Effects of elevated CO₂, warming and precipitation change on plant growth, photosynthesis and peroxidation in dominant species from North China grassland

Impacts of climate change and human activity on the potential distribution of Aconitum leucostomum in China

Aconitum leucostomum is a poisonous grass that disturbs grassland populations and livestock development, and its spread is influenced by climate change and human activities. Therefore, exploring its potential distribution area under such conditions is crucial to maintain grassland ecological security and livestock development. The present study initially selected 39 variables that may influence the spatial distribution of A. leucostomum, including bioclimate, soil, topography, solar radiation, and human footprint data; the variables were screened by Spearman's correlation coefficient and the jackknife method. Twenty variables were finally identified, and three types of models based on the maximum entropy (MaxEnt) model were constructed to predict the distribution of A. leucostomum within China under three shared economy pathways (SSP126, SSP245, and SSP585): A: prediction of environmental variables under the current climate model; B: prediction of environmental variables + human footprint under the current climate model; and C: prediction of environmental variables under the future climate model (including the 2030s, 2050s, and 2070s). The effects of human activities and climate change on the potential geographic distribution of A. leucostomum were explored separately. The results show that precipitation seasonality, human footprint, solar radiation and mean diurnal range are the main factors affecting the distribution of A. leucostomum. Human activities inhibit the spread of A. leucostomum, and climate change promotes its growth, with areas of high suitability and area variation mainly in northern Xinjiang and northern Yunnan. With climate change, in the future, the distribution center of A. leucostomum shows a tendency to migrate to the southeast on the horizontal gradient and to move to higher altitudes on the vertical gradient. This study provides a positive reference value for the control of A. leucostomum and the maintenance of grassland ecological security.

Revisiting Changes in Growth, Physiology and Stress Responses of Plants under the Effect of Enhanced CO2 and Temperature

Climate change has universally affected the whole ecosystem in a unified manner and is known to have improbable effects on agricultural productivity and food security. Carbon dioxide (CO2) and temperature are the major environmental factors that have been shown to increase sharply during the last century and are directly responsible for affecting plant growth and development. A number of previous investigations have deliberated the positive effects of elevated CO2 on plant growth and development of various C3 crops, while detrimental effects of enhanced temperature on different crop plants like rice, wheat, maize and legumes are generally observed. A combined effect of elevated CO2 and temperature has yet to be studied in great detail; therefore, this review attempts to delineate the interactive effects of enhanced CO2 and temperature on plant growth, development, physiological and molecular responses. Elevated CO2 maintains leaf photosynthesis rate, respiration, transpiration and stomatal conductance in the presence of elevated temperature and sustains plant growth and productivity in the presence of both these environmental factors. Concomitantly, their interaction also affects the nutritional quality of seeds and leads to alterations in the composition of secondary metabolites. Elevated CO2 and temperature modulate phytohormone concentration in plants, and due to this fact, both environmental factors have substantial effects on abiotic and biotic stresses. Elevated CO2 and temperature have been shown to have mitigating effects on plants in the presence of other abiotic stress agents like drought and salinity, while no such pattern has been observed in the presence of biotic stress agents. This review focuses on the interactive effects of enhanced CO2 and temperature on different plants and is the first of its kind to deliver their combined responses in such detail.

Leaf trait responses to global change factors in terrestrial ecosystems

Global change influences plant growth by affecting plant morphology and physiology. However, the effects of global change factors vary based on the climate gradient. Here, we established a global database of leaf traits from 192 experiments on elevated CO2 concentrations (eCO2), drought, N deposition, and warming. The results showed that the leaf mass per area (LMA) significantly increased under eCO2 and drought conditions but decreased with N deposition, whereas eCO2 levels and drought conditions reduced stomatal conductance and increased and decreased photosynthetic rates, respectively. Leaf dark respiration (Rd) increased in response to global change, excluding N deposition. Leaf N concentrations declined with eCO2 but increased with N deposition. Leaf area increased with eCO2, N deposition, and warming but decreased with drought. Leaf thickness increased with eCO2 but decreased with warming. eCO2 and N deposition enhanced plant water-use efficiency (WUE), eCO2 and warming increased photosynthetic N-use efficiency (PNUE), while N fertilization reduced PNUE significantly. eCO2 produced a positive relationship between WUE and PNUE, which were limited under drought but increased in areas with high humidity and high temperature. Trade-offs were observed between WUE and PNUE under drought, N deposition, and warming. These findings suggest that the effects of global change factors on plants can be altered by complex environmental changes; moreover, diverse plant water and nutrient strategy responses can be interpreted against the background of their functional traits.

Mitigating greenhouse gas emissions by replacing inorganic fertilizer with organic fertilizer in wheat-maize rotation systems in China

Combining organic and inorganic fertilizer applications can help reduce inorganic fertilizer use and increase soil fertility. However, the most suitable proportion of organic fertilizer is unknown, and the effect of combining organic and inorganic fertilizers on greenhouse gas (GHG) emissions is inconclusive. This study aimed to identify the optimum ratio of inorganic fertilizer to organic fertilizer in a winter wheat-summer maize cropping system in northern China to achieve high grain yields and low GHG intensities. The study compared six fertilizer treatments: no fertilization (CK), conventional inorganic fertilization (NP), and constant total nitrogen input with 25% (25%OF), 50% (50%OF), 75% (75%OF), or 100% (100%OF) organic fertilizer. The results showed that the 75%OF treatment increased the winter wheat and summer maize yields the most, by 7.2-25.1% and 15.3-16.7%, respectively, compared to NP. The 75%OF and 100%OF treatments had the lowest nitrous oxide (N2O) emissions, 187.3% and 200.2% lower than the NP treatment, while all fertilizer treatments decreased methane (CH4) absorption (by 33.1-82.0%) compared to CK. Carbon dioxide flux increased in the summer maize growing season (by 7.7-30.5%) compared to CK but did not significantly differ between fertilizer treatments. The average global warming potential (GWP) rankings across two wheat-maize rotations were NP > 50%OF > 25%OF > 100%OF > 75%OF > CK, and greenhouse gas intensity (GHGI) rankings were NP > 25%OF > 50%OF > 100%OF > 75%OF > CK. We recommend using 75% organic fertilizer/25% inorganic fertilizer to reduce GHG emissions and ensure high crop yields in wheat-maize rotation systems in northern China.

