Increased Drought Sensitivity Research Articles

Identifying drivers of non-stationary climate-growth relationships of European beech

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species' ecological amplitude.Our results show that drought sensitivity is highly variable and non-stationary, though consistently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees' rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech's drought sensitivity, which needs to be taken into consideration when assessing this species' response to climate change.

The ZmbHLH47-ZmSnRK2.9 Module Promotes Drought Tolerance in Maize.

Drought stress globally poses a significant threat to maize (Zea mays L.) productivity and the underlying molecular mechanisms of drought tolerance remain elusive. In this study, we characterized ZmbHLH47, a basic helix-loop-helix (bHLH) transcription factor, as a positive regulator of drought tolerance in maize. ZmbHLH47 expression was notably induced by both drought stress and abscisic acid (ABA). Transgenic plants overexpressing ZmbHLH47 displayed elevated drought tolerance and ABA responsiveness, while the zmbhlh47 mutant exhibited increased drought sensitivity and reduced ABA sensitivity. Mechanistically, it was revealed that ZmbHLH47 could directly bind to the promoter of ZmSnRK2.9 gene, a member of the subgroup III SnRK2 kinases, activating its expression. Furthermore, ZmSnRK2.9-overexpressing plants exhibited enhanced ABA sensitivity and drought tolerance, whereas the zmsnrk2.9 mutant displayed a decreased sensitivity to both. Notably, overexpressing ZmbHLH47 in the zmsnrk2.9 mutant closely resembled the zmsnrk2.9 mutant, indicating the importance of the ZmbHLH47-ZmSnRK2.9 module in ABA response and drought tolerance. These findings provided valuable insights and a potential genetic resource for enhancing the environmental adaptability of maize.

CmoPIP1-4 confers drought tolerance in pumpkin by altering hydrogen sulfide signaling

Drought is a major limiting factor for the growth and development of pumpkins. Plasma membrane intrinsic proteins (PIPs) are major water channels that play a crucial role in the regulation of cellular water status and solute trafficking during drought conditions. CmoPIP1-4 is a plasma membrane-localized protein that is significantly upregulated in roots and leaves under drought-stress conditions. In this study, the overexpression of CmoPIP1-4 enhances drought resistance in yeast. In contrast, CRISPR-mediated CmoPIP1-4 knockout in pumpkin roots increased drought sensitivity. This increased drought sensitivity of CmoPIP1-4 knockout plants is associated with a decline in the levels of hydrogen sulfide (H2S) and abscisic acid (ABA), accompanied by an increase in water loss caused by greater levels of transpiration and stomatal conductance. In addition, the sensitivity of CmoPIP1-4 CRISPR plants is further aggravated by reduced antioxidative enzyme activity, decreased proline and sugar contents, and extensive root damage. Furthermore, expression profiles of genes such as CmoHSP70s, CmoNCED3, CmoNCED4, and others involved in metabolic activities were markedly reduced in CmoPIP1-4 CRISPR plants. Moreover, we also discovered an interaction between the drought-responsive gene CmoDCD and CmoPIP1-4, indicating their potential role in activating H2S-mediated signaling in pumpkin, which could confer drought tolerance. The findings of our study collectively demonstrate CmoPIP1-4 plays a crucial role in the regulation of H2S-mediated signaling, influencing stomatal density and aperture in pumpkin plants, and thereby enhancing their drought tolerance.

The increasing drought sensitivity of silver fir (Abies alba Mill.) is evident in the last two decades

Silver fir (Abies alba Mill.) is still counted among drought-tolerant tree species. However, its ability to cope with the recent extremely dry period has not yet been sufficiently studied. The objective of research was to analyse differences in the climate-growth response between silver fir, Norway spruce (Picea abies L. Karst.) and European larch (Larix decidua Mill.) growing in areas with large-scale disintegration of spruce stands. In 2019–2021, the increment cores were sampled at 16 sites along the altitudinal gradient of 340–775 m a.s.l. in different regions of the Czech Republic affected by bark beetle outbreak. The radial growth pattern of fir was compared with that of spruce or larch growing under the same site conditions. In fir, the missing rings were frequently recorded during the period of peak SO2 pollution load in 1966–1985, but they were rarely identified in recent years. In spruce and larch, missing rings were less common and occurred mainly in the recent dry period after 2015. Fir was less sensitive to summer drought compared to larch and especially to spruce, which showed high sensitivity to summer drought regardless of the altitude. The significant positive response of fir to summer precipitation was recorded at sites up to 450 m a.s.l., however, its sensitivity to drought has increased in the last two decades. Hence, when considering the wider use of fir, it is necessary to respect its ecological requirements as much as possible in order to preserve its vitality and production potential in changing climatic conditions.

