2017 ASHS Annual Conference

Roots release chemical compounds into the rhizosphere that aid in the acquisition of resources and mediate positive and negative plant-microbial symbioses. However, these root exudates have been known to differ in composition and magnitude in response to environmental changes. For instance, roots produce mucilage, a viscous substance, in response to drying soils. This allows roots to elongate in search for water under water stress. Furthermore, recent evidence suggests that plants can select for plant growth-promoting rhizobacteria (PGPR) under water stress by altering root exudation. Drought stress is a major limiting factor to agricultural production. Therefore, observing root exudation under drought stress will contribute to better understanding whole plant drought tolerance. Sorghum (Sorghum bicolor) is an excellent model crop species to evaluate root exudation under drought stress as the species exhibits significant genetic variation in response to limited-water. We characterized the effect of pre-flowering water stress on root exudation. Gas chromatography-mass spectrometry was utilized to characterize exudates in the rhizosphere of four grain sorghum lines that were either drought tolerant (T), BTx623 and Tx7000, or susceptible (S), BTx642 and SC56, under water-stressed and well-watered conditions. Additionally, physiological and above and below ground morphological responses to water stress were examined. All traits were measured at two time points: (1) four days of withholding water (limited water) and (2) 24 hours after rewatering (24-hour recovery). At limited water, there were no differences in leaf area or root characteristics between treatments within each genotype, however assimilation and stomatal conductance rates were reduced for BTx623 (T) and SC56 (S) under water-stressed conditions compared to well-watered controls. At 24-hour recovery, leaf area was reduced for water-stressed lines BTx623 (T) and SC56 (S) and total root lengths were reduced for all water-stressed lines compared to their well-watered controls except for in Tx7000 (T). Further, all water-stressed lines had reduced assimilation rates compared to their well-watered controls at the 24-hour recovery time point. However, tolerant lines displayed more subtle changes in physiology than susceptible lines, which may be indicative of pre-flowering drought tolerance. Additionally, we describe variation in plant-mediated exudates of lines under water stress and recovery and between tolerant and susceptible lines. Characterizing root exudation trends between tolerant and susceptible lines of sorghum during water stress and recovery will contribute to a more accurate understanding of drought responses in sorghum and can be used in breeding for pre-flowering drought tolerance in the face of a changing climate.