Exogenous monoterpenes mitigate H2O2-induced lipid damage but do not attenuate photosynthetic decline during water deficit in tomato.

Although monoterpenes are suggested to mediate oxidative status, their role in abiotic stress responses is currently unclear. Here, a foliar spray of monoterpenes increased antioxidant capacity and decreased oxidative stress of Solanum lycopersicum under water deficit stress. The foliar content of monoterpenes increased with spray concentration indicating foliar uptake of exogenous monoterpenes. Exogenous monoterpene application substantially decreased foliar accumulation of hydrogen peroxide (H2O2) and lipid peroxidation (malondialdehyde). However, it appears that monoterpenes prevent the accumulation of reactive oxygen species rather than mitigating subsequent reactive oxygen species-induced damage. Low spray concentration (1.25 mM) proved most effective in decreasing oxidative stress but did not up-regulate the activity of key antioxidant enzymes (superoxide dismutase and ascorbate peroxidase) even though higher (2.5 and 5 mM) spray concentrations did, suggesting a complex role for monoterpenes in mediating antioxidant processes. Furthermore, soil drying caused similar photosynthetic limitations in all plants irrespective of monoterpene treatments, apparently driven by strong reductions in stomatal conductance as photosystem II efficiency only decreased in very dry soil. We suggest that exogenous monoterpenes may mitigate drought-induced oxidative stress by direct quenching and/or up-regulating endogenous antioxidative processes. The protective properties of specific monoterpenes and endogenous antioxidants require further investigation.

Reductions in crop yields across China from elevated ozone.

Exposure of crops to high concentrations of ozone can cause substantial reductions in yield that pose a serious threat to global food security. Here we provide comprehensive estimates of yield losses for key crops across China between 2014 and 2017 attributed to ozone using a number of new approaches. We use an air quality model at 5 km resolution and crop-specific dose-response functions developed for both concentration- and flux-based metrics. We bias correct modelled ozone concentrations and metrics using observations from more than 1000 locations. We find that on a 4-year average basis, production losses of key crops are 34-91 million metric tonnes (Mt/yr), dependent on the approach used, with highest losses in Henan province. At a national level, loss of winter wheat production derived using a China-specific dose-response function increased by 82% from 2014 to 2017, with large interannual variations in the North China Plain and in eastern China. Winter wheat losses estimated using flux-based functions, which require robust simulation of stomatal conductance and underlying vegetation physiology, are significantly lower, at 30 Mt/yr. We show that the definition of the growing season may have a greater impact on estimated losses than small biases in ozone surface concentrations. Although uncertainties remain, our findings demonstrate that increasing ozone concentrations have substantial adverse impacts on crop yields and threaten food security in China. It is important to control ozone concentrations to mitigate these negative impacts.

Temporal variability in the impacts of particulate matter on crop yields on the North China Plain.

The North China Plain (NCP) is a major agricultural region, producing 45% of China's maize. It is also vital to the Chinese economy, encompassing the Beijing-Tianjin-Hebei megacity region. Anthropogenic factors increasingly impact crop yields on the NCP, and globally. Particulate matter (PM) pollution is a significant problem in this region, where annual average PM concentrations over three times the Chinese national air quality standard were recorded for the Beijing-Tianjin-Hebei megacity region between 2013 and 18. PM absorbs light, reducing total shortwave radiation (SW), thereby limiting plant productivity. However, PM also scatters incoming SW, increasing the diffuse fraction, which has been shown to increase growth and biomass assimilation. The Joint UK Land Environment Simulator (JULES) crop model was used to assess the net impact of these competing changes in light on NCP maize yields. In contrast to some previous analyses, we find that PM-associated decreases in SW outweigh any positive impact on yield from an increasing proportion of diffuse radiation. Furthermore, carbon allocation to different portions of the growing cropchanges during the development cycle. We find significant differences between the effect on final yield of identical changes to diffuse fraction and total SW occurring during different development stages. The greatest simulated yield gains from increased SW and reduced diffuse fraction, consistent with reductions in PM, are observed during the early reproductive stage of development (July-August), when the simulated gain of yield is as much as 12.9% more than in other periods. To further assess the impact of PM-linked changes in SW and diffuse fraction on NCP crop yields, radiation profiles from different city regions were then applied across the NCP. The changes in SW associated with these city regions could increase maize yields across China by ~8 Mt. This would completely offset China's annual maize imports, increasing both national and global food security.

Plant Leaf Position Estimation with Computer Vision.

