How do the high crowns of redwoods obtain the water required for their photosynthesis? The pressure of the atmosphere (15 psi) is only sufficient to push water as high as 10 meters (33 ft.) into a vacuum, yet redwoods can reach 115 meters (379 ft.) into the sky! How is that possible? Osmosis is significant in smaller plants by producing “root pressures” of about one extra atmosphere, but that cannot explain the ascent of water into lofty redwoods.
Although rain and fog do help, the more important answer to our puzzle lies in the fact that sap, mostly water, has surprising tensile strength, due to cohesion between its molecules. The tension is caused by adhesion between sap and leaf cells and the surface tension of microscopic menisci developed within micro-pores of cell walls. Evaporation from redwood leaves pulls thin columns of water up to great heights, and this process is termed “transpiration pull.” The tensile strength of water prevents the long strings of sap from snapping under their own weight.
One way to measure the water tension in redwoods is to insert fluid-primed needles, attached to tiny pressure gauges, into their high branches. Predawn pressures as low as negative 13 atmospheres have been measured (see diagram). At midday, during active transpiration of water out of the leaves into dry air, negative 18 atmospheres have been measured. Even cold water will boil in a vacuum (zero pressure), yet redwood sap resists boiling at negative 18 atmospheres (minus 270 psi)!
Such cold boiling (“cavitation”) is prevented by water’s tensile strength, in association with an absence of seed bubbles. Confinement in fine tubes of cellulose, to which sap adheres, thwarts the formation of seed bubbles. Redwoods appear to be especially resistant to cavitation, so the maximum height to which redwoods can grow may not be limited by the threat of cavitation. The publication cited below provides evidence that as water pressure declines, the leaves’ tiny openings (stomata) close to preserve water. But closed stomata prevent the entry of carbon dioxide, thus terminating photosynthesis.
If a 100 meter giant were to suck at water through a straw, he would not succeed unless he began as an infant, used a very fine and growing straw and kept it filled as he grew. Redwood giants are likewise dependent upon the incredible tensile strength of water.
Look up in awe!
Don Garlick is a geology professor retired from HSU. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail firstname.lastname@example.org. Much of the above information was obtained from a publication by G. Koch, S. Sillett, G. Jennings and S. Davis in Plant Physiology, Essay 4.3, May 2006