A Medicago truncatula calcineurin B-like protein, MtCBL13 confers drought sensitivity in Arabidopsis through ABA-dependent pathway

The Ca2+-CBL-CIPK complex is a crucial plant-specific signaling pathway involved in plant growth, development, and stress responses. However, the functions of the CBL/CIPK protein families from Medicago truncatula remain largely unknown. Here, we characterized a novel drought-responsive calcineurin B-like protein (CBL) gene, MtCBL13, in response to drought stress. MtCBL13 is a plasma membrane-localized protein predominantly expressed in roots. The transcript level of MtCBL13 was increased at the early stage of drought stress but sharply decreased with increasing exposure time. Transgenic Arabidopsis plants overexpressing MtCBL13 were sensitive to mannitol and ABA treatments at seed germination and seedling stages. Overexpression of MtCBL13 increased drought sensitivity in Arabidopsis, as evidenced by higher ROS accumulation, electrolyte leakage, lipid peroxidation, and water loss rate compared to the WT. Moreover, overexpression of MtCBL13 dramatically decreased the contents of proline, soluble sugar, soluble protein, and relative leaf water and inhibited photosynthetic efficiency and activities of CAT and SOD enzymes. The expression levels of stress marker genes, such as RD22 and DREB2A, were significantly reduced in MtCBL13 transgenic plants. Additionally, overexpression of MtCBL13 significantly upregulated ABI1/2 but inhibited ABI4/5 under drought stress. Our results indicate that MtCBL13 could negatively influence drought tolerance through the ABA-dependent pathway. The study provides novel insights into the understanding of the underlying mechanisms of CBL-mediated drought stress response in M. truncatula and could facilitate the genetic improvement of legumes through molecular breeding.

Mutation of OsMYB60 reduces rice resilience to drought stress by attenuating cuticular wax biosynthesis.

The cuticular wax layer on leaf surfaces limits non-stomatal water loss to the atmosphere and protects against pathogen invasion. Although many genes associated with wax biosynthesis and wax transport in plants have been identified, their regulatory mechanisms remain largely unknown. Here, we show that the MYB transcription factor OsMYB60 positively regulates cuticular wax biosynthesis and this helps rice (Oryza sativa) plants tolerate drought stress. Compared with the wild type (japonica cultivar 'Dongjin'), osmyb60 null mutants (osmyb60-1 and osmyb60-2) exhibited increased drought sensitivity, with more chlorophyll leaching and higher rates of water loss. Quantitative reverse-transcription PCR showed that the loss of function of OsMYB60 led to downregulation of wax biosynthesis genes, leading to reduced amounts of total wax components on leaf surfaces under normal conditions. Yeast one-hybrid, luciferase transient transcriptional activity, and chromatin immunoprecipitation assays revealed that OsMYB60 directly binds to the promoter of OsCER1 (a key gene involved in very-long-chain alkane biosynthesis) and upregulates its expression. Taken together, these results demonstrate that OsMYB60 enhances rice resilience to drought stress by promoting cuticular wax biosynthesis on leaf surfaces.

OsWRKY114 Negatively Regulates Drought Tolerance by Restricting Stomatal Closure in Rice.

The WRKY family of transcription factors plays a pivotal role in plant responses to biotic and abiotic stress. The WRKY Group III transcription factor OsWRKY114 is a positive regulator of innate immunity against Xanthomonas oryzae pv. oryzae; however, its role in abiotic stress responses is largely unknown. In this study, we showed that the abundant OsWRKY114 transcripts present in transgenic rice plants are reduced under drought conditions. The overexpression of OsWRKY114 significantly increased drought sensitivity in rice, which resulted in a lower survival rate after drought stress. Moreover, we showed that stomatal closure, which is a strategy to save water under drought, is restricted in OsWRKY114-overexpressing plants compared with wild-type plants. The expression levels of PYR/PYL/RCAR genes, such as OsPYL2 and OsPYL10 that confer drought tolerance through stomatal closure, were also markedly lower in the OsWRKY114-overexpressing plants. Taken together, these results suggest that OsWRKY114 negatively regulates plant tolerance of drought stress via inhibition of stomatal closure, which would otherwise prevent water loss in rice.

