Fine Root Growth and Vertical Distribution in Response to Elevated CO<Subscript>2</Subscript>, Warming and Drought in a Mixed Heathland–Grassland

Belowground plant responses have received much less attention in climate change experiments than aboveground plant responses, thus hampering a holistic understanding of climate change effects on plants and ecosystems. In addition, responses of plant roots to climate change have mostly been studied in single-factor experiments. In a Danish heathland ecosystem, we investigated both individual and combined effects of elevated CO₂, warming and drought on fine root length, net production and standing biomass by the use of minirhizotrons, ingrowth cores and soil coring. Warming increased the net root production from ingrowth cores, but decreased fine root number and length in minirhizotrons, whereas there were no significant main effects of drought. Across all treatments and soil depths, CO₂ stimulated both the total fine root length (+44%) and the number of roots observed (+39%), with highest relative increase in root length in the deeper soil layers. Our results suggest that under future climate, plants may allocate considerable resources into roots compared to aboveground biomass. Increased carbon (C) allocation to roots may have a great impact on the overall ecosystem C balance and must be considered in modelling of future ecosystem responses to climate change. To provide models with necessary validation data, more studies are needed to investigate if higher C allocation to roots will lead to long-term C storage in more recalcitrant soil C pools or if this potential increase in soil carbon storage may be offset by increased priming activity and turnover rates for soil organic matter.     

The effect of drought and ultraviolet radiation on growth and stress markers in pea and wheat

It emerged recently that there is an inter‐relationship between drought and ultraviolet‐B (UV‐B) radiation in plant responses, in that both stresses provoke an oxidative burst. The purpose of this investigation was to compare the effects and interaction of drought and UV‐B in wheat and pea. The absence of changes in relative leaf water content (RWC) after UV‐B treatments indicate that changes in water content were not involved. RWC was the main factor resulting in reduced growth in response to drought. Increases in anthocyanin and phenols were detected after exposure to UV‐B. The increases do not appear to be of sufficient magnitude to act as a UV‐B screen. UV‐B application caused greater membrane damage than drought stress, as assessed by lipid peroxidation as well as osmolyte leakage. An increase in the specific activities of antioxidant enzymes was measured after UV‐B alone as well as after application to droughted plants. Proline increased primarily in drought‐stressed pea or wheat. Proline may be the drought‐induced factor which has a protective role in response to UV‐B. The physiological and biochemical parameters measured indicate the UV‐B light has stronger stress effectors than drought on the growth of seedlings of both species. The two environmental stresses acted synergistically to induce protective mechanisms in that pre‐application of either stress reduced the damage caused by subsequent application of the other stress.     

Dynamics of plant responses to combinations of air pollutants

The focus of this review is on how plants respond to combinations of multiple air pollutants. Global pollution trends, plant physiological responses and ecological perspectives in natural and agricultural systems are all discussed. In particular, we highlight the importance of studying sequential or simultaneous exposure of plants to pollutants, rather than exposure to individual pollutants in isolation, and explore how these responses may interfere with the way plants interact with their biotic community. Air pollutants can alter the normal physiology and metabolic functioning of plants. Here we describe how the phenotypic and molecular changes in response to multiple pollutants can differ compared to those elicited by single pollutants, and how different responses have been observed between plants in the field and in controlled laboratory conditions and between trees and crop plants. From an ecological perspective, we discuss how air pollution can result in greater susceptibility to biotic stressors and in direct or indirect effects on interactions with organisms that occupy higher trophic levels. Finally, we provide an overview of the potential uses of plants to mitigate air pollution, exploring the feasibility for pollution removal via the processes of bio‐accumulation and phytoremediation. We conclude by proposing some new directions for future research in the field.     

