"When we ask our students in plant physiology class why plants need water, their first answer is for growth," Sack said. "They are amazed to learn that the bulk of the water used by a plant is actually to make up for the water lost through transpiration, which would otherwise dry out the leaves. When the leaves open the small pores on their surface, the stomata, to capture carbon dioxide for photosynthesis, water is lost to the dry atmosphere. To stay moist inside, the plants need to replace the water lost by evaporation."
To do this, plants need to maintain the continuity of water in their "pipe delivery system," even as water is being pulled up by the leaves to replace water that has been lost to the air. This places tension on the water in the pipe system, known as the xylem, which runs through the roots and stem and into the leaf veins. And that continuity is challenged by dry soil, Sack explained.
"The less water in the soil, the more the leaves have to pull to get some out, so stronger tension starts building in the plant's pipes," Scoffoni said. "At a certain level of tension, an air bubble is pulled in from outside, blocking the flow of water. One way for a plant to withstand drought is to tolerate many of these embolisms."
Having more major vein routes by which water can flow around the air bubble provides this ability. Smaller leaves, possessing more major veins spaced closely together in a given square centimeter, have this ability, Sack said.
To test this idea, the UCLA team collaborated with pro
|Contact: Stuart Wolpert|
University of California - Los Angeles