One of the ways to track crop domestication is to examine the seed coat. As crops become more domesticated, so the seed coat thins. Measuring the seed coat can be a problem. One way would be to slice sections and measure them, but what if you needed a non-destructive method to measure the seed coat?
Enter Diamond Light. Diamond Light is a synchotron. It spins electrons round rapidly in magnetic fields and in doing do energises them so that they can then release photons. The kind of light varies between infra-red and X-rays, with different beamlines used for different lights. By using the high-resolution X-ray computed tomography (HRXCT) technique on Diamond’s I13-2 beamline, the researchers were able to measure for the first time the coat thickness throughout the entire seed. The results are in a new open access paper: Seed coat thinning during horsegram (Macrotyloma uniflorum) domestication documented through synchrotron tomography of archaeological seeds
“Seed coat thickness is a great indicator of domestication, as thinner coats will mean faster germination of a seed when it is watered,” explains Dorian Fuller, co-author on the paper. “But conventional methods of looking at the seed coat require breaking and destroying archaeological specimens.”
“Being able to look at the seed coat thickness without breaking the sample is possible by other methods, but you can only look at a spot on the seed,” adds Charlene Murphy, co-author on the paper. “The beamline at Diamond has allowed us to look at the entire seed, and has shown considerable variation within individual specimen’s seed coat thickness.”
Horsegram, Macrotyloma uniflorum is not a commonly-known bean in the West, but that might change with the right marketing. It’s still commonly eaten in India today, but not well studied in the archaeological record. It seems to have been an early domesticated crop, along with other beans studied by Fuller (free access).
This is the first time that HRXCT has been applied to entire archaeological seeds. What Fuller and Murphy found is that even with an individual seed, the thickness varied over the bean. They state that this may be “due to differential shrinkage during charring on different parts of the same seed and post-charring damage leading to apparent thinning.” Because of this variation, they argue that a large sample is needed for accurate measurement, which makes a non-destructive method of measuring even more valuable.
Of the twelve samples analysed, the seeds could be categorised into two distinct groups, thicker (wild type) seed coats, with averages thicknesses above 17 micrometres, and thinner (more domesticated) seed coats between 10 and 15 micrometres. The results indicated that domestication of horsegram took place during the second millennium BC, with seed coats fair fixed in thickness by the early centuries AD. The findings also show the potential for HRXCT to be used to look at a variety of domesticated grains and pulses, such as peas.
Christoph Rau, Principal Beamline Scientist on I13, where the work was carried out, says “The beamline is a unique tool and is involved in a wide range of applications from high resolution imaging of biological tissues to palaeontological research. In this case, the beamline has enabled the team to produce 3D images of the seeds with incredible micrometer scale resolution, without damaging their precious samples”
“We’re continuing to work with Diamond to look at other interesting archaeological seeds, and how they’ve become domesticated,” concludes Fuller. “Peas are a great example of this – wild peas are ejected from their pods naturally, but domesticated peas only leave the pod when the cultivator removes them – a quite symbiotic relationship.”