Maternal effects of drought stress and inbreeding in Impatiens capensis (Balsaminaceae)

Maternal effects can have substantial impacts on plant fitness and plant populations. Stressful environmental conditions can cause a maternal plant to inadequately provision its progeny, resulting in poor seedling growth, low reproductive success, and decreased competitive ability. Maternal effects consist of environmental and genetic load components, but the interactions between these two components have rarely been considered. To determine the effects of maternal drought stress and maternal inbreeding on progeny biomass (a fitness correlate) and physiological responses to drought stress, we conducted a greenhouse experiment with genetic lines from two populations (mesic site vs. dry site) of the herbaceous annual Impatiens capensis (Balsaminaceae). Seeds were collected from cleistogamous flowers of inbred or outcrossed maternal plants that were subject to either a drought or control treatment. These seeds were grown into juvenile plants that were also subject to either a drought stress or a control treatment. Plants from the mesic site had significantly reduced biomass from maternal drought stress, while plants from the dry site maintained biomass despite adverse maternal environmental conditions. Juvenile plants of both populations had reduced biomass only as a result of maternal inbreeding. Interestingly, inbreeding depression was more apparent when maternal environmental conditions were benign.     

Acclimation and interaction between drought and elevated UV‐B in A. thaliana: Differences in response over treatment,recovery and reproduction

Here, a factorial experiment was used to investigate the interactive effects of a UV‐B episode and concurrent progressive drought on the growth, chemistry, and reproductive success of A. thaliana. Both drought and UV‐B negatively affected rosette growth, although UV‐B had the greater effect. Acclimation to UV‐B involved adjustment of leaf morphology, while drought induced accumulation of soluble sugars and phenolics. All plants recovered from treatments, but the cost of recovery was a developmental delay resulting in alteration in phenological timings. Combined treatments interacted causing additive negative effects on growth following exposure. This may be linked with inhibition of soluble sugar accumulation by UV‐B, restricting the capacity for osmotic adjustment in response to drought. Following cessation of treatments, relative growth rate (RGR) and net assimilation rate (NAR) were significantly stimulated in plants treated with combined drought and UV‐B. This interaction alleviated subsequent impacts of elevated UV‐B on silique yield and reproductive timings. This study demonstrates the potential for interaction between these two common environmental factors. Furthermore, it shows the changeable nature of these interactions over the course of exposure and recovery through to reproduction, highlighting the need for sustained assessment of such interactions over a plant's lifecycle.     

UV radiation-responsive proteins in rice leaves: a proteomic analysis

Depletion of stratospheric ozone has led to increased UV radiation reaching the surface of the Earth. This may damage plants. Using physiological, proteomic and quantitative real-time PCR (qPCR) methods, we systematically studied the response of 16-day-old rice seedlings to UV [0.67 W m(-2) biologically effective UVB (UVB(BE)) and 0.28 W m(-2) UVA] exposure for 6, 12 and 24 h. UV exposure resulted in the appearance of light brown patches on leaves, a decrease in the net photosynthetic rate (Pn), lipid peroxidation, accumulation of UV-absorbing compounds (including flavonoids and other phenolic pigments) and differential expression of 22 proteins. Both physiological and molecular responses became stronger with increasing UV exposure time, indicating the effects of UV accumulation on plants. UV-induced responses included (i) phytohormone-regulative responses (up-regulation of proteins related to phytohormone synthesis such as IAA and ethylene); (ii) injurious responses (photosynthesis suppression, lipid peroxidation and visible injury); and (iii) protective responses (accumulation of UV-absorbing compounds and differential expression of proteins involved in detoxification/antioxidation, defense, protein processing, RNA processing, carbohydrate metabolism and secondary metabolism). The identification of UV-responsive proteins provided a better understanding of the molecular mechanism of plant responses to UV stress. Proteomic and qPCR analysis identified one up-regulated and two induced proteins with important functions: tryptophan synthase α chain (production of radical oxygen species), glyoxalase I (detoxification/antioxidation) and a Bet v I family protein (defense). These results will contribute to future research into their roles in UV stress responses in plants.     

Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom,Cytisus scoparius

Increases in the frequency and duration of droughts under global climate change could have implications for plant–insect interactions, and could either increase or decrease the effects of weed biological control agents. In this study, we used greenhouse and field experiments to examine the impacts of drought stress on the abundance and impacts of the adventive psyllid Arytainilla spartiophila Forster (Hemiptera: Psyllidae, Arytaininae) on its target weed Scotch broom, Cytisus scoparius (L.) Link (Fabaceae), in California, USA. The psyllid impacted plant growth in both field and greenhouse experiments, whereas drought stress affected plant growth in the greenhouse only, suggesting that other factors besides water availability may be more limiting for plants in the field. Effects on psyllid survival were consistent with the plant vigor hypothesis, which predicts that herbivores will perform better on vigorously growing plants; psyllid numbers were lower on drought-stressed plants in the greenhouse and were correlated with plant growth in both the greenhouse and the field. In the greenhouse, the combined effects of the psyllid and drought stress were additive, indicating that the effects of the psyllid were consistent across unstressed and drought-stressed plants. Although the psyllid is unlikely to control Scotch broom on its own, results suggest that it may work in conjunction with drought stress to suppress Scotch broom.     

