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<jats:p> Increasing vapor pressure deficit and drought sensitivity will limit future corn yields in the U.S. Midwest. <jats:named-content content-type="related-content"> [Also see Report by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="516" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1251423"> Lobell <jats:italic>et al.</jats:italic> </jats:related-article> ] </jats:named-content> </jats:p>

<jats:title>Optics On the Edge</jats:title> <jats:p> The ability to isolate stable materials just one atom thick has provided the impetus for a new generation of thin films technologies. Often, the materials are conceptually imaged in the form of an artist's impression as single continuous layers—a single crystal sheet of atoms. Scanning probe and transmission electron microscopies reveal the reality, however, where the material resembles a patchwork quilt with polycrystals of different orientation and size separated by grain boundaries forming a mosaic. These structures play an influential role in determining the transport and optical properties of the membrane. <jats:bold> Yin <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="488" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1250564">488</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="483" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1253531"> <jats:bold>Neshev and Kivshar</jats:bold> </jats:related-article> ) demonstrate a simple microscopy technique based on the nonlinear optical response of the materials to probe and characterize atomically thin layers of MoS <jats:sub>2</jats:sub> . The technique should prove useful during the characterization and optimization of atomically thin membranes. </jats:p>

<jats:title>Examining Supramolecular Exchange</jats:title> <jats:p> Microtubules are a natural example of a one-dimensional (1D) supramolecular structure. Synthetic examples of 1D fibrils often have monomers linked by weak noncovalent interactions that allow monomers to exchange in and out of the fibrils. <jats:bold> Albertazzi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="491" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1250945">491</jats:related-article> ) used a combination of super-resolution microscopy on individual fibrils and stochastic simulation to study monomer exchange in fibrils formed from stacked 1,3,5-benzenetricarboxamide motifs. Exchange did not require large-scale depolymerization and repolymerization, or reassembly of fragments, but proceeded through individual monomers exchanging homogeneously throughout the fibrils. </jats:p>

<jats:title>Improving Reactions at Interfaces</jats:title> <jats:p> Alloying precious metals such as platinum with more abundant transition metals, such as iron and nickel, can both improve their catalytic reactivity and lower catalyst cost. <jats:bold> Chen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="495" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1252553">495</jats:related-article> ) explored using coatings of iron oxide–hydroxide layers on supported platinum nanoparticles for CO oxidation. The presence of this layer allowed the reaction to run rapidly at room temperature by bringing together different reaction sites on the two metals. The addition of nickel improved catalyst lifetime, and an oxidative transformation created a more complex nanoparticle morphology that increased platinum utilization. </jats:p>

<jats:title>Nanotube Engineering</jats:title> <jats:p> In general, the reversible self-assembly of nanotubes through noncovalent bonding either gives fully assembled or fully disassembled products. <jats:bold> Fukino <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="499" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1252120">499</jats:related-article> , published online 10 April; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="482" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1254140">Hudson and Manners</jats:related-article> </jats:bold> ) developed a system with an intermediate possibility. Hollow nanotube structures were assembled from ferrocene-based tetratopic pyridyl ligands mixed with AgBF <jats:sub>4</jats:sub> . Through oxidation of the ferrocene groups, the tubes could be cut into stable, large rings and then reversibly reassembled into nanotubes by reduction of the ferrocene groups. </jats:p>

<jats:title>Vibrating Water Apart</jats:title> <jats:p> The main route for producing hydrogen for industrial chemical synthesis is steam reforming, in which water and methane react at high temperatures on nickel catalysts to produce hydrogen and carbon dioxide. For both water and methane, the initial dissociation step can be promoted by the translational energy of a molecule as well as its internal vibrational energy, and fundamental studies of these reactions try to determine the relative contributions of these pathways. Although the methane reaction has been well studied, only recently have lasers been available to excite the higher stretching vibrations of water. <jats:bold> Hundt <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="504" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1251277">504</jats:related-article> ) now report a joint experimental and theoretical study of D <jats:sub>2</jats:sub> O dissociation on the Ni(111) surface. For a given input of energy, vibrational energy was more effective for surmounting the reaction barrier than translational energy. </jats:p>

<jats:title>Increasing Turnover</jats:title> <jats:p> Rising concentrations of CO <jats:sub>2</jats:sub> in the atmosphere are likely to stimulate plant growth, leading to increased biomass and soil carbon stocks, thereby lessening the rate of the rise in atmospheric CO <jats:sub>2</jats:sub> levels. <jats:bold> Van Groenigen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="508" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1249534">508</jats:related-article> , published online 24 April) report the results of a meta-analysis and modeling that show that increasing the concentration of atmospheric CO <jats:sub>2</jats:sub> also stimulates microbial decomposition of organic carbon in soils, by roughly the same amount that it increases soil organic carbon, leading to lower equilibrium soil carbon inventories and limiting the accumulation of carbon. Thus, soils may not provide as much carbon storage as hoped. </jats:p>

<jats:title>Specialized Evolution</jats:title> <jats:p> Many plants make chemical compounds that are potentially of use to humans, but their evolutionary histories are unknown. <jats:bold> Chae <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="510" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1252076">510</jats:related-article> ) examined how algae and land plants have been able to evolve secondary metabolism biochemistry—those compounds produced in response to their environment—in the face of purifying evolutionary pressure to maintain primary and necessary metabolic pathways. Genomic data was used to separate the primary from the secondary metabolism pathway genes and to construct the evolutionary trajectories of secondary metabolism. Secondary metabolic pathways tend to be controlled by clustered, co-regulated sets of newly duplicated and maintained genes. </jats:p>

<jats:title>Trick and Treat</jats:title> <jats:p> Forked-tailed drongos are a particularly intelligent type of bird found in Africa. Drongos associate with many other bird and mammal species, which can learn to respond to drongo warning calls. Drongos are also exceptional mimics of the other species' alarm calls. Though the increased vigilance across these multi species associations is a benefit to all, drongos sometimes use these calls as ploys to scare associated species away from food, which the drongos then steal. However, without some approach to maintain the effectiveness of this deception, the drongos' ploy would soon be detected. <jats:bold> Flower <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="513" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1249723">513</jats:related-article> ) now show that drongos are able to fool their target species longer by flexibly varying the type of call they give. </jats:p>

<jats:title>Predicting Responses to Drought</jats:title> <jats:p> The U.S. Corn Belt accounts for a sizeable portion of the world's maize growth. Various influences have increased yields over the years. <jats:bold> Lobell <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="516" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1251423">516</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6183" page="484" related-article-type="in-this-issue" vol="344" xlink:href="10.1126/science.1253884"> <jats:bold>Ort and Long</jats:bold> </jats:related-article> ) now show that sensitivity to drought has been increasing as well. It seems that as plants have been bred for increased yield under ideal conditions, the plants become more sensitive to non-ideal conditions. A key factor may be the planting density. Although today's maize varieties are more robust to crowding and the farmer can get more plants in per field, this same crowding takes a toll when water resources are limited. </jats:p>