Adapting crop production to climate change and air pollution at different scales

Air pollution and climate change are tightly interconnected and jointly affect field crop production and agroecosystem health. Although our understanding of the individual and combined impacts of air pollution and climate change factors is improving, the adaptation of crop production to concurrent air pollution and climate change remains challenging to resolve. Here we evaluate recent advances in the adaptation of crop production to climate change and air pollution at the plant, field and ecosystem scales. The main approaches at the plant level include the integration of genetic variation, molecular breeding and phenotyping. Field-level techniques include optimizing cultivation practices, promoting mixed cropping and diversification, and applying technologies such as antiozonants, nanotechnology and robot-assisted farming. Plant- and field-level techniques would be further facilitated by enhancing soil resilience, incorporating precision agriculture and modifying the hydrology and microclimate of agricultural landscapes at the ecosystem level. Strategies and opportunities for crop production under climate change and air pollution are discussed.

Soil Moisture Content Dominates the Photosynthesis of C3 and C4 Plants in a Desert Steppe after Long-Term Warming and Increasing Precipitation

Plant photosynthesis has a non-negligible influence on forage quality and ecosystem carbon sequestration. However, the influence of long-term warming, increasing precipitation, and their interactions on the photosynthesis of dominant species in desert steppe remains unclear, and the main factors regulating plant photosynthesis in desert steppes have remained unrevealed. Therefore, we measured the photosynthetic parameters and specific leaf area of the dominant species and calculated the water and nitrogen content of leaves and soil in a desert steppe after long-term warming and increasing precipitation (air temperature, W0, air temperature increases of 2 °C and 4 °C, W1 and W2; natural precipitation, P0, natural precipitation increases of 25% and 50%, P1 and P2). Results showed that warming and increasing precipitation significantly enhanced photosynthesis in C3 and C4 species (p < 0.05). Compared to W0P0, the net photosynthetic rate of C3 and C4 species in W2P2 increased by 159.46% and 178.88%, respectively. Redundancy analysis showed that soil water content significantly explained the photosynthesis of C3 and C4 plants (the degree of explanation was 48% and 67.7%), followed by soil-available nitrogen content (the degree of explanation was 19.6% and 5.3%). Therefore, our study found that climate change enhanced photosynthesis in C3 and C4 plants, and soil water content plays a critical role in regulating photosynthesis in desert steppes.

Cultivation model and deficit irrigation strategy for reducing leakage of bundle sheath cells to CO2, improve 13C carbon isotope, photosynthesis and soybean yield in semi-arid areas

A better understanding of the photosynthesis and soil water storage regulation of soybean production will be helpful to develop a water conservation strategy under a rain-fed farming system. Reducing the leakage of CO2 bundle sheath cells and improving the photosynthesis capacity and gas exchange characteristics of soybean leaves will contribute to increase yield under the dryland agricultural system and provide a scientific basis. Therefore, during 2019 and 2020, soybean exposed to different cultivation modes to analyze the response curves of photosynthesis and CO2 under different deficit irrigation strategies. In this study, we used two cultivation models: RB: ridge covered with biodegradable film and furrow area not covered; CF: conventional flat land planting under four deficit irrigation modes (R: rainwater irrigation; IB: branch stage irrigation (220 mm); IP: Irrigation during podding (220 mm); IBP: branch stage irrigation (110 mm), podding stage irrigation (110 mm). Compared with CF-IBP treatment, RB-IBP had significant effects on rainwater collection, SWS, and soybean yield. Photo-response curve analysis showed that RB-IBP treatment a significant increase in Pn, Gs, Ci, Tr, leaf WUE, and chlorophyll ab content. Under different irrigation strategies, maximum net photosynthetic rate (Pnmax), light saturation point (LSP), and apparent quantum efficiency under RB-IBP treatment (α), Pn under respiration rate and CO2 response curve were significantly higher than that under CF cultivation mode. Compared with RB culture mode under different irrigation strategies, CF cultivation mode significantly increases Δ13C and CO2 sheath cell leakage (Փ); it also led to a significant decline in the ratio of Ci/Ca concentration. This study shows that RB-IBP treatment is the best water-saving strategy because it means reducing the leakage of CO2 from the bundle sheath, thus significantly increasing soil water storage, photosynthetic capacity, and soybean yield.

Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2

The cause of reduced leaf-level transpiration under elevated CO2 remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22-40 months under elevated (eCO2; c. 860 ppm) or ambient (aCO2; c. 410 ppm) CO2. We assessed if eCO2-triggered reductions in canopy conductance (gc) alter the response to soil or atmospheric drought and are reversible or lasting due to anatomical adjustments by exposing eCO2 seedlings to decreasing [CO2]. To quantify underlying mechanisms, we analyzed leaf abscisic acid (ABA) level, stomatal and leaf morphology, xylem structure, hydraulic efficiency, and hydraulic safety. Effects of eCO2 manifested in a strong reduction in leaf-level gc (-55%) not caused by ABA and not reversible under low CO2 (c. 200 ppm). Stomatal development and size were unchanged, while stomatal density increased (+18%). An increased vein-to-epidermis distance (+65%) suggested a larger leaf resistance to water flow. This was supported by anatomical adjustments of branch xylem having smaller conduits (-8%) and lower conduit lumen fraction (-11%), which resulted in a lower specific conductivity (-19%) and leaf-specific conductivity (-34%). These adaptations to CO2 did not change stomatal sensitivity to soil or atmospheric drought, consistent with similar xylem safety thresholds. In summary, we found reductions of gc under elevated CO2 to be reflected in anatomical adjustments and decreases in hydraulic conductivity. As these water savings were largely annulled by increases in leaf biomass, we do not expect alleviation of drought stress in a high CO2 atmosphere.

Short-term elevated temperature and CO2 promote photosynthetic induction in the C3 plant Glycine max, but not in the C4 plant Amaranthus tricolor

The continuous increases of atmospheric temperature and CO2 concentration will impact global photosynthesis. However, there are few studies considering the interaction of elevated temperature (eT) and elevated CO2 (eCO2 ) on dynamic photosynthesis, particularly for C4 species. We examine dynamic photosynthesis under four different temperature and [CO2 ] treatments: (1) 400ppm×28°C (CT); (2) 400ppm×33°C (CT+); (3) 800ppm×28°C (C+T); and (4) 800ppm×33°C (C+T+). In Glycine max L., the time required to reach 50% (T 50%A ) and 90% (T 90%A ) of full photosynthetic induction was smaller under the CT+, C+T, and C+T+ treatments than those under the CT treatment. In Amaranthus tricolor L., however, neither T 50%A nor T 90%A was not significantly affected by eT or eCO2 . In comparison with the CT treatment, the achieved carbon gain was increased by 58.3% (CT+), 112% (C+T), and 136.6% (C+T+) in G. max and was increased by 17.1% (CT+), 2.6% (C+T) and 56.9% (C+T+) in A. tricolor . The increases of achieved carbon gain in G. max were attributable to both improved photosynthetic induction efficiency (IE) and enhanced steady-state photosynthesis, whereas those in A. tricolor were attributable to enhanced steady-state photosynthesis.