The activity of the stress modulated Arabidopsis ubiquitin ligases PUB46 and PUB48 is partially redundant

ABSTRACT The Arabidopsis ubiquitin ligases PUB46, PUB47 and PUB48 are encoded by paralogus genes. Single gene pub46 and pub48 mutants display increased drought sensitivity compared to wild type (WT) suggesting that each has specific biological activity. The high sequence homology between PUB46 and PUB48 activity suggested that they may also share some aspects of their activity. Unfortunately, the close proximity of the PUB46 and PUB48 gene loci precludes obtaining a double mutant required to study if they are partially redundant by crossing the available single mutants. We thus applied microRNA technology to reduce the activity of all three gene products of the PUB46-48 subfamily by constructing an artificial microRNA (aMIR) targeted to this subfamily. Expressing aMIR46-48 in WT plants resulted in increased drought-sensitivity, a phenotype resembling that of each of the single pub46 and pub48 mutants, and enhanced sensitivity to methyl viologen, similar to that observed for the pub46 mutant. The WT plants expressing aMIR46-48 plants also revealed reduced inhibition by ABA at seed germination, a phenotype not evident in the single mutants. Expressing aMIR46-48 in pub46 and pub48 mutants further enhanced the drought sensitivity of each parental single mutant and of WT expressing aMIR46-48. These results suggest that the biological activities of PUB46 and PUB48 in abiotic stress response are partially redundant.

Molecular Evolution of Plant 14-3-3 Proteins and Function of Hv14-3-3A in Stomatal Regulation and Drought Tolerance.

Drought significantly affects stomatal regulation, leading to the reduced growth and productivity of plants. Plant 14-3-3 proteins were reported to participate in drought response by regulating the activities of a wide array of target proteins. However, the molecular evolution, expression pattern and physiological functions of 14-3-3s under drought stress remain unclear. In this study, a comparative genomic analysis and the tissue-specific expression of 14-3-3s revealed the highly conserved and early evolution of 14-3-3s in green plants and duplication and expansion of the 14-3-3s family members in angiosperms. Using barley (Hordeum vulgare) for the functional characterization of 14-3-3 proteins, the transcripts of five members out of six Hv14-3-3s were highly induced by drought in the drought-tolerant line, XZ141. Suppression of the expression of Hv14-3-3A through barley stripe mosaic virus-virus induced gene silencing resulted in significantly increased drought sensitivity and stomatal density as well as significantly reduced net CO2 assimilation (A) and stomatal conductance (gs) in barley. Moreover, we showed the functional interactions between Hv14-3-3s and key proteins in drought and stomatal responses in plants-such as Open Stomata 1 (HvOST1), Slow Anion Channel 1 (HvSLAC1), three Heat Shock Proteins (HvHSP90-1/2/5) and Dehydration-Responsive Element-Binding 3 (HvDREB3). Taken together, we propose that 14-3-3s are highly evolutionarily conserved proteins and that Hv14-3-3s represent a group of the core regulatory components for the rapid stomatal response to drought in barley. This study will provide important evolutionary and molecular evidence for future applications of 14-3-3 proteins in breeding drought-tolerant crops in a changing global climate.

CBF4/DREB1D represses XERICO to attenuate ABA, osmotic and drought stress responses in Arabidopsis.

