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How a novel plant X-ray could help farmers lock up more soil carbon

Matching chemical traces in crop leaves and soil can determine the extent of a plant's roots—with exciting consequences for plant breeding and climate change.
March 22, 2024

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Researchers can now point a handheld X-ray at a plant and determine how deep its roots have reached beneath the soil. The quick and convenient tool can provide a read on the nutrients that crops are getting from the earth, and perhaps even how much carbon they’re storing down there as well.

This elegant agricultural solution has been a long time in the making: for six years a team of Pennsylvania State University researchers has been developing this handheld tool and testing it out on corn crops in the United States. With it, scientists will be able to select new deep-rooted crops for breeding, and also check whether the experimental plants that result are indeed sinking their roots to new depths.

Plenty of research has been unfolding in recent years on plant roots, because deeper-rooting crops are seen as a panacea for several problems relating to climate change. Crops that plunge further down can access more water in the soil that could buffer them against the debilitating effects of drought. Longer roots may also mitigate climate change, because they’re able to take up more nitrogen fertilizer (which can be polluting if the excess is left unused in the soil) and also deposit more carbon that stays locked away in the ground. 

Considering the huge gains to be made from deeper roots, the researchers on the recent Crop Science study set about developing their new measurement tool. It’s built on an interesting premise: that crop leaves contain telltale clues about the nutrients the plant is getting from the soil — and that those nutrients vary by the depth of the soil. 

The work started in 2018 with 30 distinct genetic strains of corn — some of which would go on to grow deeper roots than others — which the researchers planted and grew at four research sites spread across Pennsylvania, Colorado, and Wisconsin in the US to ensure there were a variety of soil conditions in the experiment. The field experiments also included soil cores to determine what chemical elements the soil contained at varying depths at the different sites. To test the principle behind their idea, the team also ran a separate set of experiments in a greenhouse, where they grew crops in soil that had been injected with a chemical element called strontium at specific depths.

 

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Then, they collected leaf samples from the various crops, and wielded their new tool, which they’ve called a Leaf Elemental Accumulation from Deep Roots (LEADER). The device employs X-ray fluorescence spectroscopy, which uses the interaction of X-rays with ingredients in a material to figure out its chemical composition. When the X-ray interacts with calcium, for instance, it will show up with a unique pattern. “In this way, the leaves can serve as indicators or sensors of where the roots are in the soil,” the researchers explained in a press release.

When they combined these leaf readings with the associated soil samples across the various sites, the researchers found that at two of the field sites, they could accurately identify the depth of the plant roots just by matching the chemical profile in the leaf with that of the soil, which contained a variety of readable elements including silicon, potassium, copper and phosphorus. This finding was backed up by the greenhouse experiment using strontium, which showed that crops with strontium detected in their leaves were indeed growing to the depths where this chemical has been injected into the soil.

The leaf-scanning invention could dramatically streamline the process of identifying deep-rooted crops, which would usually require invasive methods — such as hauling crops out of the soil to measure their roots. “To breed deeper-rooted crops, you need to look at thousands of plants. Digging them up is expensive and time consuming because some of those roots are down two meters or more. Everybody wants deep-rooted crops — but until now, we didn’t know how to get them,” the researchers say

With the new device, scientists, plant breeders and farmers alike should be able to bleep the X-ray device through dozens of plants to get the proof that they need. The Pennsylvania researchers have entered a new patent for their technology — and while it was tested on corn crops, it could be applied to any type of plant, they say. 

This could ratchet up plant breeding with potentially huge implications for agriculture’s climate footprint, the team notes. Calculations for the US show that deep-rooted crops can “offset years of our total carbon emissions. That’s huge — think about all the acres growing crops in America. If those roots grow just a little bit deeper, then we’re storing massive amounts of carbon deeper in the soil.”

Hanlon et. al. “LEADER (Leaf Element Accumulation from DEep Roots): A nondestructive phenotyping platform to estimate rooting depth in the field.” Crop Science. 2024. 

Image: ©Anthropocene Magazine

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