Plant Growth and Physiological Responses to Various Surfactants Injected in Irrigation Water: Tween 20 as a Method for Reducing Water Use in Plant Production
Type of DegreeMaster's Thesis
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Nursery and greenhouse growers who heavily rely on groundwater and/or are located in drought prone regions should be hard-pressed to minimize water waste for the sake of their plants, their budgets, and the environment. Members of the green industry are recognizing the dangers of depleting water from the heavily relied upon aquifers and are seeking to be more efficient with natural water resources even in times when water is plentiful. The research presented in this thesis was directed at contributing to the goal of keeping water waste to a minimum and minimizing the overall water requirement of plants during production. In research previously conducted at Auburn University, the surfactant Tween 20 applied at 100 mg/L reduced the crop water demand by up to 40% for Impatiens hawkerii ‘Celebrate Salmon’ grown in Fafard 3B substrate, and reduced transpiration in Spathiphyllum floribundum ‘Viscount’ by 64% and in Impatiens hawkerii ‘Celebrate Salmon’ by 101% when grown hydroponically. Soil surfactants are generally understood to mitigate soil and substrate hydrophobicity and help soils and substrates retain more water. However, very specific data on actual quantities of water that could be saved by utilizing a soil surfactant in container production are difficult to find. To further test the validity and usefulness of Tween 20 as a means of reducing water waste, Tween 20 was compared with similar available soil surfactant products and tested on a woody plant species to determine if similar results would be observed in woody plants as was observed in prior experiments on herbaceous plants. In prior research conducted at Auburn University, the surfactant Tween 20 has shown potential for decreasing transpiration and increasing plant water use efficiency (WUE). Water use efficiency does not intrinsically equate to drought tolerance therefore the objective of this study was to determine if applying Tween 20 to herbaceous landscape plants would affect drought tolerance as well as WUE. A secondary objective was to determine how Tween 20 would affect plant growth and drought tolerance in relation to two other commercially available products (AquaGro L with PsiMatric Technology and Hydretain ES Plus). The first treatment factor consisted of four solution treatments: 100 ppm Tween 20, 100 ppm AquaGro L with PsiMatric Technology, 320 ppm Hydretain ES Plus, and plain water (Control). A second treatment factor consisting of the treatments “first-time”, “every-time”, and “last-time” were also included. The three treatments mentioned above refer to when the product solutions were injected into the irrigation stream and applied to plants. Shoot dry weight, size index, SPAD readings, drought ratings, and ET did not vary amongst individual treatments. Most data did not suggest that Tween 20 or the other two products effected the growth or drought tolerance of Solenostemon scutellarioides ‘Wasabi’. In a second experiment, three-gallon Ligustrum japonicum ‘Recurvifolium’ were irrigated with treatments of 0, 50, 100, and 200 ppm Tween 20 solutions. Whole plant evapotranspiration (ET) and transpiration (T) were measured gravimetrically on select days throughout the experiment. Leaf-level T, photosynthesis (Pn), and water use efficiency (WUE) were measured once while plants were well hydrated and once again, two days later, when plants were experiencing substantial drought stress. Plant water potential was measured on a weekly basis. Leaf level Pn, T, and WUE did not vary by treatment when plants were well hydrated, however, WUE of plants treated with 50 and 100 ppm Tween 20 decreased compared with control plants. The objective of the third study was to determine if Tween 20, when mixed with water, alters the evaporation rate of the solution. If Tween 20 were traveling through the plant and made it all the way to the leaf surface, evaporation from the leaf surface could potentially be altered by the interfacial properties of surfactants. Concentrations of 0, 25, 50, 75, 100. 125, 150, 175, and 200 ppm Tween 20 solution were placed in 12-ounce capacity open bowls on a bench in a greenhouse at Auburn University, AL. Evaporation rate was measured every morning by weighing each bowl. Results indicate that solutions containing Tween 20 have a slightly increased rate of evaporation compared with water with no Tween 20. For the fourth study, Spathiphyllum ‘Emerald Star’ liners transplanted into 6.5-inch azalea pots were treated with a single 800 mL drench of the surfactants Tween 20 (100 mg/L), AquaGro L with PsiMatric Technology (1.2 mL/L) and the humectant Hydretain ES Plus (2 oz/gal). A Control treatment (plain water) was also maintained. A second treatment factor consisting of the treatments “covered” and “non-covered” was also included. Covered-treated plants had a 4-gallon white plastic bag enclosing the container, wrapped over the substrate surface, and snugged against the base of the plant by pinning the bag into the substrate with an unfolded paperclip. Non-covered-treated plants did not have a bag around the container and substrate. The purpose for the non-covered/covered treatment was to allow (non-covered) or inhibit (covered) evaporation from the substrate surface. Data were collected on evapotranspiration, transpiration, substrate water retention, and leaching. Results indicate that AquaGro L with PsiMatric Technology and Hydretain ES Plus have certain merits regarding beneficial substrate-water relations. Tween 20 had few notable beneficial effects on water savings. More notably, covered-control-treated plants, which had no substrate evaporative loss, had an extended growing period of 21.5 days and produced 107% more dry weight than non-covered-control counterparts.