Water stress can severely impact plant growth, productivity and yield. Consequently, plants have evolved various strategies through which they can respond and adapt to their environment. XERICO (XER) is a stress-responsive RING E3 ubiquitin ligase that modulates abscisic acid (ABA) levels and promotes drought tolerance when overexpressed. To better understand the biological role of XER in stress responses, we characterized a xer-1 hypomorphic mutant and a CRISPR/Cas9-induced xer-2 null mutant in Arabidopsis. Both xer mutant alleles exhibited increased drought sensitivity, supporting the results from overexpression studies. Furthermore, we discovered that both xer mutants have greater stomatal indices and that XER is expressed in epidermal cells, indicating that XER functions in the epidermis to repress stomatal development. To explore XER spatiotemporal and stress-dependent regulation, we conducted a yeast one-hybrid screen and found that CBF4/DREB1D associates with the XER 5' untranslated region (5'-UTR). We generated three cbf4 null mutants with CRISPR/Cas9 and showed that CBF4 negatively regulates ABA responses, promotes stomatal development and reduces drought tolerance, in contrast to the roles shown for XER. CBF4 is induced by ABA and osmotic stress, and localizes to the nucleus where it downregulates XER expression via the DRE element in its 5'-UTR. Lastly, genetic interaction studies confirmed that xer is epistatic to cbf4 in stomatal development and in ABA, osmotic and drought stress responses. We propose that the repression of XER by CBF4 functions to attenuate ABA signaling and stress responses to maintain a balance between plant growth and survival under adverse environmental conditions.

The soybean CBL-interacting protein kinase, GmCIPK2, positively regulates drought tolerance and ABA signaling

Calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK) play important roles in plant environmental stress responses. However, the biological functions of the CBL-CIPK signaling pathway in the tolerance of soybean (Glycine max) to drought stress remain elusive. Here, we characterized the GmCIPK2 gene in soybean, and its expression was induced by drought stress and exogenous abscisic acid (ABA) treatments. The overexpression of GmCIPK2 enhanced drought tolerance in transgenic Arabidopsis and soybean hairy roots, whereas downregulation of GmCIPK2 expression in soybean hairy roots by RNA interference resulted in increased drought sensitivity. Further analysis showed that GmCIPK2 was involved in ABA-mediated stomatal closure in plants under drought stress conditions. GmCIPK2 increased the expression of ABA- and drought-responsive genes during drought stress. Additionally, yeast two-hybrid, pull-down, and bimolecular fluorescence complementation assays demonstrated that a positive regulator of drought stress, GmCBL1, physically interacted with GmCIPK2 on the plasma membrane. Collectively, our results demonstrated that GmCIPK2 positively regulates drought tolerance and ABA signaling in plants, providing new insights into the underlying mechanisms of how the CBL-CIPK signaling pathway contributes to drought tolerance in soybean.

Tree mortality of European beech and Norway spruce induced by 2018-2019 hot droughts in central Germany

Anthropogenic climate change pushes forest ecosystems globally beyond their limits. Widespread events of forest die-off have been attributed to direct and indirect impacts of increasingly frequent and intense droughts. Here, we focus on an extensive mortality event in Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica (L.)) forests in Germany, following the successive 2018 – 2019 hot droughts. To examine whether this die-off indeed attributed to observed trends in drought occurrence and intensity, we sampled 143 beech and 186 spruce trees at three low-elevation sites (Spessart, Hassberge, Fichtelberg) with different edaphic properties in northern Bavaria. We analysed long-term hydroclimatic sensitivity and growth responses to extreme events of five site- and species-specific tree-ring width chronologies, including a reference site for each species. Growth of beech was sensitive to drought in April to June, whereas spruce growth was strongly related to drought during June to August, except at slightly higher elevations at the Fichtelberg site, where a summer temperature signal was observed. Trees at the Spessart and Hassberge sites showed an increased response to hydroclimatic conditions in April following the extreme drought in 1976 and from the 1990s onwards at the Fichtelberg site. Spruce bark beetle (Ips typographus L.) outbreaks during the 2018 drought accelerated the high mortality rates in around 50% of the trees at the Spessart and Hassberge site. In 2018, around 7% of all beech trees died at the Hassberge site, the site with the highest clay content. Our results suggest that these widespread mortality events can be attributed to an increasing drought sensitivity and were accelerated by the consecutive recent drought years. Sustainable forest management practices for these ecologically and economically important tree species are required to mitigate the effects of global warming in the future.

