The size of leaves can vary by a factor of 1,000 across plant species, but until now, the reason why has remained a mystery. A new study by an international team of scientists led by UCLA life scientists goes a long way toward solving it.
In research federally funded by the National Science Foundation, the biologists found that smaller leaves are structurally and physiologically better adapted to dry soil because of their distinct vein systems.
The research will be published in an upcoming print issue of the journal Plant Physiology and is currently available in the journal's online edition.
"A hike in dry areas, such as the Santa Monica Mountains, proves that leaves can be small. But if you are in the tropical forest, many leaves are enormous," said Lawren Sack, a UCLA professor of ecology and evolutionary biology and senior author of the research.
This biogeographic trend—smaller leaves in drier areas—may be the best recognized in plant ecology, true at both the local and global scales, but it had evaded direct explanation, Sack said.
Sack and his research team focused on deciphering the meaning of the huge diversity in the patterns of veins across plants. They found that small leaves' major veins — those you can see with the naked eye—are spaced more closely together and are of greater length, relative to the leaf's size, than those of larger leaves.
This redundancy of major veins, the researchers say, protects the leaves from the effects of embolism—bubbles that form in their "water pipes" during drought—because it provides alternate routes for water to flow around vein blockages.
"Even with strong drought that forms embolism in the veins, a small leaf maintains function in its vein system and can keep functioning for water transport," Sack said.
"Unlike people, plants don't seem to have a complex hierarchy of needs—give them sun, water and nutrients, and they will be happy," said Christine Scoffoni, a UCLA doctoral student in the department of ecology and evolutionary biology and lead author of the research. "But when one of these three fundamental resources becomes scarce, the plant will have to find a way to cope with it or die, because there is no escape. Coping with drought can be a strong selective factor on leaf form, especially on size and their venation."
"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 professor Hervé Cochard from France's University of Clermont-Ferrand and a member of the Institut National de Recherche Agronomique, to construct three-dimensional computer models of leaves' venation systems. They then simulated the impact of embolism on water transport for leaves of different sizes and vein architectures.