Mycorrhizal symbiosis and response of sorghum plants to combined drought and salinity stresses

Arbuscular mycorrhizal (AM) symbiosis can confer increased host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salinity stress, and since drought and salinity stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be partially the result of AM influence on salinity stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices (Gi), Gigaspora margarita (Gm) or a mixture of AM species, during a sustained drought following exposure to salinity treatments (NaCl stress, osmotic stress via concentrated macronutrients, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and nonAM plants in just two instances. Days required for plants to reach stomatal closure were similar for Gi and nonAM plants exposed to drought alone, but with exposure to combined NaCl and drought stress, stomates of Gi plants remained open 17-22% longer than in nonAM plants. Promotion of stomatal conductance by Gm occurred with exposure to NaCl/drought stress but not with drought alone or with soil leaching before drought. In other instances, however, the addition of salt tended to nullify an AM-induced change in drought response. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their nonAM counterparts.     

植物抗旱性中的补偿效应及其在农业节水中的应用

在论述植物补偿效应存在类型和研究范畴的基础上,详细评述了植物抗旱性中根系形态结构功能及地上部干物质积累、产量和水分利用效率方面的补偿效应及其影响因素,并对植物抗旱作用中补偿生长的可能生理学机制作了探讨。同时,对补偿效应在提高农业水分利用效率中的应用进行了讨论     

Meta‐analysis of drought and heat stress combination impact on crop yield and yield components

Episodes of prolonged drought coupled with heat waves (i.e. drought and heat combination) can have a devastating impact on agricultural production and crop yield. It is therefore not surprising that improving tolerance to drought and heat combination has been a major goal for breeders and biotech companies. Although much is known about the physiological and molecular responses of vegetative tissues to a combination of drought and heat stress, less is known about the impact of this stress combination on yield and different yield components. Here, we used a meta‐analysis approach to synthesize results from over 120 published case studies of crop responses to combined drought and heat stress. Our findings reveal that drought and heat stress combination significantly impacts yield by decreasing harvest index, shortening the life cycle of crops, and altering seed number, size and composition. Furthermore, these impacts are more severe when the stress combination is applied during the reproductive stage of plants. We further identify differences in how legumes and cereals respond to the stress combination and reveal that utilizing C3 or C4 metabolism may not provide an advantage to plants during stress combinations. Taken together our study highlights a need to focus future studies, as well as breeding efforts, on crop responses to drought and heat combination at the reproductive stage of different crop species.     

Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta‐analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.     

Additive and non-additive responses of seedlings to simulated herbivory and drought

Drought is a global threat, increasing in severity and frequency throughout tropical ecosystems. Although plants often face drought in conjunction with biotic stressors, such as herbivory or disease, experimental studies infrequently test the simultaneous effects of drought and biotic stress. Because multiple simultaneous stressors may have non-additive and complex effects on plant performance, it is difficult to predict plant responses to multiple threats from research examining one stress at a time. Using an experimental approach in the greenhouse, we investigated potential non-additivity in seedling growth and survival to simulated drought and herbivory across a phylogenetically diverse pool of ten Hawaiian plant species. Overall, seedlings showed limited tolerance, defined as similar growth and survival in stressed compared with control (non-stressed) plants, to simulated herbivory and drought, with the combined effects of both stressors to be generally additive and negative across species. Significant variation in stress tolerance was detected among species, and species variation was explained, at least in part, by functional traits such that species with larger root/shoot ratios and smaller seeds, tended to demonstrate greater herbivory and drought tolerance. Future research incorporating additional trait analysis and different stressors could shed light on mechanisms underlying seedling stress tolerance and clarify whether additivity, as detected in this study, extends across other combinations of stressors. Such work will provide needed insights into the regeneration of seedlings in tropical forests under threats of herbivory and climate change.     