Tree growth is more limited by drought in rear-edge forests most of the times

BackgroundEquatorward, rear-edge tree populations are natural monitors to estimate species vulnerability to climate change. According to biogeographical theory, exposition to drought events increases with increasing aridity towards the equator and the growth of southern tree populations will be more vulnerable to drought than in central populations. However, the ecological and biogeographical margins can mismatch due to the impact of ecological factors (topography, soils) or tree-species acclimation that can blur large-scale geographical imprints in trees responses to drought making northern populations more drought limited.MethodsWe tested these ideas in six tree species, three angiosperms (Fagus sylvatica, Quercus robur, Quercus petraea) and three gymnosperms (Abies alba, Pinus sylvestris and Pinus uncinata) by comparing rear-edge tree populations subjected to different degrees of aridity. We used dendrochronology to compare the radial-growth patterns of these species in northern, intermediate, and southern tree populations at the continental rear edge.Results and conclusionsWe found marked variations in growth variability between species with coherent patterns of stronger drought signals in the tree-ring series of the southern populations of F. sylvatica, P. sylvestris, and A. alba. This was also observed in species from cool-wet sites (P. uncinata and Q. robur), despite their limited responsiveness to drought. However, in the case of Q. petraea the intermediate population showed the strongest relationship to drought. For drought-sensitive species as F. sylvatica and P. sylvestris, southern populations presented more variable growth which was enhanced by cool-wet conditions from late spring to summer. We found a trend of enhanced vulnerability to drought in these two species. The response of tree growth to drought has a marked biogeographical component characterized by increased drought sensitivity in southern populations even within the species distribution rear edge. Nevertheless, the relationship between tree growth and drought varied between species suggesting that biogeographical and ecological limits do not always overlap as in the case of Q. petraea. In widespread species showing enhanced vulnerability to drought, as F. sylvatica and P. sylvestris, increased vulnerability to climate warming in their rear edges is forecasted. Therefore, we encourage the monitoring and conservation of such marginal tree populations.

Recent CO2 rise has modified the sensitivity of tropical tree growth to rainfall and temperature.

Atmospheric CO2 (c a) rise changes the physiology and possibly growth of tropical trees, but these effects are likely modified by climate. Such c a × climate interactions importantly drive CO2 fertilization effects of tropical forests predicted by global vegetation models, but have not been tested empirically. Here we use tree‐ring analyses to quantify how c a rise has shifted the sensitivity of tree stem growth to annual fluctuations in rainfall and temperature. We hypothesized that c a rise reduces drought sensitivity and increases temperature sensitivity of growth, by reducing transpiration and increasing leaf temperature. These responses were expected for cooler sites. At warmer sites, c a rise may cause leaf temperatures to frequently exceed the optimum for photosynthesis, and thus induce increased drought sensitivity and stronger negative effects of temperature. We tested these hypotheses using measurements of 5,318 annual rings from 129 trees of the widely distributed (sub‐)tropical tree species, Toona ciliata. We studied growth responses during 1950–2014, a period during which c a rose by 28%. Tree‐ring data were obtained from two cooler (mean annual temperature: 20.5–20.7°C) and two warmer (23.5–24.8°C) sites. We tested c a × climate interactions, using mixed‐effect models of ring‐width measurements. Our statistical models revealed several significant and robust c a × climate interactions. At cooler sites (and seasons), c a × climate interactions showed good agreement with hypothesized growth responses of reduced drought sensitivity and increased temperature sensitivity. At warmer sites, drought sensitivity increased with increasing c a, as predicted, and hot years caused stronger growth reduction at high c a. Overall, c a rise has significantly modified sensitivity of Toona stem growth to climatic variation, but these changes depended on mean climate. Our study suggests that effects of c a rise on tropical tree growth may be more complex and less stimulatory than commonly assumed and require a better representation in global vegetation models.

Increased Drought Sensitivity Results in a Declining Tree Growth of Pinus latteri in Northeastern Thailand

