Physiological and structural traits contribute to thermotolerance in wild Australian cotton species

Background and Aims: Five species of cotton (Gossypium) were exposed to 38 °C days during early vegetative development. Commercial cotton (Gossypium hirsutum) was contrasted with four wild cotton species (Gossypium australe, G. bickii, G. robinsonii and G. sturtianum) that are endemic to central and northern Australia. 

Methods: Plants were grown at daytime maxima of 30 or 38 °C for 25 days, commencing at the four-leaf stage. Leaf areas and shoot biomass were used to calculate relative rates of growth and specific leaf areas. Leaf gas exchange measurements revealed assimilation and transpiration rates, in addition to electron transport rates and carboxylation efficiency in steady-state conditions. Finally, leaf morphological traits (mean leaf area and leaf shape) were quantified, along with leaf surface decorations, imaged using scanning electron microscopy. 

Key Results: Shoot morphology was differentially affected by heat, with three of the four wild species growing faster at 38 than at 30 °C, whereas early growth in G. hirsutum was severely inhibited by heat. Areas of individual leaves and the number of leaves both contributed to these contrasting growth responses, with fewer, smaller leaves at 38 °C in G. hirsutum. CO₂ assimilation and transpiration rates of G. hirsutum were also dramatically reduced by heat. Cultivated cotton failed to achieve evaporative cooling, contrasting with the transpiration-driven cooling in the wild species. Heat substantially reduced electron transport rates and carboxylation efficiency in G. hirsutum, with much smaller effects in the wild species. We speculate that leaf shape, as assessed by invaginations of leaf margins, and leaf size contributed to heat dispersal differentially among the five species. Likewise, reflectance of light radiation was also highly distinctive for each species. 

Conclusions: These four wild Australian relatives of cotton have adapted to hot days that are inhibitory to commercial cotton, deploying a range of physiological and structural adaptations to achieve accelerated growth at 38 °C.