UCLA Biologists Uncover The Secrets Involved In Leaf Size, Shape

Alan McStravick for redOrbit.com – Your Universe Online

Trees, and more specifically, the leaves that adorn them, having predated human’s foray into scientific learning, should harbor no mysteries at this point. However, nature continues to surprise us.

Whether it is news of gold in eucalyptus leaves, as we learned last week with news out of Australia, or the simple yet profound truth of the math that governs internal leaf structure, human understanding continues to expand with the study of some of the world’s oldest and simplest structures.

This week, researchers from UCLA’s College of Letters and Science have discovered the fundamental rules that govern internal leaf design. These rules grant plants the ability to produce leaves that vary greatly in size. According to Lawren Sack, professor of ecology and evolutionary biology and senior author of the research, the study findings show how leaves are, in fact, the “perfect machines.”

Allometric analysis was employed to help the team discover the mathematical relationships which show how the proportions of parts of an organism change with differences in total size. This study marks the first time allometric analysis has been employed to study the interior of leaves.

The study results, published in this month’s issue of the American Journal of Botany, focus on how leaf anatomy varies across leaves of different sizes. Each species studied was grown on the UCLA campus even though they are native to differing regions around the world.

The team notes the study of leaf surface area among different species is easy to observe. However, differences in leaf thickness are less obvious but equally important.

“Once you start rubbing leaves between your fingers, you can feel that some leaves are floppy and thin, while others are rigid and thick,” said Grace John, a UCLA doctoral student in ecology and evolutionary biology and lead author of the research. “We started with the simplest questions — but ones that had never been answered clearly — such as whether leaves that are thicker or larger in area are constructed of different sizes or types of cells.”

Every leaf consists of three basic tissues. Each tissue contains cells meant to carry out its own unique and particular set of functions. Whether looking at the epidermis of the leaf or the mesophyll or vascular tissues, the team discovered that the thickness of the leaf had a direct correlation to the size of the cells in the epidermis and mesophyll. This did not hold true, however, for the cells in the vascular tissue.

Most interesting to the team was the “extraordinary” strength of the relationships linking cell size and thickness and leaf thickness, regardless of leaf species and the region on earth where it is native. The mathematical equations developed by the team apply across the board and will allow scientists in future studies to accurately predict cell and cell wall dimensions based solely on the thickness of the leaf they are studying. In most cases, the relationships the team found were isometric.

“This means that if a leaf has a larger cell in one tissue, it has a larger cell in another tissue, in direct proportion, as if you blew up the leaf and all its cells using Photoshop,” said Christine Scoffoni, a doctoral student at UCLA and member of the research team.

While the thickness of the leaf is the determinant of cell and cell wall thickness, a leaf’s area is completely unrelated to that relationship. This means plants are able to produce leaves with a huge range of surface areas without the need for larger cells in some and smaller cells in others. This, say the researchers, would be completely inefficient for the plant function.

Because some leaves receive more light than others on a plant, the team hypothesized this mathematical relationship most likely stems from leaf development – the process by which leaves form on the branch, growing from a few cells that divide into many, with cells then expanding until the leaf is fully mature. Leaves are unable to affect the number of cells they arrange vertically. Cell and cell wall expansion occurs simultaneously and is reflected in the thickness of the whole leaf, regardless of leaf area. Therefore, the cells arranged horizontally in the leaves continue to increase as the leaf expands; the size of the individual cells is irrelevant.

Sack stated the real-world benefit of the team’s findings will allow researchers the ability to make predictions of the function of the leaf based on leaf thickness. This is because leaf thickness affects not only the photosynthetic rate, but also the lifespan.

“A minor difference in thickness tells us more about the layout inside the leaf than a much more dramatic difference in leaf area,” John said.

Other disciplines, such as engineering, could gain new insight into the potential design and construction of larger structures without losing function or stability.

“Fundamental discoveries like these highlight the elegant solutions evolved by natural systems,” Sack said. “Plant anatomy often has been perceived as boring. Quantitative discoveries like these prove how exciting this science can be. We need to start re-establishing skill sets in this type of fundamental science to extract practical lessons from the mysteries of nature.

“There are so many properties of leaves we cannot yet imitate synthetically,” he added. “Leaves are providing us with the blueprints for bigger, better things. We just have to look close enough to read them.”

The discovery of these new allometric equations is, according to John, an important step toward understanding the design of leaves on a cellular basis. The overall diversity of leaf structures on earth also means there is still so much more to learn.

The research team next intends to study species that are very closely related in an effort to uncover any evolutionary relationships between leaf design and function. Funding for the current study was provided by the National Science Foundation.

“What makes the cross-sections especially exciting is the huge variation from one species to the next,” John said. “Some have relatively enormous cells in certain tissues, and cell shapes vary from cylindrical to star-shaped. Each species is beautiful in its distinctiveness. All of this variation needs decoding.”