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To retain water, succulents pump up leaves

BROWN (US) — To hang on to water in arid conditions, succulent plants have remodeled the vein structure of their leaves and gone 3D.

Leaves are food factories complete with plumbing to transport water and sugar. The farther those veins are from cells performing photosynthesis, the less efficient the process will be.

Researchers wondered how plants managed to evolve fat leaves, given the hydraulic challenges of gaining girth. Of all plants, succulents could sustain photosynthesis in fat leaves with sparse venation because they store so much water right where it’s needed. But even succulents are constrained by 2D veins.


Veins, internal to the dark purple-stained deposits of this cross-section of a succulent Phemeranthus teretifolius leaf, are arranged in a ring shape that reduces the distance between veins and photosynthetic cells. (Credit: Matt Ogburn)

“There must be some kind of a tradeoff in a fat leaf that’s really different from most flat leaves,” says Matt Ogburn, formerly a graduate student at Brown University and now a postdoctoral fellow at Yale.

“There’s a benefit to that in storing water in the leaf, but it’s going to have a cost to it in terms of the other things the leaf has to do.”

For a new study published in the journal Current Biology, Ogburn and Erika Edwards, assistant professor of biology at Brown, looked deeply into the catalog of two dry-climate plant groups, Portulacineae and Molluginaceae. They pieced together the plants’ evolutionary history, and measured the water storage of 83 species, and the vein structure of 42.

Up to a certain level of thickness, leaves retained the planar vein structure of flat leaves. As a consequence, their vein structure was sparser and path lengths were longer—but only up to a point.

After a certain threshold of thickness, vein structure evolves to become 3D, abandoning the single plane layout of flatter leaves for either an oval or circular orientation in cross-section.

That 3D vein structures could independently evolve more than 10 times, in two ways, in this limited sample of two lineages suggests that the different vein structure is no coincidence. Instead it seems conclusively to be a functional trait that emerged to allow leaves to become thicker.

“If you had just a 2D-veined pile of species and a 3D-veined pile of species and you didn’t know how they were related to each other, you might say, maybe 3D venation just evolved once a long time ago and had absolutely nothing to do with succulence,” Edwards says.

“But when you can lay them out on a phylogeny and reconstruct how many times this transition happened—the more times you see this repeated correlation between these two traits the more power you have to say that this is actually adaptive.”

Because they produce a low density of veins in thicker leaves, 2D arrangements of veins likely imposed a hard limit on leaf thickness. Evolution of 3D veins allowed them to burst through those constraints.

“Increased leaf thickness (when veins are planar) negates the hydraulic benefits of dense leaf venation,” the researchers write in the paper. “It also predicts an upper limit to leaf thickness that would be set by the minimal functional vein density.

“However, the repeated evolution of 3D venation allowed for further increases in succulence while maintaining moderate hydraulic path lengths.”

The National Science Foundation supported the research.

Source: Brown University

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