Coordination of leaf functional traits under climatic warming in an arid ecosystem

BACKGROUND

Climatic warming is increasing regionally and globally, and results concerning warming and its consequent drought impacts have been reported extensively. However, due to a lack of quantitative analysis of warming severities, it is still unclear how warming and warming-induced drought influence leaf functional traits, particularly how the traits coordinate with each other to cope with climatic change. To address these uncertainties, we performed a field experiment with ambient, moderate and severe warming regimes in an arid ecosystem over 4 years.

RESULTS

Severe warming significantly reduced the specific leaf area and net photosynthetic rate with a relatively stable change and even enhancement under moderate warming, especially showing species-specific performance. The current results largely indicate that a coordinated trade-off can exist between plant functional traits in plant communities in a dryland ecosystem under ambient temperature conditions, which is strongly amplified by moderate warming but diminished or even eliminated by severe warming. Based on the present findings and recent results in the relevant literature, we advance the ecological conceptual models (e.g., LES and CSR) in the response to climatic warming in arid grassland communities, where the few key species play a crucial role by balancing their functional performances to cope with environmental change.

CONCLUSION

Our results highlight the importance of coordination and/or trade-off between leaf functional traits for understanding patterns of climatic change-induced vegetation degradation and suggest that the plant community composition in these drylands could be shifted under future climate change.

Proline, a multifaceted signalling molecule in plant responses to abiotic stress: understanding the physiological mechanisms

Abiotic stresses have a detrimental impact on plant growth and productivity and are a major threat to sustainable crop production in rapidly changing environments. Proline, an important amino acid, plays an important role in maintaining the metabolism and growth of plants under abiotic stress conditions. Many insights indicate a positive relationship between proline accumulation and tolerance of plants to various abiotic stresses. Because of its metal chelator properties, it acts as a molecular chaperone, an antioxidative defence molecule that scavenges reactive oxygen species (ROS), as well as having signalling behaviour to activate specific gene functions that are crucial for plant recovery from stresses. It also acts as an osmoprotectant, a potential source to acquire nitrogen as well as carbon, and plays a significant role in the flowering and development of plants. Overproduction of proline in plant cells contributes to maintaining cellular homeostasis, water uptake, osmotic adjustment and redox balance to restore the cell structures and mitigate oxidative damage. Many reports reveal that transgenic plants, particularly those overexpressing genes tailored for proline accumulation, exhibit better adaptation to abiotic stresses. Therefore, this review aims to provide a comprehensive update on proline biosynthesis and accumulation in plants and its putative regulatory roles in mediating plant defence against abiotic stresses. Additionally, the current and future directions in research concerning manipulation of proline to induce gene functions that appear promising in genetics and genomics approaches to improve plant adaptive responses under changing climate conditions are also highlighted.

Leaf Physiological Responses of Three Psammophytes to Combined Effects of Warming and Precipitation Reduction in Horqin Sandy Land, Northeast China

The decreasing precipitation with global climate warming is the main climatic condition in some sandy grassland ecosystems. The understanding of physiological responses of psammophytes in relation to warming and precipitation is a possible way to estimate the response of plant community stability to climate change. We selected Lespedeza davurica, Artemisia scoparia, and Cleistogenes squarrosa in sandy grassland to examine the effect of a combination of climate warming and decreasing precipitation on relative water content (RWC), chlorophyll, proline, and antioxidant enzyme activities. We found that all experimental treatments have influenced RWC, chlorophyll, proline, and antioxidant enzyme activities of three psammophytes. L. davurica has the highest leaf RWC among the three psammophytes. With the intensification of precipitation reduction, the decreasing amplitude of chlorophyll from three psammophytes was L. davurica > C. squarrosa > A. scoparia. At the natural temperature, the malondialdehyde (MDA) content of the three psammophytes under severe drought treatment was much higher than other treatments, and their increasing degree was as follows: A. scoparia > C. squarrosa > L. davurica. At the same precipitation gradient, the proline of three psammophytes under warming was higher than the natural temperature. The differences in superoxide dismutase (SOD) among the three psammophytes were A. scoparia > L. davurica > C. squarrosa. Moreover, at natural temperature, more than 40% of precipitation reduction was most significant. Regardless of warming or not, the catalase (CAT) activity of A. scoparia under reduced precipitation treatments was higher than natural temperature, while the response of L. davurica was opposite. Correlation analyses evidenced that warming (T) was significant in L. davurica and precipitation (W) was significant in A. scoparia and C. squarrosa according to the Monte-Carlo permutation test (p = 0.002, 0.004, and 0.004). The study is important in predicting how local plants will respond to future climate change and assessing the possible effects of climate change on sandy grassland ecosystems.

Bioinoculants-Natural Biological Resources for Sustainable Plant Production

Agricultural sustainability is of foremost importance for maintaining high food production. Irresponsible resource use not only negatively affects agroecology, but also reduces the economic profitability of the production system. Among different resources, soil is one of the most vital resources of agriculture. Soil fertility is the key to achieve high crop productivity. Maintaining soil fertility and soil health requires conscious management effort to avoid excessive nutrient loss, sustain organic carbon content, and minimize soil contamination. Though the use of chemical fertilizers have successfully improved crop production, its integration with organic manures and other bioinoculants helps in improving nutrient use efficiency, improves soil health and to some extent ameliorates some of the constraints associated with excessive fertilizer application. In addition to nutrient supplementation, bioinoculants have other beneficial effects such as plant growth-promoting activity, nutrient mobilization and solubilization, soil decontamination and/or detoxification, etc. During the present time, high energy based chemical inputs also caused havoc to agriculture because of the ill effects of global warming and climate change. Under the consequences of climate change, the use of bioinputs may be considered as a suitable mitigation option. Bioinoculants, as a concept, is not something new to agricultural science, however; it is one of the areas where consistent innovations have been made. Understanding the role of bioinoculants, the scope of their use, and analysing their performance in various environments are key to the successful adaptation of this technology in agriculture.

