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Spotlights Diagnosis: Drought G LO BALLY, D R O U G H T S AR E B ECO M I N G M O R E FR EQ U E N T and more severe. Because the availability of water controls a plant’s ability to exchange gas with the atmosphere, taking in carbon dioxide and releasing oxygen—both processes that humans rely on plants to do—scientists need to understand how different kinds of plants respond to drought. That means they need instruments that can measure water in plants over long periods of time. However, most methods in use today involve puncturing the plant and probing the inside, which damages the plant and creates confounding dryness at the site being measured. There aren’t many options for objective, non‑destructive, low-cost, lightweight, and scalable water measurement in the field. Nuclear magnetic resonance (NMR) is the leading technology for non- destructively measuring plant water content (though it misses most of the other marks above). NMR is not an imaging tool, but it can measure the total amount of hydrogen atoms in a given section of tree trunk or branch. The technique relies on the magnetic properties of hydrogen nuclei: When placed in a magnetic field, the hydrogen nuclei align themselves with the field according to their magnetic spin properties. In this stage, the nuclei absorb energy from electromagnetic radiation. When the magnetic field is removed, the nuclei return to their normal, unaligned state, and emit the absorbed energy at a specific resonance frequency. (NMR can be tuned to measure different elements, each of which has its own resonance frequency.) The energy emitted by the hydrogen atoms, when compared to previous measurements, reveals whether there were any changes in the hydrogen content of the sample. More hydrogen generally means more water. 2 1663  October 2017 A B A 15 mm 15 mm B D C C Neutron images of a tree branch before and after taking up heavy water. (Left) A tree branch (A) is shown with the NMR device (B) wrapped around it and standing in a vessel of heavy water (C) after having first taken up normal water. (Right) Subtracting subsequent neutron images from the first image creates a picture of water transport and reveals how much (and how quickly) the heavy water is taken up by the branch (D). The bright green regions in the image on the right are the areas of greatest difference from the image on the left.