Physiological traits for tolerance to post-anthesis drought and heat stress in wheat

Drought and heat waves are projected to increase in frequency and severity in the light of a changing climate. Both stresses commonly occur simultaneously in wheat growing regions and challenge wheat production, especially during the grain filling stage when grain weight is being determined. Given the importance of wheat as a global food crop and its value as an export commodity, fluctuations in wheat productivity cause both social and economic issues. One of the strategies to alleviate such consequences is to continually develop wheat varieties with better performance in a dry and hot climate. This work aimed to identify novel physiological traits underlying wheat tolerance to the combination of drought and high temperatures to help identifying drought and heattolerant wheat varieties. To this end, the presented studies investigated four potential indicators of tolerance to combined drought and heat stress (D&H): plant water use, carbohydrate partitioning, xylem embolism and reactive oxygen species (ROS) modulation following combined drought and high temperature stress. Experiments were designed to mimic post-anthesis drought including a three-day heat stress. The first experiment examined plant water use, carbohydrate partitioning and ROS response to combined drought and heat stress in eight wheat genotypes contrasting for grain weight under drought or combined drought and heat stress. Water-soluble carbohydrates (WSC) were mainly stored in the spike of modern varieties, unlike older varieties where most of the WSC were stored in the stem. Glucose and fructose concentrations in grains measured 12 days after anthesis were associated with total grain weight. Three-day heat stress differentially affected transpiration response to vapour pressure deficit, subsequently reducing daily water use in some genotypes while other genotypes were able to maintain water use. A genotypic difference in the modulation of the ROS scavenging system was observed following combined D&H; the activity of glutathione reductase, a ROS scavenging enzyme, was specifically reduced by combined D&H, while non-enzymatic antioxidant capacity was induced in the same genotypes. An in planta method was develop to quantify H₂O₂, one of the ROS, and oxidative stress in vivo using optical fibres coupled with probes that could detect physiological ROS concentrations. Reduction in transpiration following D&H could reduce water flow to the spike, alter its hydraulic properties and subsequently the grain filling process. The second experiment investigated the hydraulic properties of the wheat spike following combined D&H. Hydraulic conductance, water potential and water flow to the spike were measured in two wheat varieties contrasting for grain weight under combined D&H. Reduced daily water use following combined D&H, used as a surrogate for transpiration, did not negatively affect water flow to the spike unlike leaf transpiration that was reduced by drought and combined D&H. This suggested differential regulation of transpiration under stress depending on the transpiring organ. Although there was a genotypic difference in the hydraulic conductance of the peduncle and water potential of the spike, these hydraulic properties were not associated with sap flow to the spike. Xylem embolism could possibly explain reduction in whole plant transpiration following D&H. The third experiment explored xylem embolism in the peduncle and the flag leaf of one wheat genotype. Visualisation of cavitation using the optical method revealed a delayed embolism in the peduncle compared to the flag leaf. Vulnerability curves showed that the peduncle was more resistant to embolism than the flag leaf. This hydraulic vulnerability segmentation in wheat may protect the spike from variations in water availability in the rest of the plant. The findings arising from this work contribute significantly to the body of knowledge on the physiological response of wheat plants to drought and to combined drought and heat stress and the differences between them. We investigated partitioning of water use and carbohydrates between tissues following stress and gained significant, new insights into tissue variability. We explored, in detail, potential physiological traits that could assist in the selection of wheat varieties well-adapted to a dry and hot environment.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 202