Long-Term Warming and Nitrogen Addition Have Contrasting Effects on Ecosystem Carbon Exchange in a Desert Steppe

Desert steppe, a unique ecotone between steppe and desert in Eurasia, is considered highly vulnerable to global change. However, the long-term impact of warming and nitrogen deposition on plant biomass production and ecosystem carbon exchange in a desert steppe remains unknown. A 12-year field experiment was conducted in a Stipa breviflora desert steppe in northern China. A split-design was used, with warming simulated by infrared radiators as the primary factor and N addition as the secondary factor. Our long-term experiment shows that warming did not change net ecosystem exchange (NEE) or total aboveground biomass (TAB) due to contrasting effects on C₄ (23.4% increase) and C₃ (11.4% decrease) plant biomass. However, nitrogen addition increased TAB by 9.3% and NEE by 26.0% by increasing soil available N content. Thus, the studied desert steppe did not switch from a carbon sink to a carbon source in response to global change and positively responded to nitrogen deposition. Our study indicates that the desert steppe may be resilient to long-term warming by regulating plant species with contrasting photosynthetic types and that nitrogen deposition could increase plant growth and carbon sequestration, providing negative feedback on climate change.

Photosynthetic resistance and resilience under drought, flooding and rewatering in maize plants

Abnormally altered precipitation patterns induced by climate change have profound global effects on crop production. However, the plant functional responses to various precipitation regimes remain unclear. Here, greenhouse and field experiments were conducted to determine how maize plant functional traits respond to drought, flooding and rewatering. Drought and flooding hampered photosynthetic capacity, particularly when severe and/or prolonged. Most photosynthetic traits recovered after rewatering, with few compensatory responses. Rewatering often elicited high photosynthetic resilience in plants exposed to severe drought at the end of plant development, with the response strongly depending on the drought severity/duration. The associations of chlorophyll concentrations with photosynthetically functional activities were stronger during post-tasseling than pre-tasseling, implying an involvement of leaf age/senescence in responses to episodic drought and subsequent rewatering. Coordinated changes in chlorophyll content, gas exchange, fluorescence parameters (PSII quantum efficiency and photochemical/non-photochemical radiative energy dissipation) possibly contributed to the enhanced drought resistance and resilience and suggested a possible regulative trade-off. These findings provide fundamental insights into how plants regulate their functional traits to deal with sporadic alterations in precipitation. Breeding and management of plants with high resistance and resilience traits could help crop production under future climate change.

The effects of elevated CO2, elevated O3, elevated temperature, and drought on plant leaf gas exchanges: a global meta-analysis of experimental studies

Global change significantly influences plant leaf gas exchange, which affects the carbon-water cycle of terrestrial ecosystems. However, the magnitudes of the effects of multiple global change factors on leaf gas exchanges are currently lacking. Therefore, a global meta-analysis of 337 published articles was conducted to determine the effects of elevated CO₂ (eCO₂), elevated O₃ (eO₃), elevated temperature (eT), and drought on plant leaf gas exchanges. The results indicated that (1) the overall responses of photosynthesis rate (P_n) and instantaneous water use efficiency (WUE_i) to eCO₂ increased by 28.6% and 58.6%. But transpiration rate (T_r) and stomatal conductance (g_s) responded negatively to eCO₂ (- 17.5% and - 17.2%, respectively). Furthermore, all P_n, g_s, and WUE_i responded negatively to eO₃ (- 32.7%, - 24.6%, and - 27.1%), eT (- 23.2%, - 10.8%, and - 28.9%), and drought (- 53.6%, - 59.3%, and - 4.6%, respectively), regardless of functional groups and various complex experimental conditions. (2) Elevated CO₂ increased WUE_i combined with eO₃, eT, and drought (26.6%, 36.0%, and 58.6%, respectively, for eCO₂ + eO₃, eCO₂ + eT, and eCO₂ + drought) and mitigated their negative impacts on P_n to some extent. (3) Plant form and foliage type play an important role in the responses of leaf gas exchanges. Trees responded mostly to eCO₂, but responded least to eT in P_n, T_r, g_s, and WUE_i compared with shrubs and herbs. Evergreen broad-leaved species were more responsive to eCO₂ and drought. (4) The stress level of each factor can also significantly influence the responses of leaf gas exchanges to environment change. Hopefully, the quantitative results are helpful for the further assessments of the terrestrial carbon-water cycle.

Transcriptomic Leaf Profiling Reveals Differential Responses of the Two Most Traded Coffee Species to Elevated [CO2]

As atmospheric [CO₂] continues to rise to unprecedented levels, understanding its impact on plants is imperative to improve crop performance and sustainability under future climate conditions. In this context, transcriptional changes promoted by elevated CO₂ (eCO₂) were studied in genotypes from the two major traded coffee species: the allopolyploid Coffea arabica (Icatu) and its diploid parent, C. canephora (CL153). While Icatu expressed more genes than CL153, a higher number of differentially expressed genes were found in CL153 as a response to eCO₂. Although many genes were found to be commonly expressed by the two genotypes under eCO₂, unique genes and pathways differed between them, with CL153 showing more enriched GO terms and metabolic pathways than Icatu. Divergent functional categories and significantly enriched pathways were found in these genotypes, which altogether supports contrasting responses to eCO₂. A considerable number of genes linked to coffee physiological and biochemical responses were found to be affected by eCO₂ with the significant upregulation of photosynthetic, antioxidant, and lipidic genes. This supports the absence of photosynthesis down-regulation and, therefore, the maintenance of increased photosynthetic potential promoted by eCO₂ in these coffee genotypes.

Retrospective analysis of biochemical limitations to photosynthesis in 49 species: C4 crops appear still adapted to pre-industrial atmospheric [CO2 ]

Leaf CO₂ uptake (A) in C₄ photosynthesis is limited by the maximum apparent rate of PEPc carboxylation (V_pmax ) at low intercellular [CO₂ ] (c_i ) with a sharp transition to a c_i -saturated rate (V_max ) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to c_i has been widely used to determine these two parameters. V_max and V_pmax depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when V_max and V_pmax are co-limiting. We collected published A/c_i curves in 49 C₄ species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO₂ ]. The balance of V_max -V_pmax varied among evolutionary lineages and C₄ subtypes. Operating A was strongly V_max -limited, such that re-allocation of resources from V_pmax towards V_max was predicted to improve A by 12% in C₄ crops. This would not require additional inputs but rather altered partitioning of existing leaf nutrients, resulting in increased water and nutrient-use efficiency. Optimal partitioning was achieved only in plants grown at pre-industrial atmospheric [CO₂ ], suggesting C₄ crops have not adjusted to the rapid increase in atmospheric [CO₂ ] of the past few decades.