The crucial role of plant mitochondria in orchestrating drought tolerance

BACKGROUND: Around the world, the frequency and intensity of droughts is increasing as a result of global climate change, with important consequences for the growth and survival of agricultural and native plant species. Understanding how plants respond to water stress is thus crucial for predicting the impacts of climate change on the crop productivity and ecosystem functioning. In contrast to the large number of studies assessing drought impacts on photosynthesis, relatively little attention has been devoted to understanding how mitochondrial respiratory metabolism is altered under water stress conditions. SCOPE: This review provides an overview of the impacts of water stress on mitochondrial respiration (R), combining studies at the whole-plant, individual organ, cellular and organelle levels. To establish whether there are clear patterns in the response of in vivo R to water stress, a wide range of root, leaf and whole-plant studies are reviewed. It is shown that water stress almost always inhibits R in actively growing roots and whole plants. However, in fully expanded, mature leaves the response is more variable, with water stress reducing R in near two-thirds of reported studies, with most of the remainder showing no change. Only a few studies reported increases in leaf R under severe water stress conditions. The mechanisms responsible for these variable responses are discussed. Importantly, the fact is highlighted that irrespective of whether drought increases or decreases respiration, overall the changes in R are minor compared with the large decreases in photosynthetic carbon gain in response to drought. Based on recent work highlighting the link between chloroplast and mitochondrial functions in leaves, we propose a model by which mitochondrial R enables survival and rapid recovery of productivity under water stress conditions. Finally, the effects of water stress on mitochondrial function, protein abundance and overall metabolism are reviewed.     

Droughting a megadrought: Ecological consequences of a decade of experimental drought atop aridification on the Colorado Plateau

Global dryland vegetation communities will likely change as ongoing drought conditions shift regional climates towards a more arid future. Additional aridification of drylands can impact plant and ground cover, biogeochemical cycles, and plant–soil feedbacks, yet how and when these crucial ecosystem components will respond to drought intensification requires further investigation. Using a long-term precipitation reduction experiment (35% reduction) conducted across the Colorado Plateau and spanning 10 years into a 20+ year regional megadrought, we explored how vegetation cover, soil conditions, and growing season nitrogen (N) availability are impacted by drying climate conditions. We observed large declines for all dominant plant functional types (C₃ and C₄ grasses and C₃ and C₄ shrubs) across measurement period, both in the drought treatment and control plots, likely due to ongoing regional megadrought conditions. In experimental drought plots, we observed less plant cover, less biological soil crust cover, warmer and drier soil conditions, and more soil resin-extractable N compared to the control plots. Observed increases in soil N availability were best explained by a negative correlation with plant cover regardless of treatment, suggesting that declines in vegetation N uptake may be driving increases in available soil N. However, in ecosystems experiencing long-term aridification, increased N availability may ultimately result in N losses if soil moisture is consistently too dry to support plant and microbial N immobilization and ecosystem recovery. These results show dramatic, worrisome declines in plant cover with long-term drought. Additionally, this study highlights that more plant cover losses are possible with further drought intensification and underscore that, in addition to large drought effects on aboveground communities, drying trends drive significant changes to critical soil resources such as N availability, all of which could have long-term ecosystem impacts for drylands.     

Soybean nodulation and N2 fixation response to drought under carbon dioxide enrichment

The combined effects of carbon dioxide (CO₂) enrichment and water deficits on nodulation and N₂ fixation were analysed in soybean [Glycine max (L.) Merr.]. Two short-term experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO₂ atmospheres of either 360 or 700 μmol CO₂ mol^–1. Under drought-stressed conditions, elevated [CO₂] resulted in a delay in the decrease in N₂ fixation rates associated with drying of the soil used in these experiments. The elevated [CO₂] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO₂]. The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO₂ had much higher TNC levels than those under ambient CO₂. For both [CO₂] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N₂ fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO₂] resulted in substantially less drought-induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO₂] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N₂ fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO₂ enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N₂ rates under soil water deficits. We suggest that future [CO₂] increases are likely to benefit soybean production by increasing the drought tolerance of N₂ fixation.     

