| Description |
Stomata are critical valves responsible for gas exchange on the leaf level. They 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 conditions. To reduce water loss due to transpiration, plants typically close stomatal pores during the night, when photosynthetic carbon requirements are reduced. However, many plants maintain partial stomatal conductance and transpiration at night. This research investigates the hypothesis that a pattern of nocturnal conductance occurs despite water loss to induce favorable hydraulic redistribution in the soil. 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 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. The trees were arranged into four planting groups with no competition, strong and weak intraspecific competition, and interspecific competition. To intensify the plant water competition, an 8-week drought treatment was induced. Measurements of nighttime and daytime stomatal conductance and leaf water potential were recorded weekly during drought stress conditions. Competitive and non-competitive plants alike increased nighttime stomatal conductance as water supply decreased, and trees planted in competition showed significantly greater increases in nighttime stomatal conductance than non-competitive cottonwoods. However, stomatal behavior was more strongly iii correlated with competition type than with hydraulic stress. This experiment supports the hypothesis that trees may 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. |
