Changes in root characteristics, gas exchange and water use efficiency following water stress and rehydration of Alfalfa and Sorghum

Soil water stress has paramount effects on water uptake by roots and its use by shoots. In this study, we determined changes in root hydraulic conductivity (Lp(r)) and morphology under drought stress and how they regulate shoot gas exchange and water use efficiency in alfalfa and sorghum, two crops with contrasting adaptations to drought, the former exhibiting strong drought avoidance and the later strong drought tolerance. Seedlings of two alfalfa cultivars differing in drought avoidance and one highly drought tolerant sorghum cultivar were subjected to PEG-6000-induced water stress and then rehydrated. The Lp(r) declined rapidly after the PEG treatment but largely recovered within 48 hours of rehydration; however, inter-specific and cultivar differences were significant. The rapid change in Lp(r) also led to equally rapid changes in leaf water potential, gas exchange and consequently, the instantaneous water use efficiency (WUEi) in both species. A reasonable correlation was found between Lp(r) and WUEi. Treatment with Hg2+ indicated that the water stress-induced changes in Lp(r) were due to the involvement of aquaporins. One year old alfalfa and sorghum cultivars subjected to moderate and severe drought stress at the field level had altered root morphology and reduced biomass production and water dissipation but increased water use efficiency of biomass production (WUEb). These changes were dependent on stress level, species and cultivars. However, not all the root morphological changes contributed to improved water use. For example, the decrease in taproot length negatively affected the WUEb of alfalfa whereas the increase in root surface area was positively related only to the WUEb of sorghum. The difference in drought tolerance between species or cultivars was related to their ability to recover the lost Lp(r) and CO2 assimilation after rehydration, as well as the ability to effectively regulate root morphological changes to increase WUE.