Climate change may lead to alterations in tree growth and carbon cycling. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of tree species. However, the effects of climate change on pine forest dynamics in tropical region of Thailand remain poorly understood. This study develops three new tree ring-width chronologies of Pinus latteri (Tenasserim pine) in northern and northeastern Thailand and analyzes their climate-growth relationships and temporal stability. Ring-width chronologies of P. latteri at three sites showed significantly positive correlations with precipitation, relative humidity and self-calibrated Palmer Drought Severity Index (scPDSI) during the dry season (previous November to current April) and early rainy season (May–June). Conversely, significantly negative correlations were found between ring-width site chronologies and air temperatures (mean, maximum and minimum) from April to August. Therefore, our results revealed that radial growth of Tenasserim pines from northern and northeastern Thailand was mainly limited by moisture availability during the dry-to-wet transition season from April to June. Moving correlations revealed that Tenasserim pines in the lowland area of northeastern Thailand became more sensitive to moisture availability in recent 30 years (1985–2017) as compared with early period (1951–1984). Accompanying the shifted growth sensitivity to climate change, growth synchrony among trees was increasing and tree growth rates of Tenasserim pines have been declining during recent decades at two more moisture-limited sites in northeastern Thailand. Recent rapid warming and increasing drought during the transition season (April–June) together intensify climatic constrains on tree growth of Tenasserim pines in the lowland area of northeastern Thailand. Considering continued regional climate change, pine forests in tropical lowland areas may encounter intensified drought stresses, and thus, become more vulnerable to future climate change. Our results serve as an early indicator of potential effects of climate change on tropical pine species and raise concerns about sustainable managements of pine forests under a changing climate.

Effect of Deficit Irrigation and Planting Method on Maize Plants under Middle Delta Conditions of Egypt

Two field experiments were carried out at El-Gemmeiza Agricultural Research Station Farm, Egypt, during 2017 and 2018 summer seasons to study the effect of soil water deficit and the planting method on maize plant under Middle Delta conditions of Egypt. Irrigation at 65% available soil moisture depletion (AVSMD) did not significantly decrease chlorophyll content, dry matter (DM), leaf area index (LAI), crop growth rate (CGR), relative water content (RWC), osmotic potential (OP), ear length, ear diameter, rows per ear, kernels per row,100-kernel weight and grain yield. 50 % irrigation treatment decreased crop water productivity (CWP) by 9.93 % across the two seasons compared to irrigation at 80 % AVSMD. Irrigation at 80 % of AVSMD increased drought sensitivity (YR/ SD) and the drought susceptibility index (S) and CWP in both seasons. The reduction in WCU due to irrigation at 65 % of AVSMD reached 19.34 to 20.30 % and the improvement in CWP was 15.42 to 14.37 % compared to the irrigation at 50 % (control) in both seasons, respectively. Sowing maize plants on beds140 cm apart on both sides of beds and the distance between the hills was 25 cm (24000 plants per feddan) resulted in the highest chlorophyll content, DM, LAI, CGR, RWC, ear height, ear length, 100-kernel weight and grain yield in the two seasons, This treatment decreased YR/SD and S. Decreasing YR/SD and S. such decreases were accompanied with increasing CWP. It can be recommend that sowing maize plants on beds140 cm apart on both sides of beds, and the distance between hills 25 cm and irrigation at 65 % of AVSMD decreased WCU and improved CWP, in addition this treatment did not significantly decrease yield and its components and attained the lowest YR/S and S values.

Effects of drought and N level on the interactions of the root hemiparasite Rhinanthus alectorolophus with a combination of three host species.

Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species. We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design. Biomass of the hemiparasite and host community increased with N level and was reduced by drought to a similar degree. Larger plants in fertilised pots started to wilt earlier, and the presence of a hemiparasite further increased drought sensitivity. The hemiparasite strongly reduced biomass of the host community and overall productivity, and affected the competitive balance among host plants because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. The hemiparasite increased the root mass fraction of the hosts at all levels of N and water availability, indicating that the effect of the hemiparasite on the hosts was mainly due to loss of belowground resources. Our results indicate that hemiparasites will not always respond more strongly to increased N availability and drought than autotrophic plants, and that hemiparasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species the hemiparasites might alleviate the negative effects of nutrient enrichment on grassland diversity.

