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Changes in Crested Wheatgrass Root Exudation Caused by Flood, Drought, and Nutrient Stress

^a Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan UT 84321
^b Dep. of Civil and Environmental Engineering, Utah State Univ., Logan UT 84321

^* Corresponding author (bugbee{at}cc.usu.edu)

Received for publication October 3, 2006. Root exudates can chelate inorganic soil contaminants, change rhizosphere pH, and may increase degradation of organic contaminants by microbial cometabolism. Root-zone stress may increase exudation and enhance phytoremediation. We studied the effects of low K⁺, high NH₄⁺/NO₃^– ratio, drought, and flooding on the quantity and composition of exudates. Crested wheatgrass (Agropyron cristatum) was grown in Ottawa sand in sealed, flow-through glass columns under axenic conditions for 70 d. Root exudates were collected and analyzed for total organic carbon (TOC) and organic acid content to compare treatment effects. Plants in the low K⁺ treatment exuded 60% more TOC per plant per day (p = 0.01) than the unstressed control. Drought stress increased cumulative TOC exuded per gram dry plant by 71% (p = 0.05). The flooded treatment increased TOC exuded per gram dry plant by 45%, although this was not statistically significant based on the two replicate plants in this treatment. Exudation from the high NH₄⁺/NO₃^– ratio treatment was 10% less than the control. Exudation rates in this study ranged from 8 to 50% of rates in four other published studies. Gas chromatography-mass spectrometry (GC–MS) analysis indicated that malic acid was the predominant organic acid exuded. Fumaric, malonic, succinic, and oxalic acids were also detected in the exudates of all treatments. These results demonstrate that nutrient and water stress have significant effects on the quantity and composition of root exudates. Cultural manipulations to induce stress may change the quantity of root exudates and thus increase the effectiveness of phytoremediation.

Abbreviations: ANOVA, analysis of variance • EC, electrical conductivity • GC–MS, gas chromatography-mass spectrometry • GLM, general linear model • HEPA, high efficiency particulate air • HPLC, high performance liquid chromatography • ICP-ES, inductively coupled plasma emission spectrophotometry • PPF, photosynthetic photon flux • RGR, relative growth rate • TOC, total organic carbon • WUR, water use requirement

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