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At the Root of It: Investigating the Effects of Noise Pollution on Directional Root Growth Shayna Engdahl, Molly Williams, Dr. William Quinn Department of Biological Sciences, St. Edward’s University, Austin, Texas
Abstract: It has long been understood that roots exhibit directional growth in response to stimuli such as moisture, gravity, and touch. However, a recent study found that in the absence of a moisture gradient or any other known stimuli, Pisum sativum roots use vibrations to locate water sources. This has led to questions regarding any correlations between noise pollution and the effectiveness of plant roots to locate water sources using solely vibrations. Previous research has demonstrated the devastating effects of noise pollution on aquatic life, humans, and other organisms but has yet to investigate the effects on plant life. In this study, we tested the impact of competing noise on the roots’ effectiveness to acoustically locate a water source. Our findings suggest that when competing noise is present, the accuracy is diminished, therefore potentially illuminating consequences of noise pollution on plant life. Introduction: It is widely believed that plants invaded land 460 million years ago1, succeeding mainly through effective water acquisition and conservation. Natural selection has produced a number of easily recognized mechanisms for these adaptations 2 including the CAM photosynthetic pathway3, the abundance of thick cuticles and reduction of leaf size in dry environments4, and development of leaf hairs5, among others. Relatively little is known about how natural selection has selected for plants whose roots are better able to grow toward water. Recently, Gagliano et al.6 have indicated that the radicle of Pisum sativum seedlings appears to respond to acoustic cues that come from running water, directing root growth toward the source. That is, the radicles may “hear” and respond to the vibration of running water, even in the absence of a moisture gradient. Given the potential importance of this acoustic cue, the increase in anthropogenic background noise associated with urbanization may affect this suggested ability to “hear” water. Therefore, we intend to provide information about the potential for human noise pollution to impact this phenomenon. The agronomic and economic consequences may be substantial. Specifically, this study is intended to clarify this relationship by testing whether the ability to locate a water source solely using acoustic cues is diminished when competing noise is present. It is hypothesized that if plants locate water sources using acoustic vibrations, then when a P. sativum seedling is exposed to co-occurring sounds, the difference in the tendency of roots to grow in the direction of the water source will be offset. It is believed that the human-made sound will create a “masking” effect, essentially drowning out the vibrations of the running water. Methods: Experiment 1: First, we attempted to validate the results reported by Gagliano et al. that pea seedling radicle directional growth is affected by the presence of running water. In this set of experiments, no competing noise was included. P. sativum seedlings were exposed solely to the vibrations of running water and the sides of the manipulation and placement were assigned randomly. Seeds were germinated using the rag-doll technique and once the radicle was 5 mm or
2 longer, they were placed with the straight down in a PVC pipe in the shape of a U-Maze (see Figure 1).
Figure 1: U-Maze The control was opposed on the other side by water running through aquarium tubing. A Percival growth chamber was programmed to run a diurnal cycle with a 12/12 hour day/night and the following settings; day temperature at an average of 23 °C (± 0.1 (SE)); night temperature at 18.3 °C (± 0.1 (SE)); humidity at an average of 60% ( ± 0.3 (SE)); light measurement 17,400 LUX (± 5%rdg + 10dgt (SE)); sound measurement 74.1dBA (± 1.5bD (SE)). After 5 days, the seedling was removed from the maze, the potting soil gently washed away, and the direction of root growth against the manipulation recorded. In all experiments, ambient noise was not controlled. Experiment 1 was compared to Gagliano’s data using a two-population proportion test. The remaining experiments were also compared using this equation.
Figure 4: Experiment 2
Figure 2: Experiment 1
Experiment 2: This portion of the study investigated the effect of noise competing with the water source. Following the methods described above, P. sativum seedlings were tested with
3 water and electronic noise from a vibration speaker on opposing sides of the U-Maze. Experiment 3: This portion of the study investigated the effects of co-occurring noise on directional root growth by placing the speaker on the same side as the water.
Figure 5: Proportion Test Calculation
Results: Experiment 1 (n=16) showed a percentage of 75% (12/16) growing toward the side with the water running. Experiment 2 (n=16) (8/16) showed a percentage of 50% growing toward the side with water running. Experiment 3 (n=18) showed a percentage of 39% (7/18) growing toward the side with water running. Experiment 1 had a p-value of 0.38591, which was not significant at the 0.05 level. Experiment 2 had a p-value of 0.07215, also not significant. Experiment 3 had a p-value of 0.017 which is significant at the 0.05 level.
Figure 6: Experiment 1
Figure 7: Experiment 3
Figure 8: Two-Population Proportion Test Results
4 Discussion: Experiment 1 suggests that Gagliano’s surprising positive effect of water vibrations affecting directional root growth is replicable. Given a p-value of 0.07 for experiment 2 and 0.017 for Experiment 3, this study suggests that human-made noise may, in fact, be impacting directional root growth. By essentially overloading P. sativum plants with background electronic sound and vibrations from running water, the plants did not show the preference they seem to have shown with water vibrations alone. In a location where water is scarce, plants may, therefore, be unable to prevent unnecessary energy expenditure in locating water. This could prove detrimental for economically important plants, as well as for the people who rely on them. It is important to continue researching these findings to better understand and manage the impacts humans have on the plant world. Works Cited: Heckman DS, Geiser DM, Eidell BR, Stauffer RL. Molecular Evidence for the Early Colonization of Land by Fungi and Plants. Science. 2001;293(5532):1129–1133. 1
Brodribb TJ, Feild TS, Sack L. Viewing leaf structure and evolution from a hydraulic perspective. Functional Plant Biology. 2010;37(6):488–498. 2
Keeley JE, Rundel PW. Evolution of CAM and C4 Carbon‐Concentrating Mechanisms. International Journal of Plant Sciences. 2003;164(S3). 3
Bacelar EA, Correia CM, Moutinho-Pereira JM, Goncalves BC, Lopes JI, Torres-Pereira JMG. Sclerophylly and leaf anatomical traits of five field-grown olive cultivars growing under drought conditions. Tree Physiology. 2004;24(2):233–239. 4
Pedrol N, Ramos P, Reigosa MJ. Phenotypic plasticity and acclimation to water deficits in velvet-grass: a long-term greenhouse experiment. Changes in leaf morphology, photosynthesis and stress-induced metabolites. Journal of Plant Physiology. 2000;157(4):383–393. 5
Gagliano, M. and Grimonprez, M. and Depczynski, M. and Renton, M. 2017. Tuned In: Plants Use Sound to Locate Water. Oecologia. 2017;184:151–160. 6