Testing transpiration factors

Explanation

In a bubble potometer, water uptake by a leafy shoot moves an air bubble along a narrow tube. Bubble distance per unit time compares uptake rates when the tube cross-section is unchanged. With known cross-sectional area, distance multiplied by area gives volume. Uptake estimates transpiration because most absorbed water replaces water lost, but some water is used or retained by the plant.

Cut and assemble the shoot under water to reduce air entering the xylem, and ensure airtight, watertight connections. Keep leaf area and the apparatus comparable, allow adjustment to each condition and repeat readings. Change only one environmental factor at a time.

More air movement removes humid air around a leaf, maintaining a steeper water-vapour concentration gradient. Lower humidity also increases that gradient. Higher temperature generally increases evaporation and diffusion, provided water supply and stomatal opening are not restricting the process.

A second method weighs a whole potted plant. Enclose the pot and soil in a plastic bag tied around the stem, so water can only be lost from the leaves and stem, not by evaporation from the soil. Stand the plant on a balance and record its mass every hour. The loss in mass per hour estimates the rate of transpiration. Compare plants under one changed condition, such as still air and a fan, keeping the plant size, light and temperature the same.

Greater light intensity often increases transpiration because stomata open for gas exchange, but other conditions must be controlled. Severe water stress can cause stomata to close, so these predictions are conditional rather than unlimited rules. Compare actual data before assigning a mechanism.