Breeding processes based on yield data from stress environments alone, may lead to low heritabilities of drought stress tolerance. The crop water stress index (CWSI) (Jackson et. al. 1988) indicating the relation between actual transpiration and potential transpiration may serve as an additional, more sensitive and specific measure for drought stress. Its application may lead to a better understanding of plant physiological traits responsible for drought stress tolerance.

Two biparental populations consisting of 480 inbred lines and hybrids of rye were evaluated under irrigated and rainfed regime on 2 locations resulting in 4 drought stress prone environments. A georeferenced tractor based measurement system was used to measure canopy temperature (Apogee IR 120 sensors), leaf area index (calculated from spectral reflection) and weather conditions simultaneously and repeatedly between stem elongation and ripening. CWSI was calculated from canopy temperature and ambient meteorological conditions and was compared to yield reduction between irrigated and rainfed conditions.

CWSI values were high during drought stress phases and our analysis revealed a significant positive correlation between yield difference (irrigated-rainfed) and CWSI at flowering. We further analysed measurements at several growth stages towards a possible differentiation between genotypes. The contribution of CWSI measurements in identifying different mechanisms of drought resistance will be discussed.