Autonomous analysis of plants, such as for phenotyping and health monitoring etc., often requires the reliable identification and localization of single leaves, a task complicated by their complex and variable shape. Robotic sensor platforms commonly use depth sensors that rely on either infrared light or ultrasound, in addition to imaging. However, infrared methods have the disadvantage of being affected by the presence of ambient light, and ultrasound methods generally have too wide a field of view, making them ineffective for measuring complex and intricate structures. Alternatives may include stereoscopic or structured light scanners, but these can be costly and overly complex to implement. This article presents a fully computer-vision based solution capable of estimating the three-dimensional location of all leaves of a subject plant with the use of a single digital camera autonomously positioned by a three-axis linear robot. A custom trained neural network was used to classify leaves captured in multiple images taken of a subject plant. Parallax calculations were applied to predict leaf depth, and from this, the three-dimensional position. This article demonstrates proof of concept of the method, and initial tests with positioned leaves suggest an expected error of 20 mm. Future modifications are identified to further improve accuracy and utility across different plant canopies.

Current and future impacts of drought and ozone stress on Northern Hemisphere forests.

Rising ozone (O3 ) concentrations, coupled with an increase in drought frequency due to climate change, pose a threat to plant growth and productivity which could negatively affect carbon sequestration capacity of Northern Hemisphere (NH) forests. Using long-term observations of O3 mixing ratios and soil water content (SWC), we implemented empirical drought and O3 stress parameterizations in a coupled stomatal conductance-photosynthesis model to assess their impacts on plant gas exchange at three FLUXNET sites: Castelporziano, Blodgett and Hyytiälä. Model performance was evaluated by comparing model estimates of gross primary productivity (GPP) and latent heat fluxes (LE) against present-day observations. CMIP5 GCM model output data were then used to investigate the potential impact of the two stressors on forests by the middle (2041-2050) and end (2091-2100) of the 21st century. We found drought stress was the more significant as it reduced model overestimation of GPP and LE by ~11%-25% compared to 1%-11% from O3 stress. However, the best model fit to observations at all the study sites was obtained with O3 and drought stress combined, such that the two stressors counteract the impact of each other. With the inclusion of drought and O3 stress, GPP at CPZ, BLO and HYY is projected to increase by 7%, 5% and 8%, respectively, by mid-century and by 14%, 11% and 14% by 2091-2100 as atmospheric CO2 increases. Estimates were up to 21% and 4% higher when drought and O3 stress were neglected respectively. Drought stress will have a substantial impact on plant gas exchange and productivity, off-setting and possibly negating CO2 fertilization gains in future, suggesting projected increases in the frequency and severity of droughts in the NH will play a significant role in forest productivity and carbon budgets in future.

Using green infrastructure to improve urban air quality (GI4AQ).

As evidence for the devastating impacts of air pollution on human health continues to increase, improving urban air quality has become one of the most pressing tasks facing policy makers world-wide. Increasingly, and very often on the basis of conflicting and/or weak evidence, the introduction of green infrastructure (GI) is seen as a win-win solution to urban air pollution, reducing ground-level concentrations without imposing restrictions on traffic and other polluting activities. The impact of GI on air quality is highly context dependent, with models suggesting that GI can improve urban air quality in some situations, but be ineffective or even detrimental in others. Here we set out a novel conceptual framework explaining how and where GI can improve air quality, and offer six specific policy interventions, underpinned by research, that will always allow GI to improve air quality. We call GI with unambiguous benefits for air quality GI4AQ. However, GI4AQ will always be a third-order option for mitigating air pollution, after reducing emissions and extending the distance between sources and receptors.

Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland.

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state-of-the-art FORest Canopy-Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave-drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress-induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model-measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long-term ecosystem-scale observations to enable improved model representation of atmosphere-biosphere interactions in a changing global climate.

Impact of Biofuel Poplar Cultivation on Ground-Level Ozone and Premature Human Mortality Depends on Cultivar Selection and Planting Location.

Isoprene and other volatile organic compounds emitted from vegetation play a key role in governing the formation of ground-level ozone. Emission rates of such compounds depend critically on the plant species. The cultivation of biofuel feedstocks will contribute to future land use change, altering the distribution of plant species and hence the magnitude and distribution of emissions. Here we use relationships between biomass yield and isoprene emissions derived from experimental data for 29 commercially available poplar hybrids to assess the impact that the large-scale cultivation of poplar for use as a biofuel feedstock will have on air quality, specifically ground-level ozone concentrations, in Europe. We show that the increases in ground-level ozone across Europe will increase the number of premature deaths attributable to ozone pollution each year by up to 6%. Substantial crop losses (up to ∼9 Mt y(-1) of wheat and maize) are also projected. We further demonstrate that these impacts are strongly dependent on the location of the poplar plantations, due to the prevailing meteorology, the population density, and the dominant crop type of the region. Our findings indicate the need for a concerted and centralized decision-making process that considers all aspects of future land use change in Europe, and not just the effect on greenhouse gas emissions.