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'Plumbing' key to flowering success
Flowering plants managed to speed up their growth and transform the planet thanks to a boost in their leaf plumbing, say Australian researchers.

Dr Tim Brodribb, a plant scientist at the University of Tasmania in Hobart, says the findings may be useful in developing faster-growing plants.

"The question of how angiosperms [flowering plants] became such a dominant group on Earth has been hotly debated for a long time," says Brodribb, whose research with Assistant Professor Taylor Feild of University of Tennessee appears this month in Ecology Letters.

"Flowers have been very commonly cited as the key to angiosperm success, so our suggestion that leaf evolution is the key to explaining the rise of the angiosperms is significant and controversial."

Brodribb describes the rise of angiosperms as "one of the most dramatic and profound evolutionary events in the history of Earth".

"Now angiosperms constitute 99% of land plant diversity. As such they provide energetic and ecological foundation for most of the life on land," he says.

"They are also at the foundation of human culture because without their high productivity most of the human race would starve to death."

But for a plant to grow fast it must produce food quickly, so how does it do this?
Plant plumbing

During photosynthesis, the plant opens up pore-like structures in its leaves so it can absorb carbon dioxide from the air.

But while the pores are open, water is lost from the plant and it has has to be replaced.

The rate at which a plant can photosynthesise will be limited by how fast it can do this, says Brodribb.

A plant's 'plumbing' system consists of dead hollowed-out cells called xylem, which allow water to be sucked up into the plant from the soil.

And a crucial part of this plumbing is in the leaf, where water must make its way from the xylem, through living tissue.

"If you can look at the density of the veins in a leaf, you can tell its capacity to deliver water," says Brodribb.

"The capacity to transpire water is directly proportional to the capacity to photosynthesise. So it's a proxy for the productivity of the plant."

But until this study, no one had ever looked at how leaf vein density has changed throughout evolution.
Vein study

Brodribb and Feild studied the density of veins in the leaves of over 500 plants, including representatives from the most primitive to the most advanced groups of angiosperms.

They found that shortly after angiosperms evolved, there was a fourfold increase in leaf vein density.

"We demonstrate how this major shift in leaf vein architecture potentially allowed the maximum photosynthetic capacity in angiosperms to rise above competing groups 140 to 100 [million years ago]," say the researchers.

To measure the density of leaf veins, Brodribb and Feild had to pull each leaf apart and measure the total length of its veins.

"It's easy to describe, but it's a horrifyingly laborious process," says Brodribb.

But he says the findings could open a whole new world up to plant scientists.

"If we can work out how to increase the density of veins in a plant then we can actually make a plant grow a lot bigger faster."

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