In plants, the transpiration stream is the uninterrupted stream of water which is taken up by the roots and, via the xylem vessels, transported to the leaves where it will eventually evaporate into the air/apoplast-interface of the substomatal cavity. It is driven in part by root pressure and capillary action. However, the most important cause is the difference in water potential between the soil and the substomatal cavity. This difference occurs as a result of transpiration, which in turn can be regulated through stomatal closure or opening. It allows for plants to efficiently transport water up to their highest body organs, regulate the temperature of stem and leaves during heat-generating photosynthesis and it allows for upstream signalling such as the dispersal of an apoplastic alkalinization during local oxidative stress.
References
Felle HH, Herrmann A, Hückelhoven R, Kogel K-H (2005) Root-to-shoot signalling: apoplastic alkalinization, a general stress response and defence factor in barley (Hordeum vulgare). Protoplasma 227, 17 - 24.
Salibury F, Ross C (1991) Plant Physiology. Brooks Cole, pp 682, ISBN 0-534-15162-0.
