Linking QTLs That Regulating the Distinct Cuticular Layers in the Spike Glume, and Its Variable Composition to Improve Reproductive Stage Heat Tolerance in Wheat.

Wheat leaf epicuticular wax has been shown to improve tolerance to heat and drought stress in wheat. Specifically, the epicuticular layer has been shown to affect agronomic characteristics by altering transpirational cooling needs by acting as a reflective surface to excess photosynthetic and infrared energy, dissipating heat during high light, high temperatures, and drought. The spike glum also contains a potentially equally important epicuticular layer which appears to be independently inherited in terms of content and composition versus leaf epicuticular wax.  As in leaf epicuticular wax, this variation in spike wax may influence its role in spectral reflectance, water loss and spike cooling, for optimial grain maturation.  If so, as an adaptive trait for heat and drought tolerance genetic loci regulating it composition will need to be defined for use as marker-assisted selection tools. This study aims to address this question using a recombinant inbred line (RIL) population derived from the heat tolerant Australian cultivar ‘Halbred’ which we have previously identified as having a unique genetic loci regulating spike cooling. The RIL mapping panel contains 180 lines derived from ‘Halberd’ and a heat susceptible cultivar ‘Len’.  The population was grown at multiple field locations at Uvalde in Texas, at CIMMYT in Mexico, and at Agricultural Research Center ARC in Egypt. Glume wax was extracted using published methods (Richardson et al, 2007). The waxes are being quantified and characterized using GC-MS.  The wax thickness and layers will be determined by light microscopy designed to be as high throughput as possible. Spike temperature depression, grain-fill rate will be estimated as will harvest index, single kernel weight and kernel number per main spike.  Each yield component will be expressed in terms of a heat susceptibility index. QTLs associated with variation in the chemical composition of the epicuticular wax layer, and their association with resistance to abiotic stress in terms of these yield components will be characterized.