X-rays and leaves
X-rays aren’t just for peering at broken bones. A few years ago I wondered if they could also be used to look at the structure of leaves – especially the venation network, which is the plant’s equivalent of a skeleton combined with a circulatory system. Looking at leaf veins with x-rays is an attractive idea because many scientists, including me, are interested in understanding how the structure of the veins reflects each species’ adaptation to its environment. However looking at the veins means a lot of chemical work that takes dozens of hours and ends up destroying the leaf. What about x-rays instead? Maybe they could penetrate the leaf without destroying it, and make an image in milliseconds instead of days.
So I spent a lot of time thinking about how to do this. It turns out that the physics of how x-rays interact with plants is very different than the physics of how x-rays interact with dense materials like bone or metal. But with enough tinkering the idea should work.
I want to share a few vignettes from the process of testing these ideas. I started by testing how the x-ray methods would work using plants which are typically very difficult to study – the silverswords of Hawai’i. Here are a few individuals of a Dubautia species growing on recent cinder cones formed by the Haleakala volcano. These plants have very thick leaves that make seeing the veins very difficult.
To image these leaves, I developed some new approaches that are best implemented with very powerful x-ray sources – for example, the kind that can be generated at a synchrotron light source. This is a kind of particle accelerator that brings electrons very close to the speed of light and then passes them through large magnets, a process which generates many x-rays. Here is an image of the exterior of the Advanced Photon Source, where the work was conducted.
Despite working in such a big facility, I spent most of my time interacting with a very small set of components. I put the leaves in the path of the x-rays and a very sensitive detector. All of this technology – and much more you don’t see – is needed to test the original idea!
One of the coolest parts about working with x-rays is seeing how they are detected. Here you can see a lutetium-aluminum-garnet crystal (LuAG). X-rays that strike this crystal are absorbed and re-emitted as visible green light. A long way of saying that the experiment glows in the dark!
You’re seeing images from the second attempt to make this work – I got several things wrong on the first try, and had to wait the better part of a year to figure out what went wrong and try another time. Fortunately the second time was a success, so I now am happy to share a new publication describing exactly how it does work!
So please check out the next issue of New Phytologist for Blonder et al. X-ray imaging of leaf venation networks. You can also use this free PDF link. X-rays will have a bright future in the study of plants!