Genetic and physiological studies of heat tolerance in hexaploid wheat (Triticum aestivum L.)

High temperature is one of the major environmental constraints for wheat production globally. It puts significant pressure on the wheat industry around the world, compromising both the quantity and quality of wheat grain produced. The current study focussed on the impact of brief episodes of very high temperatures during vegetative and grain-filling stages of wheat development using a combined approach of plant physiology and quantitative trait loci (QTL) mapping. At grain-filling stage, wheat plants were exposed to a brief heat stress (3 days, 37/27 ºC) 10 days after anthesis and the plants evaluated for a number of morphological and physiological traits (Chapters 3, 4, and 6). At the vegetative stage (~ 4 weeks after sowing) plants were challenged with a brief heat treatment (2 days, 40/30 ºC), and growth and senescence related characters were monitored using automated imaging facilities and a SPAD chlorophyll meter (Chapter 7). In total, 37 bread wheat genotypes were evaluated for different heat responses during the grain-filling stage. Genetic variation was observed among wheat genotypes for various heat responses, particularly for single grain weight, chlorophyll retention, rate and duration of grain-filling, and water soluble carbohydrate content and mobilization (Chapters 3 and 4). Overall, the findings suggested that more than one adaptation process contributed to tolerance. Generally, genotypes with more stable grain weight under heat tended to have particular traits under stress, including the ability to maintain chlorophyll content and rate and duration of grain-filling, and stronger water soluble carbohydrate mobilization efficiency (Chapters 3 and 4). Therefore, these traits may provide appropriate selection criteria for improving heat tolerance in wheat. A genetic linkage map of a Drysdale/Waagan population was constructed using a 9K SNP array (Chapter 5) and used for QTL analysis (Chapter 6) of heat responses (evaluated using heat susceptibility index) at the grain-filling stage. A region on chromosome 3BS strongly affected heat responses of grain weight, stay-green related traits, grain-filling duration, shoot dry weight and harvest index, explaining 10 to 40% of the phenotypic variation, with Waagan contributing the tolerance allele. Most notably, the results indicated a strong genetic link between stay-green and grain weight maintenance under brief episodes of terminal high temperatures but a lack of a significant association between the Rht-B1 and Rht-D1 dwarfing loci and heat tolerance. Using high-throughput automated imaging facilities in The Plant Accelerator, considerable variation among 77 bread wheat genotypes was observed for growth rate and senescence responses to a brief heat stress at the vegetative stage (Chapter 7). A subset of 32 genotypes was also screened at the grain-filling stage (Chapter 3) which allowed a comparison of heat responses at these two developmental stages. Growth rate and senescence responses at the vegetative stage showed significant associations with grain weight maintenance and senescence responses at the grain-filling stage. These results suggested a physiological/genetic link between heat responses at the different growth stages, with implications for developing more efficient heat tolerance screening methods. The present work contributes to the understanding of physiological mechanisms of heat tolerance and its genetic basis in hexaploid wheat, and identifies assays with potential to assist heat tolerance studies and in breeding programs.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2015