Re: What factors affect transpiration in dicotyledons?

Transpiration is the loss of water from the leaves of plants. Transpiration occurs because plants must keep the surface of the mesophyll cells in the interior of the leaves moist so they can absorb carbon dioxide from the air to be converted into carbohydrates. The inevitable result of keeping these cells moist AND exposed to the air is evaporation.

There are two major factors contributing to the rate of transpiration. First is the difference in water vapor concentration between the inside and outside of the leaf. Inside the leaf, the air is nearly saturated (100% relative humidity), so the closer the outside air is to 100% relative humidity, the slower the transpiration will be. On a humid day (fog, rain, high humidity), transpiration will be slower. In a desert, transpiration is very rapid, so desert plants have special ways of preserving their limited supply of water. The gradient in water vapor concentration is the driving force of transpiration (water vapor diffusing from an area of high concentration to an area of lower concentration). Outside the leaf is always lower or equal to inside, unless the plant is having severe problems.

The second category of factors affecting transpiration rate is "diffusional resistance", things that slow down the diffusion which would occur naturally based on the water vapor concentration gradient. There are two main components to this factor: the stomatal pore and the "boundary layer". Essentially all transpiration occurs through the stomatal pores. Plants can control their water loss by closing their stomatal pores when water is in short supply.

The boundary layer is the blanket of unstirred air on the outer surface of the leaf. It insulates the leaf against water loss because it becomes a nearly water-saturated layer of air. The thickness of the boundary layer (a few millimeters at most) depends on the amount of wind, which blows away the boundary layer. No wind gives a thicker layer and slower transpiration, high wind gives a thinner layer and rapid transpiration. At high wind speeds, the stomata usually close to prevent this rapid water loss.

Other plants, particularly those in the desert, increase their boundary layer by having leaf hairs or sunken stomata. Still others (the crassulacean acid metabolism plants or CAM plants) keep their stomata closed all day and only open them at night when the humidity is higher.

For a full review of transpiration and other aspects of plant water relations see a plant physiology text, such as Plant Physiology by Taiz and Zeiger.

For a cyber review of transpiration: http:// www.hcs.ohio-state.edu/hcs/TMI/hort300/anat3.htm

To find out about some of the latest research on transpiration or CAM (or any other plant topic), search at the American Society of Plant Physiologists.