Responses of plant biomass and yield component in rice, wheat, and maize to climatic warming: a meta-analysis

Responses of plant biomass and yield components to warming are species-specific and are shifted as increased warming magnitude rises; this finding improves the results of IPCC AR5. The responses of crop yields to climatic warming have been extensively reported from experimental results, historical yield collections, and modeling research. However, an integrative report on the responses of plant biomass and yield components of three major crops to experimental warming is lacking. Here, a meta-analysis based on the most recent warming experiments was conducted to quantify the climatic warming responses of the biomass, grain yield (GY), and yield components of three staple crops. The results showed that the wheat total aboveground biomass (TAGB) increased by 6.0% with general warming, while the wheat GY did not significantly respond to warming; however, the responses shifted with increases in the mean growing season temperature (MGST). Negative effects on wheat TAGB and GY appeared when the MGSTs were above 15 °C and 13 °C, respectively. The wheat GY and the number of grains per panicle decreased by 8.4% and 7.5%, respectively, per degree Celsius increase. Increases in temperature significantly reduced the rice TAGB and GY by 4.3% and 16.6%, respectively, but rice straw biomass increased with increasing temperature. However, the rice grain weight and the number of panicles decreased with continuous increasing temperature (ΔT_a). The maize biomass, GY, and yield components all generally decreased with climatic warming. Finally, the crop responses to climatic warming were significantly influenced by warming time, warming treatment facility, and methods. Our findings can improve the assessment of crop responses to climatic warming and are useful for ensuring food security while combating future global climate change.

Proteomic changes may lead to yield alteration in maize under carbon dioxide enriched condition

In the present study, the effect of elevated CO2 on growth, physiology, yield and proteome was studied on two maize (Zea mays L.) varieties grown under Free-air CO2 enrichment. Growth in high CO2 (530 ppm) did not affect either photosynthesis or pigment contents in both varieties. Reduced MDA content, antioxidant and antioxidative enzymes levels were observed in both varieties in response to high CO2. PEHM-5 accumulated more biomass than SMH-3031 under eCO2. PEHM-5 also had more seed starch and total soluble sugar than SMH-3031. However, SMH-3031 had increased number of seed per cob than PEHM-5. Interestingly, thousand seed weight was significantly increased in PEHM-5 only, while it was decreased in SMH-3031 under eCO2. We observed increased seed size in PEHM-5, while the size of the SMH-3031 seeds remained unaltered. Leaf proteomics revealed more abundance of proteins related to Calvin cycle, protein synthesis assembly and degradation, defense and redox homeostasis in PEHM-5 that contributed to better growth and yield in elevated CO2. While in SMH-3031 leaf, proteins related to Calvin cycle, defense and redox homeostasis were less abundant in elevated CO2 resulting in average growth and yield. The results showed a differential response of two maize varieties to eCO2.

Effects of Warming and N Deposition on the Physiological Performances of Leymus secalinus in Alpine Meadow of Qinghai-Tibetan Plateau

Warming and Nitrogen (N) deposition are key global changes that may affect eco-physiological process of territorial plants. In this paper, we examined the effects of warming, N deposition, and their combination effect on the physiological performances of Leymus secalinus. Four treatments were established in an alpine meadow of Qinghai-Tibetan plateau: control (CK), warming (W), N deposition (N), and warming plus N deposition (NW). Warming significantly decreased the photosynthetic rate (A_net ), stomatal conductance (g_s ), intercellular CO₂ concentration (C_i ), and transpiration rate (T_r ), while N deposition and warming plus N deposition significantly increased those parameters of L. secalinus. Warming significantly increased the VPD and L_s , while N deposition and warming plus N deposition had a significant positive effect. Warming negatively reduced the leaf N content, Chla, Chlb, and total Chl content, while N deposition significantly promoted these traits. Warming, N deposition, and their combination significantly increased the activity of SOD, POD, and CAT. Besides, warming and warming plus N deposition significantly increased the MDA content, while N deposition significantly decreased the MDA content. N deposition and warming plus N deposition significantly increased the Rubisco activity, while warming showed no significant effect on Rubisco activity. N deposition and warming plus N deposition significantly increased the Fv/Fm, ΦPSII, qP, and decreased NPQ, while warming significantly decreased the Fv/Fm, ΦPSII, qP, and increased NPQ. N deposition strengthened the relations between g_s , Chl, Chla, Chlb, Rubisco activity, and A_net . Under warming, only g_s showed a significantly positive relation with A_net . Our findings suggested that warming could impair the photosynthetic potential of L. secalinus enhanced by N deposition. Additionally, the combination of warming and N deposition still tend to lead positive effects on L. secalinus.

Comparing the impacts of climate change on the responses and linkages between terrestrial and aquatic ecosystems

Aquatic and terrestrial organisms are being exposed to a number of anthropogenically-induced environmental stresses as a consequence of climate change. In addition, climate change is altering various linkages that exist between ecosystems on land and in water. Here we compare and contrast how climate change is altering aquatic and terrestrial environments and address some of the ways that the organisms in these ecosystems, especially the primary producers, are being affected by climate change factors, including changes in temperature, moisture, atmospheric carbon dioxide and solar UV radiation. Whereas there are some responses to climate change in common between terrestrial and aquatic ecosystems (e.g., changes in species composition and shifting geographic ranges and distributions), there are also responses that fundamentally differ between these two (e.g., responses to UV radiation). Climate change is also disrupting land-water connections in ways that influence biogeochemical and hydrologic cycles, and biosphere-atmosphere interactions in ways that can modify how aquatic and terrestrial ecosystems are affected by climate change and can influence climate change. The effects of climate change on these ecosystems are having wide-ranging effects on ecosystem biodiversity, structure and function and the abilities of these systems to provide essential services.

