BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. TRANSPORT IN PLANTS

13.3. Transpiration and Water Movement Through Leaves

13.3.8. Physiological Role of Transpiration

Transpiration is sometimes referred to as a "necessary evil" because it is an unavoidable yet potentially harmful consequence of the requirement that Cell walls remain moist and that Water evaporates from them. Water is lost primarily through the Stomata, which are also essential for the plant to exchange gases with the environment. Through gas exchange, plants obtain carbon dioxide from the outside, which is indispensable for Photosynthesis. (Since atmospheric oxygen is much more abundant, it diffuses into the plant even when the stomata are closed, meaning Respiration encounters no obstacles in the dark.) Were it not for the cuticle, gas exchange could occur without stomata, and its efficiency would increase. However, in that case, water loss would be completely uncontrolled. The cuticle reduces water loss, while further regulation is carried out by the stomata, which in most plants are highly sensitive to water stress and close under dry conditions. Stomata also typically close in the dark when photosynthesis ceases. Water loss poses the risk of wilting, desiccation, and plant death. It is well known that even mild drought can retard crop development and lead to significant yield losses.

Despite the apparent inevitability of transpiration, It is interesting to consider whether it confers any benefits upon the plant. There are two potential possibilities.

1. Transpiration is accompanied by the evaporation of water from mesophyll Cells. This process is endothermic, leading to leaf cooling in much the same way that perspiration cools mammalian Skin. This can sometimes be important, particularly under direct sunlight, when leaves absorb large amounts of radiant energy and can overheat, with severe overheating inhibiting photosynthesis. However, such a cooling effect is unlikely to play a significant role under normal conditions. In hot climates, plants employ alternative mechanisms to protect against heat stress.

2. Furthermore, it has been suggested that the transpiration stream is necessary for the distribution of dissolved mineral salts throughout the plant. Although this hypothesis is logical, a very low rate of transpiration would be entirely sufficient for Mineral Salt Transport. For instance, the uptake of mineral nutrients into leaves is not reduced at night when transpiration is minimal; this occurs because xylem sap becomes more concentrated at night than during the day. Moreover, the Absorption of mineral salts from the soil is largely independent of the transpiration stream.