Plant water relations at elevated CO2 -- implications for water-limited environments

Long-term exposure of plants to elevated [CO2] leads to a number of growth and physiological effects, many of which are interpreted in the context of ameliorating the negative impacts of drought. However, despite considerable study, a clear picture in terms of the influence of elevated [CO2] on plant water relations and the role that these effects play in determining the response of plants to elevated [CO2] under water-limited conditions has been slow to emerge. In this paper, four areas of research are examined that represent critical, yet uncertain, themes related to the response of plants to elevated [CO2] and drought. These include (1) fine-root proliferation and implications for whole-plant water uptake; (2) enhanced water-use efficiency and consequences for drought tolerance; (3) reductions in stomatal conductance and impacts on leaf water potential; and (4) solute accumulation, osmotic adjustment and dehydration tolerance of leaves. A survey of the literature indicates that the growth of plants at elevated [CO2] can lead to conditions whereby plants maintain higher (less negative) leaf water potentials. The mechanisms that contribute to this effect are not fully known, although CO2-induced reductions in stomatal conductance, increases in whole-plant hydraulic conductance and osmotic adjustment may be important. Less understood are the interactive effects of elevated [CO2] and drought on fine-root production and water-use efficiency, and the contribution of these processes to plant growth in water-limited environments. Increases in water-use efficiency and reductions in water use can contribute to enhanced soil water content under elevated [CO2]. Herbaceous crops and grasslands are most responsive in this regard. The conservation of soil water at elevated [CO2] in other systems has been less studied, but in terms of maintaining growth or carbon gain during drought, the benefits of CO2-induced improvements in soil water content appear relatively minor. Nonetheless, because even small effects of elevated [CO2] on plant and soil water relations can have important implications for ecosystems, we conclude that this area of research deserves continued investigation. Future studies that focus on cellular mechanisms of plant response to elevated [CO2] and drought are needed, as are whole-plant investigations that emphasize the integration of processes throughout the soil--plant--atmosphere continuum. We suggest that the hydraulic principles that govern water transport provide an integrating framework that would allow CO2-induced changes in stomatal conductance, leaf water potential, root growth and other processes to be uniquely evaluated within the context of whole-plant hydraulic conductance and water transport efficiency.     

Regulation of citrus responses to the combined action of drought and high temperatures depends on the severity of water deprivation

Plants grown in natural environment are regularly subjected to different combinations of abiotic stresses. Recent studies revealed that citrus plants subjected to a combination of severe drought and high temperatures displayed specific physiological, hormonal, molecular and metabolic responses. In the present study, we have performed a long‐term experiment combining moderate drought and heat in Cleopatra mandarin to evaluate the impact of the stress‐sequence, intensity and duration. Our results support previous observation of high sensitivity of Cleopatra mandarin to abiotic stresses that include high temperatures. In this sense, a combination of drought and heat stress negatively impacts Cleopatra seedlings independently of the drought intensity. However, some responses to combined drought and heat depend on drought intensity, especially those involved in stomatal regulation. The intricate natural environment, abiotic stress combinations and global climatic changes increase the complexity of studying plant responses to stress factors in the laboratory. Consequently, new experimental approaches taking in consideration different stress combinations should be implemented to study the viability of Cleopatra mandarin as a rootstock in a rapidly changing environment.     

Infection by powdery mildew Erysiphe cruciferarum (Erysiphaceae) strongly affects growth and fitness of Alliaria petiolata (Brassicaceae)

Garlic mustard (Alliaria petiolata) is an invasive biennial that negatively impacts plant and animal communities throughout North America and lacks significant herbivory in its invasive range. Throughout Ohio, many garlic mustard populations support the powdery mildew fungus Erysiphe cruciferarum, although disease incidence varies among populations and environments. Effects of infection on plant growth, as well as both plant and fungal responses to drought and light conditions, were examined on greenhouse-grown, first-year garlic mustard plants. Also, the effects of the fungus on plant growth and fitness were studied in a naturally growing population of second-year plants in the field. Powdery mildew significantly reduced growth of first-year plants in the greenhouse, eventually causing complete mortality. Simulated drought slowed both plant growth and disease development, independent of light conditions. In the field, plants with little incidence of disease after their first year grew taller during their second year, producing significantly more siliques and twice as many seeds as heavily diseased plants did. Seed germination rates did not differ between plants with different levels of disease severity. Consistent reductions in survival, growth, and fitness caused by fungal infection may reduce populations of garlic mustard. These effects may be more evident in moist sites that favor fungal development.