Sir

Melvin T. Tyree in his Concepts essay on plant hydraulics “The ascent of water” (Nature 423, 923; 2003) mentions the well-known comparison of the mammalian heart, the pump circulating the blood, to the movement of water in plants by “the pulling force generated at the evaporative surface of leaves” (transpiration). One interesting question arising from this comparison is whether transpiration is as essential for a plant as is the heart for an animal?

Although it is generally accepted that transpiration for plants is essential, doubts have often been raised. For example, Paul Kramer on page 293 of his book Water Relations of Plants (Academic, Orlando, 1983) states: “transpiration can be best regarded as an unavoidable evil” — unavoidable because no plant 'skin' has evolved that is permeable to CO2 but impermeable to water vapour. Which view is correct?

There are several distinct aspects to long-distance water transport. First, water lost via the evaporative surface of plant leaves has to be replenished to prevent the plant wilting, as outlined by Tyree. Second, 'growth water' has to be lifted to the top of plants (including trees) for the expansion growth of leaves, fruits and shoots. And third, all the water flowing downwards in the phloem has to be replaced by an equal volume of water moving upwards.

Only the water flow in the first of these is transpiration-driven. It does not take place if transpiration is zero or almost zero (if relative humidity is 100% or at night, when the stomata are closed). Evaporation generates a pulling force sufficient to replenish the amount of water lost by transpiration, thus minimizing the problem it creates for the plant.

The water movements caused by the second two processes do not depend on transpiration. The evaporative loss of, say, 100 molecules of water could at the very best generate a force sufficient to lift 100 molecules, but not 110 (those lost by evaporation plus those required for growth). So how is the amount of water in the second two processes moved against gravitational force? By the difference in water potential created by growth, volume flow of assimilates and root pressure. H. Beevers and I have estimated that in less than four days a negative water potential large enough to pull water up a 100-m tree would arise in the absence of any transpiration (W. Tanner and H. Beevers Proc. Natl Acad. Sci. USA 98, 9443–9447; 2001). Transpiration is not essential for net water movement: that is, the amount of water the plant requires. This calls into question the analogy with the mammalian heart.