Growth and photosynthetic responses in Brassica napus differ during stress and recovery periods when exposed to combined heat, drought and elevated CO2

This study was intended to investigate how an agronomically important crop Brassica napus will be able to cope with the combined impact of a heatwave (21/14 °C vs. 33/26 °C day/night) and drought under ambient or elevated CO₂ (800 vs. 400 μmol mol^-1) and to what degree their recovery will be ensured after the stress, when additional CO₂ is also removed. The obtained results revealed that, in the presence of an adequate water supply, B. napus performed well under heatwave conditions. However, drought fully negated all the advantages gained from hotter climate and led to a slower and incomplete recovery of gas exchange and retarded growth after the stress, regardless mitigating the effect of elevated CO₂ during the stress. The mechanism by which the elevated CO₂ diminished the adverse effect of a combined heat and drought stress on photosynthetic rate at saturating light (A_sat) was attributed to the improved plant water relations. However, it had little effect on the recovery of A_sat. In contrast, the mechanism by which photosynthesis was more impaired under the combination of heatwave and drought, compared to single drought treatment, was attributed mainly to the faster soil drying as well as faster and sharper decrease in stomatal conductance and subsequent in C_i/C_a. Keeping in mind that photosynthesis can acclimatize by downregulation to higher CO₂, the results of this study, showing a weak memory of mitigating the effect of elevated CO₂, highlight a potential risk of more intense and frequent heatwaves and droughts on B. napus.

Climate, Life Form and Family Jointly Control Variation of Leaf Traits

Variation in leaf traits may represent differences in physiological processes and environmental adaptative strategies. Using multivariate analyses, we investigated 13 leaf traits to quantify the trade-off in these traits and the trait-climate/biome relationships based on the China Plant Trait Database, which contains morphometric and physiological character information on 1215 species for 122 sites, ranging from the north to the tropics, and from deserts and grasslands to woodlands and forests. Leaf traits across the dataset of Chinese plants showed different spatial patterns along longitudinal and latitudinal gradients and high variation. There were significant positive or negative correlations among traits; however, with the exception of the leaf 13C:12C stable isotope ratio, there were no significant correlations between leaf area and other traits. Climate, life form, and family jointly accounted for 68.4% to 95.7% of trait variance. Amongst these forms of variation partitioning, the most important partitioning feature was the family independence of climate and life form (35.6% to 57.2%), while the joint effect of family and climate was 4.5% to 26.2%, and the joint effect of family and life form was 2.4% to 21.6%. The findings of this study will enhance our understanding of the variation in leaf traits in Chinese flora and the environmental adaptative strategies of plants against a background of global climate change, and also may enrich and improve the leaf economics spectrum of China.

Contrasting responses of steppe Stipa ssp. to warming and precipitation variability

Climate change, characterized by warming and precipitation variability, restricted the growth of plants in arid and semiarid areas, and various functional traits are impacted differently. Comparing responses of functional traits to warming and precipitation variability and determining critical water threshold of dominate steppe grasses from Inner Mongolia facilitates the identification and monitoring of water stress effects. A combination of warming (ambient temperature, +1.5°C and +2.0°C) and varying precipitation (-30%, -15%, ambient, +15%, and +30%) manipulation experiments were performed on four Stipa species (S. baicalensis, S. bungeana, S. grandis, and S. breviflora) from Inner Mongolia steppe. The results showed that the functional traits of the four grasses differed in their responses to precipitation, but they shared common sensitive traits (root/shoot ratio, R/S, and specific leaf area; SLA) under ambient temperature condition. Warming increased the response of the four grasses to changing precipitation, and these differences in functional traits resulted in changes to their total biomass, with leaf area, SLA, and R/S making the largest contributions. Critical water thresholds of the four grasses were identified, and warming led to their higher optimum precipitation requirements. The four steppe grasses were able to adapt better to mild drought (summer precipitation decreased by 12%-28%) when warming 1.5°C rather than 2.0°C. These results indicated that if the Paris Agreement to limit global warming to 1.5°C will be accomplished, this will increase the probability for sustained viability of the Stipa steppes in the next 50-100 years.

Stomatal movements are involved in elevated CO2 -mitigated high temperature stress in tomato

Climate changes such as heat waves often affect plant growth and pose a growing threat to natural and agricultural ecosystems. Elevated atmospheric CO₂ can mitigate the negative effects of heat stress, but the underlying mechanisms remain largely unclear. We examined the interactive effects of elevated CO₂ (eCO₂ ) and temperature on the generation of the hydrogen peroxide (H₂ O₂ ) and stomatal movement characteristics associated with heat tolerance in tomato seedlings grown under two CO₂ concentrations (400 and 800 µmol mol^-1 ) and two temperatures (25 and 42°C). eCO₂ ameliorated the negative effects of heat stress, which was accompanied by greater amounts of RESPIRATORY BURST OXIDASE 1 (RBOH1) transcripts, apoplastic H₂ O₂ accumulation and decreased stomatal aperture. Silencing RBOH1 and SLOW-TYPE ANION CHANNEL, impeded eCO₂ -induced stomatal closure and compromised the eCO₂ -enhanced water use efficiency as well as the heat tolerance. Our findings suggest that RBOH1-dependent H₂ O₂ accumulation was involved in the eCO₂ -induced stomatal closure, which participate in maintaining balance between water retention and heat loss under eCO₂ concentrations. This phenomenon may be a contributor to eCO₂ -induced heat tolerance in tomato, which will be critical for understanding how plants respond to both future climate extremes and changes in CO₂ .

Plant carbon metabolism and climate change: elevated CO2 and temperature impacts on photosynthesis, photorespiration and respiration

Contents Summary 32 I. The importance of plant carbon metabolism for climate change 32 II. Rising atmospheric CO2 and carbon metabolism 33 III. Rising temperatures and carbon metabolism 37 IV. Thermal acclimation responses of carbon metabolic processes can be best understood when studied together 38 V. Will elevated CO2 offset warming-induced changes in carbon metabolism? 40 VI. No plant is an island: water and nutrient limitations define plant responses to climate drivers 41 VII. Conclusions 42 Acknowledgements 42 References 42 Appendix A1 48 SUMMARY: Plant carbon metabolism is impacted by rising CO₂ concentrations and temperatures, but also feeds back onto the climate system to help determine the trajectory of future climate change. Here we review how photosynthesis, photorespiration and respiration are affected by increasing atmospheric CO₂ concentrations and climate warming, both separately and in combination. We also compile data from the literature on plants grown at multiple temperatures, focusing on net CO₂ assimilation rates and leaf dark respiration rates measured at the growth temperature (A_growth and R_growth , respectively). Our analyses show that the ratio of A_growth to R_growth is generally homeostatic across a wide range of species and growth temperatures, and that species that have reduced A_growth at higher growth temperatures also tend to have reduced R_growth , while species that show stimulations in A_growth under warming tend to have higher R_growth in the hotter environment. These results highlight the need to study these physiological processes together to better predict how vegetation carbon metabolism will respond to climate change.

