2014 ASHS Annual Conference

Controlling the elongation of ornamental plants is commonly needed for shipping and aesthetic purposes. Plant growth regulators are typically used to do this; however drought stress can also limit elongation. Many growers are reluctant to expose their plants to drought stress because they do not want it to negatively affect overall plant quality. Knowing how and when stem elongation is affected by water availability will help to increase our understanding of how elongation can be controlled without reducing plant quality. Hibiscus acetosella rooted cuttings were grown in a growth chamber with 12 hours of light at 25 °C. Two plants of similar size were used for each run of the experiment to compare growth under well-watered and water-stressed conditions. Plants were kept well watered for the first four days of the study, after which one plant remained well-watered while the other was allowed to become water-stressed over the remaining eight to sixteen days. Time lapse photography was used to compare the diurnal patterns of elongation over the course of the experiment. Evapotranspiration and substrate water status were measured using load cells and soil moisture sensors.  Over the course of the 12- 20 d runs, the increase in plant height, number of internodes, and number of leaves of drought-stressed plants was 34, 30, and 55% less than for the well-watered plants. Under well watered conditions, plant height increased by approximately 10% per day, but this decreased to 5% within three days of the start of the water stress. The well-watered plants had more visible internodes; however average length of visible internodes were similar for both the well-watered and water-stressed plants. We hypothesize that the difference in the number of visible internodes between well-watered and drought-stressed is due to un-elongated internodes near the growing point of the water-stressed plants. Stem elongation was greatest at night, when evapotranspiration was lowest, suggesting that elongation is greatest at night due to rehydration and increasing turgor pressure of the plant. The results of this study show that controlling stem elongation and growth of plants is possible via irrigation management. Further manipulation of irrigation timing and substrate moisture will provide additional information about controlling plant growth via irrigation.