Matthew Reynolds
Global Wheat Program, International Maize and Wheat Improvement Centre, Mexico

A new generation of stress-adapted wheat lines has been distributed worldwide by the International Maize and Wheat Improvement Centre (CIMMYT) based on physiological trait (PT) based breeding. Lines were derived from crosses designed to combine complementary PTs to generate cumulate gene action for agronomic performance under water deficit. For example, to increase water uptake, lines with deeper roots were selected using as a selection criterion cooler canopy temperatures expressed under drought. Water use efficiency was measured indirectly by estimating -under well watered conditions- poor discrimination against the heavy 13C isotope of CO2 which indicates conservative expression of stomatal conductance. A trait associated with the ability to partition assimilates to grain yield under stress is the capacity for accumulation in the stem and subsequent remobilization to the grain of water soluble carbohydrates. One of the challenges of applying plant physiology in crop improvement is scale, therefore, in addition to thermal screening, other remote sensing approaches were applied in parental and progeny selection such as spectral radiometry to screen for hydration status of leaves and their concentration of photo-protective pigments. The same kinds of tools are used to screen among genetic resources -such as landraces- for high levels of expression of desirable stress adaptive traits, as well as in phenotyping mapping populations. The latter has led to the discovery of QTL associated with cooler canopies as well as the identification of a common genetic basis for adaptation to drought and heat stress, most probably associated with improved root capacity in both environments.