It's the pits. Water flowing up the xylem must cross ever-changing pit membranes like this one to reach thirsty leaves.

How Plants Water Themselves

Inside plant stems, tiny pipes supply water to thirsty leaves. Researchers always thought this so-called xylem was a passive column of dead tissue. But a surprising new study, published online today by Science, suggests that the xylem may actively adjust the flow of mineral-rich water coursing toward leaves.

Last year, while researching xylem repair, Harvard University plant biologist Michele Holbrook and her colleagues stumbled across the inspiration for their current study: a 1978 paper by Harvard biologist Martin Zimmerman. In that paper, Zimmerman's team noted that when they pumped tap water--full of everyday salts--into the xylem, it flowed much faster than deionized water. The paper didn't explain why.

Maybe, Holbrook's group reasoned, the added salts somehow alter the xylem. To test that idea, they pumped water into pieces of stems from laurel (Laurus nobilis), steadily increasing the amount of potassium chloride (KCl). Sure enough, by the time the KCl concentration had increased from 0 to 50 mM, the water was flowing up to 2.5 times faster. Other salts had the same effect, and it also happened in stems of 18 other angiosperms, five conifers, and three ferns. By contrast, when the team tried deionized water, the xylem's flow rate dropped considerably.

But how does the xylem do it? Engineers have shown that hydrogels, jellylike substances that can shrink and expand, influence the flow of water through a material. And plants are full of hydrogels, in the form of pectins that glue cell walls together. So the team injected the xylem with solutions of varying pH and polarity, factors known to activate hydrogels. Low pH and nonpolar solvents did, indeed, spur immediate increases in xylem flow rate--a similar effect, the researchers say, to the xylem's uptake of salty water from soil.

Further experiments localized this activity to the xylem's "pit" membranes--a sievelike mesh of cellulose fibers and pectins. Water flowing up the xylem must pass through these membranes. As a plant soaks up soil minerals, the researchers suggest, the pectins can either swell or shrink. When pectins swell, pores in the membranes are squeezed, slowing water flow to a trickle. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above.

"This is the first good evidence I know of that the xylem regulates water transport in plants," says John Boyer, a plant biologist at the University of Delaware, Lewes.

Related sites
Michele Holbrook's lab