Negative effects of climate change on upland grassland productivity and carbon fluxes are not attenuated by nitrogen status

Effects of climate change on managed grassland carbon (C) fluxes and biomass production are not well understood. In this study, we investigated the individual and interactive effects of experimental warming (+3 °C above ambient summer daily range of 9-12 °C), supplemental precipitation (333 mm +15%) and drought (333 mm -23%) on plant biomass, microbial biomass C (MBC), net ecosystem exchange (NEE) and dissolved organic C (DOC) flux in soil cores from two upland grasslands of different soil nitrogen (N) status (0.54% and 0.37%) in the UK. After one month of acclimation to ambient summer temperature and precipitation, five replicate cores of each treatment were subjected to three months of experimental warming, drought and supplemental precipitation, based on the projected regional summer climate by the end of the 21st Century, in a fully factorial design. NEE and DOC flux were measured throughout the experimental duration, alongside other environmental variables including soil temperature and moisture. Plant biomass and MBC were determined at the end of the experiment. Results showed that warming plus drought resulted in a significant decline in belowground plant biomass (-29 to -37%), aboveground plant biomass (-35 to -77%) and NEE (-13 to -29%), regardless of the N status of the soil. Supplemental precipitation could not reverse the negative effects of warming on the net ecosystem C uptake and plant biomass production. This was attributed to physiological stress imposed by warming which suggests that future summer climate will reduce the C sink capacity of the grasslands. Due to the low moisture retention observed in this study, and to verify our findings, it is recommended that future experiments aimed at measuring soil C dynamics under climate change should be carried out under field conditions. Longer term experiments are recommended to account for seasonal and annual variability, and adaptive changes in biota.

Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment

Theory predicts and evidence shows that plant species that use the C₄ photosynthetic pathway (C₄ species) are less responsive to elevated carbon dioxide (eCO₂) than species that use only the C₃ pathway (C₃ species). We document a reversal from this expected C₃-C₄ contrast. Over the first 12 years of a 20-year free-air CO₂ enrichment experiment with 88 C₃ or C₄ grassland plots, we found that biomass was markedly enhanced at eCO₂ relative to ambient CO₂ in C₃ but not C₄ plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO₂ relative to ambient CO₂ in C₄ but not C₃ plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO₂ first enhanced but later depressed rates in C₃ plots, with the opposite true in C₄ plots, partially explaining the reversal of the eCO₂ biomass response. These findings challenge the current C₃-C₄eCO₂ paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Sensitive indicators of Stipa bungeana response to precipitation under ambient and elevated CO2 concentration

Precipitation is a primary environmental factor in the semiarid grasslands of northern China. With increased concentrations of atmospheric greenhouse gases, precipitation regimes will change, and high-impact weather events may be more common. Currently, many ecophysiological indicators are known to reflect drought conditions, but these indicators vary greatly among species, and few studies focus on the applicability of these drought indicators under high CO₂ conditions. In this study, five precipitation levels (- 30%, - 15%, control, + 15%, and + 30%) were used to simulate the effects of precipitation change on 18 ecophysiological characteristics in Stipa bungeana, including leaf area, plant height, leaf nitrogen (N), and chlorophyll content, among others. Two levels of CO₂ concentration (ambient, 390 ppm; 550 ppm) were used to simulate the effects of elevated CO₂ on these drought indicators. Using gray relational analysis and phenotypic plasticity analysis, we found that total leaf area or leaf number (morphology), leaf water potential or leaf water content (physiology), and aboveground biomass better reflected the water status of S. bungeana under ambient and elevated CO₂ than the 13 other analyzed variables. The sensitivity of drought indicators changed under the elevated CO₂ condition. By quantifying the relationship between precipitation and the five most sensitive indicators, we found that the thresholds of precipitation decreased under elevated CO₂ concentration. These results will be useful for objective monitoring and assessment of the occurrence and development of drought events in S. bungeana grasslands.

Interactive Effects of CO2 Concentration and Water Regime on Stable Isotope Signatures, Nitrogen Assimilation and Growth in Sweet Pepper

Sweet pepper is among the most widely cultivated horticultural crops in the Mediterranean basin, being frequently grown hydroponically under cover in combination with CO2 fertilization and water conditions ranging from optimal to suboptimal. The aim of this study is to develop a simple model, based on the analysis of plant stable isotopes in their natural abundance, gas exchange traits and N concentration, to assess sweet pepper growth. Plants were grown in a growth chamber for near 6 weeks. Two [CO2] (400 and 800 μmol mol-1), three water regimes (control and mild and moderate water stress) and four genotypes were assayed. For each combination of genotype, [CO2] and water regime five plants were evaluated. Water stress applied caused significant decreases in water potential, net assimilation, stomatal conductance, intercellular to atmospheric [CO2], and significant increases in water use efficiency, leaf chlorophyll content and carbon isotope composition, while the relative water content, the osmotic potential and the content of anthocyanins did change not under stress compared to control conditions support this statement. Nevertheless, water regime affects plant growth via nitrogen assimilation, which is associated with the transpiration stream, particularly at high [CO2], while the lower N concentration caused by rising [CO2] is not associated with stomatal closure. The stable isotope composition of carbon, oxygen, and nitrogen (δ13C, δ18O, and δ15N) in plant matter are affected not only by water regime but also by rising [CO2]. Thus, δ18O increased probably as response to decreases in transpiration, while the increase in δ15N may reflect not only a lower stomatal conductance but a higher nitrogen demand in leaves or shifts in nitrogen metabolism associated with decreases in photorespiration. The way that δ13C explains differences in plant growth across water regimes within a given [CO2], seems to be mediated through its direct relationship with N accumulation in leaves. The changes in the profile and amount of amino acids caused by water stress and high [CO2] support this conclusion. However, the results do not support the use of δ18O as an indicator of the effect of water regime on plant growth.