A Nck‐associated protein 1‐like protein affects drought sensitivity by its involvement in leaf epidermal development and stomatal closure in rice

SummaryWater deficit is a major environmental threat affecting crop yields worldwide. In this study, a drought stress‐sensitive mutant drought sensitive 8 (ds8) was identified in rice (Oryza sativa L.). The DS8 gene was cloned using a map‐based approach. Further analysis revealed that DS8 encoded a Nck‐associated protein 1 (NAP1)‐like protein, a component of the SCAR/WAVE complex, which played a vital role in actin filament nucleation activity. The mutant exhibited changes in leaf cuticle development. Functional analysis revealed that the mutation of DS8 increased stomatal density and impaired stomatal closure activity. The distorted actin filaments in the mutant led to a defect in abscisic acid (ABA)‐mediated stomatal closure and increased ABA accumulation. All these resulted in excessive water loss in ds8 leaves. Notably, antisense transgenic lines also exhibited increased drought sensitivity, along with impaired stomatal closure and elevated ABA levels. These findings suggest that DS8 affects drought sensitivity by influencing actin filament activity.

Fire deficits have increased drought sensitivity in dry conifer forests: Fire frequency and tree-ring carbon isotope evidence from Central Oregon.

A century of fire suppression across the Western United States has led to more crowded forests and increased competition for resources. Studies of forest thinning or stand conditions after mortality events have provided indirect evidence for how competition can promote drought stress and predispose forests to severe fire and/or bark beetle outbreaks. Here, we demonstrate linkages between fire deficits and increasing drought stress through analyses of annually resolved tree-ring growth, fire scars, and carbon isotope discrimination (Δ13 C) across a dry mixed-conifer forest landscape. Fire deficits across the study area have increased the sensitivity of leaf gas exchange to drought stress over the past >100years. Since 1910, stand basal area in these forests has more than doubled and fire-return intervals have increased from 25 to 140years. Meanwhile, the portion of interannual variation in tree-ring Δ13 C explained by the Palmer Drought Severity Index has more than doubled in ca. 300-500-year-old Pinus ponderosaas well as in fire-intolerant, ca. 90-190-year-old Abies grandis. Drought stress has increased in stands with a basal area of ≥25m2 /ha in 1910, as indicated by negative temporal Δ13 C trends, whereas stands with basal area ≤25m2 /ha in 1910, due to frequent or intense wildfire activity in decades beforehand, were initially buffered from increased drought stress and have benefited more from rising ambient carbon dioxide concentrations, [CO2 ], as demonstrated by positive temporal Δ13 C trends. Furthermore, the average Δ13 C response across all P.ponderosa since 1830 indicates that photosynthetic assimilation rates and stomatal conductance have been reduced by ~10% and ~20%, respectively, compared to expected trends due to increasing [CO2 ]. Although disturbance legacies contribute to local-scale intensity of drought stress, fire deficits have reduced drought resistance of mixed-conifer forests and made them more susceptible to challenges by pests and pathogens and other disturbances.

Upland rice breeding led to increased drought sensitivity in Brazil

The upland rice (UR) crop system located in the Brazilian savannas (states of Mato Grosso, Rondônia, Tocantins and Goiás) is the main upland rice growing area of Brazil and the largest rainfed rice growing area in Latin America. It plays an important social and economic role in central Brazil. UR cropped area, however, has decreased by 70% in the last two decades, partly due to high drought risk. Here, we hypothesize that the UR breeding program’s focus on direct grain selection for wide adaptation has led to an increase in yield potential but a reduction in the capacity of modern cultivars to maintain yield under drought stress. Using the ORYZAv3 rice crop model, we analyzed changes in relative and absolute drought impact for cultivars representative of three decades of breeding: 1980s (1986–1992), 1990s (1993–2002), and 2000s (2003–2013). We found a mean increase in relative drought impact of 12% (0.35% per year) between 1980s and 2000s. Most of the increase occurred during 1980s and 1990s (7%), where a major shift from landraces (i.e. Douradão) to modern cultivars (i.e. cv. BRS Primavera) occurred in the breeding program. Drought adaptation traits such as greater root length density, shorter cycle, and lower stomatal sensitivity to drought have been systematically bred out of the UR material in the period 1980–2013. We recommend the UR breeding program to be adjusted by better targeting to specific drought environments, and by taking into account drought stress profiles and environmental co-variables in statistical analyses for genotype selection. Moreover, physiological studies on tradeoffs between yield potential and drought tolerance should be performed for each target environment to dissect plant traits that confer both high yield potential and low drought sensitivity. These strategies will ultimately ensure that newly released cultivars are adapted to the environmental conditions of the study region, therefore reducing agro-climatic risk for UR farmers.