Borrell, A.K., McIntyre, C.L. and Chapman, S.C. (1997). Research on drought resistance in grain sorghum in Australia. International Sorghum and Millets Newsletter 38,1-9.

Muchow, R.C., Borrell, A.K. and Hammer, G.L. (1996). Recent advances and current needs in sorghum physiology. In (M. Foale and R.G. Henzell, eds). Proceedings of the Third Australian Sorghum Conference, Tamworth, 20 to 22 February 1996.

Muchow, R.C., Bidinger, F.R., Hammer, G.L., Cooper, M., Borrell, A.K. and Chapman, S.C. (1996). Targeting sorghum improvement in drought-prone environments: approaches and progress. Proceedings of the International Conference on Genetic Improvement of Sorghum and Pearl Millet, Lubbock, Texas, USA, pp. 429-442.

Rosenow, D.T., Ejeta, G., Clark, L.E., Gilbert, M.L., Henzell, R.G., Borrell, A.K. and Muchow, R.C. (1996). Breeding for pre- and post-flowering drought stress resistance in sorghum. Proceedings of the International Conference on Genetic Improvement of Sorghum and Pearl Millet, Lubbock, Texas, USA, pp. 400-411.

 

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