Question

What is the effect of transpiration on the surface tension of the water, i.e. does it increase or decrease it? How does changing surface tension affect water molecules below the surface? How do you think this affects the water molecules in the xylem cells that touch these spongy mesophyll cell walls or water in xylem cells farther back in the leaf or stem?

Answer 1

Surface tension is a phenomenon of cohesive forces between liquid molecules. On the other hand transpiration is a process of evaporation of water from the leaf surface. As the transpiration is start, it creates negative tension on the xylem. During this process the water molecules stick to one another (with the strong hydrogen bond) and it flows upward in xylem tissue. Hence as the cohesive forces between liquid molecules are increased (surface tension), transpiration increased.   

If surface tension is decrease, water molecules below the surface, cannot flow in xylem tissue and transpiration rate will decrease.

The spongy mesophyll helps the plant to exchange gases during transpiration. But the cohesive forces between water molecules are stronger than gas and water molecules hence water flows upward in xylem (like capillary movement).

In simple words, when the stomata will open, it creates surface tension on water; the water flows upward from root to xylem and from xylem to surface of leaf which leads to transpiration of water.