What effect does overlapping of diffusion shells have on transpiration? Does it increase it or decrease it?

Question

If the stomata in a leaf are very close together, there is an overlapping of gaseous diffusion shells, but i a dont know if this mechanism explains how tranpiration rate is increased or decreased. extra info: decreased stomata density prevents the overlapping of diffusion shells and increases the rate of transpiration but i am not entirely sure about this.

Answer 1

It is actually the conductivity of the stomata - how easy it is for water molecules to diffuse into the atmosphere -  that affects transpiration and the shorter the distance the molecules have to travel, then the higher the conductivity, and the faster the rate of transpiration.

If the stomata are too close together, the 'dome' of saturated air (or air with a high water potential) around each one overlaps, and the water molecules have to travel further to reach unsaturated air (or air with a lower water potential).  This means the water molecules take longer to disperse away from the stomatal opening, so diffusion through the inter-cellular air spaces is slower, so evaporation from the mesophyll cells is slower, so the transpiration rate is slower.

Answer 2

There will be an increase in transpiration rate with larger numbers of more closely spaced stomata, but this increase is much less than would be expected because of the interference effect of the near neighbours.

Experiments, and calculations, have shown that a scattering of holes in an impermeable membrane over a sheet of water will reduce evaporation a lot less than expected. A sheet with small holes making up only 1% of the surface area, for example, will allow evaporation  at 50% the rate of an open surface.

As the number of holes increase, and the proximity of these holes decreases, the evaporation rate WILL increase but increasingly less effectively. So, doubling the number of holes will not get anywhere near to doubling the rate of evaporation.