Ever since Australian wheat breeding began over 140 years ago, breeders have either deliberately or unintentionally developed varieties with improving tolerance to heat during the critical stem elongation to grain fill period.
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Today's research includes programs that deliberately select for heat tolerance, and lines with improvement are passed on to the various breeding companies. Success from this research is already apparent with some of these heat-tolerant lines incorporated into released varieties.
Dr Rebecca Thistlethwaite and Professor Richard Trethowan, lead researcher based at The University of Sydney Plant Breeding Institute (NSW farmer-owned facility) with GRDC funding, have identified and bred lines that yield 10 to 30 per cent higher than similar varieties without added heat tolerance.
In early Australian wheat breeding days, William Farrer made the first enormous progress in combating periodic heat and water stress as crops approached maturity by developing the variety Federation. Its earlier maturity compared to other varieties of that day (released 1901) accounted for a lot of its yield advantage and westward move of wheat growing in NSW, as well as into drier areas of other states.
Further improvements in heat stress tolerance occurred from selecting varieties that performed best in trials assessing new breeding lines across the ever-expanding wheat belt. Hot, dry springs have always been an almost regular occurrence, and varieties that coped best under these conditions were commonly selected by breeders.
Today's research dedicated to finding traits with superior heat stress tolerance, combined with high yield in non-stressful situations, aims to accelerate the development of more hardy durable varieties. Dr Thistlethwaite notes that research shows when temperatures exceed 28 degrees during wheat reproductive growth stages of pollen formation and flowering, yields decline. For every 1.0C rise in temperature above this level, yields decline by 4.0 per cent.
Dr Thistlethwaite highlights that wheat (Triticum aestivum) is a complex genetic plant with enormous potential for finding new traits like improved heat tolerance. It is based on two natural separate hybridisation events that occurred thousands of years ago with three grass species (T. Urartu, T. speltoides and Aegilops tauchii).
Lines derived from crosses between ancestors of bread wheat with current durum wheat varieties are providing a great source of genetic variability and improved traits like high yield, heat tolerance and water use efficiency having at least 30 per cent higher yielding potential than existing wheat cultivars. Some 2500 selected lines, for example, are part of the heat tolerance program under assessment.
Rebecca Thistlethwaite and colleagues' research includes a three-tiered phenotyping process where they identify heat-tolerant material in the field at multiple locations and multiple times of sowing (Narrabri, VIC and WA). Selected promising lines from field trials are then assessed comprehensively in field-based heat chambers. Superior lines are then more intensely heat tested under glasshouse conditions.
High temperatures at flowering cause pollen sterility and affect the number of viable florets. High grain filling temperatures influence seed number and seed weight. Mechanisms of improved tolerance to both stresses are critical in germplasm development. This research is linked with other Australian programs and the world. Breeding material is accessed from various international programs also seeking attributes like improved heat tolerance.
Part of the research has identified molecular markers closely linked with improved heat tolerance. Molecular markers are the modern way of breeding improvement and allow plant breeders to rapidly assess new crosses or potential breeding material for given traits. This speeds up the breeding process and increases selection accuracy.
Heat tolerance research linked with other University of Sydney (Narrabri) research targets combining a number of other traits that improve heat stress and improved water use efficiency. These include varieties with faster growing roots, more efficient leaf types, higher biomass, rapid grain filling, and high transpiration efficiency.
For further information on this research, contact rebecca.thistlethwaite@sydney.edu.au
Next week: What is a practical farm tree density?
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
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