Why does the mean rate of water uptake increase on a hot day?

The mean rate of water uptake increases on a hot day primarily because more kinetic energy leads to faster transpiration. When temperatures rise, the energy of water molecules increases, causing them to move more rapidly. This increased kinetic energy enhances the rate at which water evaporates from the surface of leaves during transpiration.

As water evaporates, it creates a negative pressure within the plant's xylem vessels, which ultimately encourages more water from the soil to be drawn up through the roots and into the plant. This process is essential for transporting nutrients and maintaining turgor pressure in plant cells.

In contrast, while humidity levels might influence transpiration rates, a hot day is typically characterized by lower humidity, leading to greater evaporation rates. Additionally, roots do not generally absorb less water on hot days; they work to maximize water uptake in response to increased transpiration demands. Therefore, the increase in kinetic energy due to higher temperatures is the critical factor driving the increased rate of water uptake in hot conditions.