Phenotyping for Heat Adaptive Traits.

As agricultural environments become generally warmer and subject to increased frequency of heat shocks, it is imperative to ensure that genetic gains in breeding are robust to these conditions. Wheat –the world’s widest grown crop- while typically showing a linear response to water availability is relatively susceptible to heat stress. One approach to tackle adaptation to abiotic stress is to push yield potential as high as possible so that crops take best advantage of environmentally favorable parts of the cycle or whole seasons. To this end, the International Wheat Yield Partnership (IWYP),has been established and CIMMYT is responsible for translating products of research into new higher yielding germplasm. However, it is also important to tackle heat stress directly and several physiological traits -that are amenable to high throughput phenotyping- have been shown to drive yield under heat stress. Ground cover is a key early indicator of productivity under chronic heat stress, and in-season and final biomass are reliably correlated with yield (unlike under more favorable conditions). One of the bases for superior growth -under hot, low relative humidity conditions- has been shown to be associated with larger investment in root mass, enabling adapted genotypes to match evaporative demand at high vapor pressure deficit (VPD). Though itself not measurable on a breeding scale, root mass is well estimated by high throughput proxies like canopy temperature and water index, themselves being driven by the interaction of stomatal conductance and VPD. Stay-green also seems to be driven by root health in wheat and sorghum and can be estimated -using spectral indices- in a more robust way than canopy temperature, being independent of atmospheric conditions or boundary layer effects. Membrane thermos-stability has long been associated with heat tolerance but remains relatively low throughput; though large genetic variation is expressed among elite materials, and new and better sources have been identified among exotic wheat genetic resources. One of the greatest challenges facing field-based research in the area of heat tolerance is to control heat shocks. A few labs have experimented with portable heat tents that, if carefully managed, can increase temperature around the canopy in a repeatable fashion. This is a cost-effective way of simulating heat shocks at discrete phenological stages and can be tailored to individual plots in the field. Physiological breeding at CIMMYT –employing several of these traits in strategic crosses- has already demonstrated increased genetic gains at dozens of heat stressed international testing sites in Asia and Africa.