*Correspondence: rajeev.varshney@murdoch.edu.au

Terminal heat and drought stresses have greatly affected wheat production since the high temperature and dry weather are in company with the stages of wheat flowering and grain filling. Global warming enhances the importance of improving the tolerance of heat and drought stresses in wheat for sustaining wheat industry.

For quantifying the impact of the heat and drought stresses and understanding the genetic basis of the tolerant genotypes, the heat and drought stresses were applied for 345 wheat genotypes for five consecutive days at anthesis in a heat chamber. Substantial genotypic differences in heat and drought tolerant levels were found.

The grain weight loss was arranged from 0 to 91% while the average was 52%. The tolerant lines showed almost equal flag leaf area and similar chlorophyll content of the flag leaf to the control plants, whereas, in general, the flag leaf area went down 21% due to the narrowed down leaf width by 20%, and the chlorophyll content of the flag leaf was decreased by 40%.

On average, the plant height was shortened due to the great reduction of the peduncle length under the stress treatment while the effective tiller number, the biomass and spike weight per plant were significantly reduced. The large loss of grain weight per plant was caused by the large deduction of thousand grain weight and grain number per spike. Especially, the seed setting rate was greatly decreased.

However, the tolerant genotypes with no or less grain weight deduction did not show the decrease of the leaf chlorophyll content and the shrink of the flag leaf area, which indicates the tolerant mechanisms in response to the heat and drought stresses.

Under the stresses, the large osmotic potential within the plants induced by small molecules, eg. proline, polysaccharides etc. may contribute to the tolerant levels, and the high remobilization efficiency of the stem water soluble carbohydrate is strongly suggested as another tolerant mechanism for those tolerant genotypes.

Further GWAS analysis are highly recommended to be carried out in this population for identifying the candidate genes in those mechanism pathways. In addition, the flag leaf area (in particular, leaf width), and the leaf chlorophyll content are suggested as valuable indicators for heat and drought tolerant genotype screening.