Wheat flag leaf epicuticular wax morphology and composition in response to moderate drought stress are revealed by SEM, FTIR-ATR and synchrotron X-ray spectroscopy.

Wheat (Triticum aestivum L.) is the largest cereal crop grown in Western Canada where drought during late vegetative and seed filling stages affects plant development and yield. To identify new physiochemical markers associated with drought tolerance, epidermal characteristics of the flag leaf of two wheat cultivars with contrasting drought tolerance were investigated. The drought resistant 'Stettler' had a lower drought susceptibility index, greater harvest index and water-use efficiency than the susceptible 'Superb'. Furthermore, flag leaf width, relative water content and leaf roll were significantly greater in Stettler than in Superb at moderate drought stress (MdS). Visible differences in epicuticular wax density on the adaxial flag leaf surfaces and larger bulliform cells were identified in Stettler as opposed to Superb. Mid-infrared attenuated total internal reflectance spectra revealed that Stettler flag leaves had increased asymmetric and symmetric CH but reduced carbonyl esters on its adaxial leaf surface compared to Superb under MdS. X-ray fluorescence spectra revealed a significant increase in total flag leaf Zn concentrations in Stettler in response to MdS. Such information on the microstructural and chemical features of flag leaf may have potential as markers for drought tolerance and thereby accelerate the selection and release of more drought-resistant cultivars.