Presenter: Karrin Tennant, College of Science, School of Biological Sciences
Authors: Karrin Tennant
Faculty Advisor: William Anderegg, College of Science, School of Biological Sciences
Institution: University of Utah
Stomata, critical valves responsible for gas exchange on the leaf level, control rates of transpiration and as such play a crucial role in terrestrial water cycling. When stomata open, water molecules in the intracellular leaf space are subject to evaporation, especially in dry, hot conditions. To reduce water loss due to transpiration, plants typically close stomatal openings during the night, when photosynthetic carbon requirements are reduced. However, some plants maintain partial stomatal conductance and transpiration at night. It is hypothesized that a pattern of night-time stomatal conductance occurs despite water loss to induce favorable hydraulic redistribution in the soil. Partially open stomata maintain tension within the plant’s water transport system, drawing below-ground water closer to the plant’s roots. Thus, partially open stomata would benefit the plant by keeping a viable water source in close proximity, and away from competitors. To test this hypothesis, black cottonwood (Populus trichocarpa) and water birch (Betula occidentalis) seedlings were planted in a controlled environment growth chamber, using black cottonwoods as a focal species. The trees were arranged into four planting groups with no competition, strong and weak intraspecific competition, and strong interspecific competition. To intensify the plants’ water use strategies, an 8-week drought treatment was induced. Measurements of night-time and day-time stomatal conductance and leaf water potential were taken weekly during drought stress. As the drought progressed, all trees planted in competition showed significantly greater increases in night-time stomatal conductance than non-competitive cottonwoods. Competitive and non-competitive plants alike increased night-time stomatal conductance as water supply decreased. However, stomatal behavior correlated much more with competition type. This experiment supports the hypothesis that Populus trichocarpa increase stomatal conductance at night in the presence of competition. The observed pattern of competitive stomatal behavior at night has considerable implications for models of transpiration and plant water cycling in ecosystems.