The latest climate models predict an increase in global mean temperature by as much as 1.5°C by 2035. Higher maximum temperatures and extreme transient heatwaves will heighten the challenge of breeding crops that can keep pace with global food demand. Cotton is a subtropical crop and the world’s pre-eminent source of plant fibre. Above-optimal temperatures reduce growth and alter developmental processes, thereby adversely affecting productivity. Genetically diverse wild Australian cotton germplasm was investigated as a source of thermotolerance traits that can guide the path to improved phenotypes of cultivated cotton. Four Australian Gossypium species and G. hirsutum were grown in glasshouses at daytime temperatures up to 38°C and compared with plants at 30°C. The first experimental chapter reports on growth, morphology and physiology of these five cotton species. Growth of three wild relatives (G. australe, G. robinsonii and G. sturtianum) was essentially resistant to 38°C whereas growth, canopy development and gas exchange variables were severely impaired in domesticated cotton (G. hirsutum). In the second experimental chapter, leaf surface anatomy and reflective properties were investigated, revealing wide contrasts between species. The diverse leaf shapes were also quantified in order to determine the importance of lobed margins for heat loss.