Heat stress tolerance examined
Heat stress is a significant abiotic stress and inhibitor of crop production in Australia. The wheat plant is most sensitive to heat stress during flowering and grain filling, and ‘heat-stress events’ often coincide with drought stress and frost events. This makes it difficult to quantify the effects of heat stress, specifically, on grain yield under field conditions.
At Australian Grain Technologies (AGT) we were able to demonstrate the negative impact of the stress on farm profitability. That in turn allowed us to calculate the potential gains if heat stress tolerance could be improved.
To do that, grain yield data was collected from National Variety Trials (NVTs) across southern Australia and analysed with reference to climatic data from the nearest Australian Bureau of Meteorology weather station for each experiment.
The analysis looked at the effects of various climatic variables on grain yield in more than 600 field experiments during 2005–10 growing seasons. Data on climatic variables was provided by Dr Scott Chapman and Dr Bangyou Zheng of CSIRO Plant Industry.
The most adverse effects on yield were found to result from stress during flowering as opposed to grain fill. Day temperatures over 35ºC decreased yields by an average of 837 kilograms per hectare, whereas the impact of similar levels of heat stress during grain filling was a loss of 179kg/ha. The average grain yield across all trials was 2530kg/ha.
The effects from all the analysed climatic variables are listed in Table 1. To recoup some of these production losses, AGT is managing two research projects investigating ways to boost heat-stress tolerance in wheat.
The first is a controlled-environment investigation funded by the South Australian Grain Industry Trust and the second is a GRDC-funded field validation project.
The GRDC-funded project aims to evaluate varietal responses to heat-stress tolerance under field conditions and assess the value of genetics previously identified by various studies as important in influencing heat-stress tolerance.
This work is being carried out at the GRDC-funded managed environment facilities (MEFs) at Merredin (Western Australia), and Narrabri and Yanco (New South Wales). These facilities allow experiments to be conducted in field conditions but enable greater flexibility to induce or alleviate stresses such as drought through the use of irrigation and rainout shelters. They also enable intensive field measurements that can provide detailed plant trait analysis and evaluation.
Importantly, using the MEFs allows for sowing times that delay plant development in order to increase exposure to heat-stress conditions later in the season. Irrigation is used to offset any drought effects, which may mask the true heat-tolerance levels of the evaluated germplasm.
To date, the project appears to be using this strategy successfully. The number of heat-stress events during the flowering to grain-fill growth stages at each location in 2012 is shown in Table 2.
The aim is to develop a field assay to accurately test the heat-stress tolerance of wheat varieties. This will enable us to develop a better understanding of the levels of heat stress tolerance within current Australian germplasm, understand the physiological traits involved with plant responses to heat stress, and to begin to explore the genetics of the heat-stress tolerance.
Ultimately, this work may contribute to the development of genetic markers that breeders can use to select for superior heat-stress tolerance within a breeding program.
In the short term, the data collected has highlighted that current best practice in terms of sowing time and appropriately matching it to crop maturity are the best ways to minimise the effects of heat stress on crop production.
GRDC Project Code AGP00010