Precision phenotyping of a barley diversity set reveals distinct drought response strategies

Plants exhibit a wide array of responses and adaptive mechanisms to drought. During drought, the trade-off between water loss and CO2uptake for growth is mediated by the regulation of stomatal aperture in response to soil water content (SWC), among other factors. We earlier identified, in a few reference varieties of barley that differed by the SWC at which transpiration was curtailed, two divergent water use strategies: water-saving ("isohydric") and water-spending ("anisohydric"). We proposed that an isohydric strategy may reduce risk from early droughts in climates where the probability of precipitation increases during the growing season, whereas an anisohydric strategy is consistent with environments having terminal droughts, or with those where dry periods are short and show little seasonal variation. Here, we have examined drought response in an 81-line barley (Hordeum vulgare) diversity set that spans 20th-century European barley breeding and identified a several lines with a third, dynamic transpirational response to drought. We found a strong positive correlation between vigor and transpiration, the dynamic group being highest for both. However, these lines curtailed daily transpiration at a higher SWC than the isohydric group. While the dynamic lines, particularly cv Hydrogen and Baronesse, were not the most resilient in terms of restoring initial growth rates, their strong initial vigor and high return to initial transpiration rates meant that their growth nevertheless surpassed more resilient lines during recovery from drought. The results will be of use for defining barley physiological ideotypes suited to future climate scenarios.