ASU biologist Benjamin Blonder looks deep into different plants' unique vein patterns
In his poem “Song of Myself,” Walt Whitman wrote: “I believe a leaf of grass is no less than the journey-work of the stars.”
For Arizona State University biologist Benjamin Blonder, the wonder of a single leaf begins upon a closer examination of its veins under the microscope. These fingerprint-like patterns — many unique to a plant species — turn out to have a tremendous potential toward understanding the ecological health of plants, trees and forests.
“If you are an organism, how should you deploy your resources?” asks Blonder, an assistant professor in ASU’s School of Life Sciences. “If you are a plant or animal and you are trying to grow, and you’ve got a certain amount of energy and mass to throw around, are you better off reproducing right away, are you better off growing really big first or are you better off growing really quickly and then reproducing later?”
“In plants, one of the critical axes for that is in their leaves.”
Leaf venation network traits are vital to plant life, from nutrient transport to mechanical support to damage control and pest resistance.
Connecting the patterns found in the veins of leaves drives much of the research focus in Blonder's lab.
Economies of scale
Leaves are a basic energy unit of biology. But they come at a cost to make. Elements like carbon and nitrogen flow through, costing energy to produce enough nutrients to sustain a single leaf.
“And you can think about a leaf as a vehicle for investment sort of like you would invest in a mutual fund or anything else that could return some interest,” said Blonder. “So, you have some initial outlay of costs to buy the thing which is effectively constructing it, and then photosynthesis returns some resources over time, but it also depreciates. …
“With a plant, you’ve sort of got these trade-offs between resource return rate and initial resource costs and then, the longevity of the resource.”
This “leaf economic spectrum” theory was first developed in a seminal Nature paper by Ian Wright. Blonder, while under the tutelage of doctoral adviser Brian Enquist at the University of Arizona, wanted to explain why it exists. Their hypothesis is that there is a cost and benefit to making a leaf, and when observed repeatedly across the scale of biomes, the trade-offs influence leaf form and function, and the different kinds of vein architectures and patterns seen.
Under the skin
Postdoctoral researcher Luiza Teophilo Aparecido, a Brazilian tropical forest ecologist, has helped Blonder develop and refine a suite of standardized basic leaf vein measurements.
They start by peeling away the delicate outer layer of leaf skin, the tough lignum. They soak the leaves in a chemical solution to get at the heart of the nourishing waterways.
After an hour, the chemical clearing and a safranin staining reveals a reddish vein network, tied together by a thin, translucent skin of connective tissue. When they examine them under the microscope, there are patterns that jump out, unique and widely varying among different species.
Blonder’s team measures three key leaf vein traits — distance, density and loopiness — and has developed a way to standardize the measurements for these traits. Under the microscope, they take images and use digital-image analysis programs to precisely measure the venation patterns.
“We are usually on a 4X objective for the maximum scale, 500 pixels per millimeter, or 2 microns per pixel when we look at these samples,” said Blonder. “The smallest feature is about 10 microns wide, and the biggest feature, the whole leaf, in the case of some palms, can be up to several meters long.”
Turning a new leaf
The pattern of the leaves' vein transportation networks provides a new window into plant form and function.
It can reveal the relationship between the plant water supply and available environmental water supply, and an optimized nutrient and water transport system.
The venation traits mediate different economic strategies across environments. For the longest time, plants were classified based on just form, like maple leaves from oaks. Now, Blonder’s predictive model has the potential to provide insights into leaf form and function across broad climate gradients, as well as broad biogeographic patterns of plant species’ distributions.
“It’s trade-offs in transportation networks within leaves that may force all of these other economic trade-offs downstream,” said Blonder.