Nondestructive root exudate sampling using aeroponics throughout progressive drought stress and recovery

Background: Sampling root exudates in situ is challenging because root systems are often damaged while exudates are prone to microbial decomposition and sorption in the soil environment. Hydroponic systems provide the capability to capture root exudates without root damage or soil sorption; however, they lack the utility to provide drought stress. We hypothesized that a high-pressure aeroponic system could induce gradual drought stress and recovery by manipulating misting cycles, and that collecting “rinseate” from washed root systems would contain detectable quantities of exudates without destructively sampling roots. The objectives were to: (1) evaluate the ability of the aeroponic system to impose drought conditions on plants by measuring their metabolic, physiological, and morphological responses; and (2) test the efficacy of the aeroponic system to collect root exudates of cotton under progressive drought and recovery conditions. Progressive drought stress was induced for two weeks in aeroponically grown cotton at the match-head square stage (34 days after planting) followed by a 1-week recovery phase at full irrigation. Results: The progressive drought treatment limited plant development by decreasing canopy height, number of green leaves, leaf fluorescence measurements, tissue biomass, and plant water content, while increasing visual drought severity. Abscisic acid (ABA) was chosen as a root exudate indicator sensitive to drought that could be monitored in the collected root “rinseate”. After initiating the drought treatment, ABA increased in the rinseate from drought-treated plants and subsequently declined in the recovery phase. Some response variables improved by the end of recovery phase (i.e., ABA, fluorescence, drought severity index, and water content), yet others expressed more longstanding effects (i.e., canopy height and number of green leaves). Conclusion: Using the aeroponic system, detectable quantities of root exudates were captured at each sampling time throughout simulated drought, while cotton demonstrated rapid adjustment to drought and recovery. This method expands root exudate collection strategies by offering continuous sampling of target crops throughout a diverse array of simulated stressors.