The interaction between drought and elevated CO2 in water relations in two grassland species is species-specific

Climate change can have major consequences for grassland communities since the different species of the community utilize different mechanisms for adaptation to drought and elevated CO₂ levels. In addition, contradictory data exist when the combined effects of elevated CO₂ and drought are analyzed because the soil water content is not usually similar between CO₂ concentrations. Thus, the objectives of this work have been to examine the effect of water stress on plant water relations in two grassland species (Trifolium pratense and Agrostis capillaris), analyzing the possible differences between the two species when soil water content is equal in all treatments, and to elucidate if development under elevated CO₂ increases drought tolerance and if so, which are the underlying mechanisms. At ambient CO₂, when soil volumetric water content was 15%, both species decreased their water potential in order to continue taking up water. Trifolium pratense performed osmotic adjustment, while Agrostis capillaris decreased the rigidity of its cell wall; moreover, both species increased the root to shoot ratio and decreased leaf area. However, these mechanisms were not sufficient to maintain cell turgor. Elevated CO₂ partially mitigated the negative impact of drought on turgor potential in Trifolium pratense through a higher osmotic adjustment and root to shoot ratio and in Agrostis capillaris through a higher leaf relative water content caused by higher hydraulic conductance, but the impact of drought was not mitigated in either species by higher soil water conservation.

Elevated CO2 Induces Root Defensive Mechanisms in Tomato Plants When Dealing with Ammonium Toxicity

An adequate carbon supply is fundamental for plants to thrive under ammonium stress. In this work, we studied the mechanisms involved in tomato (Solanum lycopersicum L.) response to ammonium toxicity when grown under ambient or elevated CO2 conditions (400 or 800 p.p.m. CO2). Tomato roots were observed to be the primary organ dealing with ammonium nutrition. We therefore analyzed nitrogen (N) and carbon (C) metabolism in the roots, integrating the physiological response with transcriptomic regulation. Elevated levels of CO2 preferentially stimulated root growth despite the high ammonium content. The induction of anaplerotic enzymes from the tricarboxylic acid (TCA) cycle led to enhanced amino acid synthesis under ammonium nutrition. Furthermore, the root transcriptional response to ammonium toxicity was improved by CO2-enriched conditions, leading to higher expression of stress-related genes, as well as enhanced modulation of genes related to signaling, transcription, transport and hormone metabolism. Tomato roots exposed to ammonium stress also showed a defense-like transcriptional response according to the modulation of genes related to detoxification and secondary metabolism, involving principally terpenoid and phenolic compounds. These results indicate that increasing C supply allowed the co-ordinated regulation of root defense mechanisms when dealing with ammonium toxicity.

Effects of salinity and short-term elevated atmospheric CO2 on the chemical equilibrium between CO2 fixation and photosynthetic electron transport of Stevia rebaudiana Bertoni

The effect of water salinity on plant growth and photosynthetic traits of Stevia rebaudiana was investigated to determine its level and mechanisms of salinity tolerance. It was also attempted to assess how short-term elevated CO₂ concentration would influence the boundaries and mechanisms of its photosynthetic capacity. The plants were grown in gravel/hydroponic system under controlled greenhouse conditions and irrigated with four different salinity levels (0, 25, 50 and 100 mol m^-3NaCl). Low salinity did not significantly alter the plant fresh weight, which was substantially decreased by 67% at high salinity treatment. Salinity tolerance threshold was reached at 50 mol m^-3 NaCl while C50 was between 50 and 100 mol m^-3 NaCl, indicating that S. rebaudiana is a moderate salt tolerant species. Salt-induced growth reduction was apparently linked to a significant decline of about 47% in the photosynthetic rates (A_net) at high salinity treatment, leading consequently to a disequilibrium between CO₂-assimilation and electron transport rates (indicated by enhanced ETR_max/A_gross ratio). Elevated atmospheric CO₂ enhanced CO₂ assimilation rates by 65% and 80% for control and high-salt-stressed plants respectively, likely due to significant increases in intercellular CO₂ concentration (indicated by enhanced C_i/C_a). The priority for Stevia under elevated atmospheric CO₂ was not to save water but to maximize photosynthesis so that the PWUE was progressively improved and the threat of oxidative stress was diminished (decline in ETR_max/A_gross). The results imply that elevated CO₂ level could ameliorate some of the detrimental effects of salinity, conferring higher tolerance and survival of S. rebaudiana, a highlydesired feature with the forthcoming era of global changes.

Radiation Interception, Chlorophyll Fluorescence and Senescence of Flag leaves in Winter Wheat under Supplemental Irrigation

Water shortage threatens agricultural sustainability in China, effective water-saving technologies urgently need to be developed. In this study, five treatments were conducted: rainfed (W0), a local supplemental irrigation (SI) practice (W1), and three treatments in which soil water content was tested prior to SI, specifically at 0-20 (W2), 0-40 (W3) and 0-60 cm (W4) soil layers. Soil water consumption in W3 had no differ with W2 but was higher than W1 and W4. Crop evapotranspiration in W1, W3 and W4 treatments were higher than that in W2. W3 treatment had higher leaf area index than W1 and W4 at later grain filling stages. The mean photosynthetically active radiation capture ratio in W3, especially at 20, 40 and 60 cm plant heights, were significantly higher than those in W1, W2 and W4. The chlorophyll content index, actual photosynthetic activities, catalase and superoxide dismutase activities of flag leaves from W3 were the highest after the middle grain filling stages. W3 treatment obtained the highest grain yield (9169 kg ha^-1) and water use efficiency (20.8 kg ha^-1 mm^-1) in the two seasons. These benefits likely accrued through created a suitable soil moisture environment in W3 treatment.

Amelioration of drought-induced negative responses by elevated CO2 in field grown short rotation coppice mulberry (Morus spp.), a potential bio-energy tree crop

Present study describes the responses of short rotation coppice (SRC) mulberry, a potential bio-energy tree, grown under interactive environment of elevated CO₂ (E) and water stress (WS). Growth in E stimulated photosynthetic performance in well-watered (WW) as well as during WS with significant increases in light-saturated photosynthetic rates (A _Sat), water use efficiency (WUE_i), intercellular [CO₂], and photosystem-II efficiency (F _V/F _M and ∆F/F _M') with concomitant reduction in stomatal conductance (g _s) and transpiration (E) compared to ambient CO₂ (A) grown plants. Reduced levels of proline, H₂O_2, and malondialdehyde (MDA) and higher contents of antioxidants including ascorbic acid and total phenolics in WW and WS in E plants clearly demonstrated lesser oxidative damage. Further, A plants showed higher transcript abundance and antioxidant enzyme activities under WW as well as during initial stages of WS (15 days). However, with increasing drought imposition (30 days), A plants showed down regulation of antioxidant systems compared to their respective E plants. These results clearly demonstrated that future increased atmospheric CO₂ enhances the photosynthetic potential and also mitigate the drought-induced oxidative stress in SRC mulberry. In conclusion, mulberry is a potential bio-energy tree crop which is best suitable for short rotation coppice forestry-based mitigation of increased [CO₂] levels even under intermittent drought conditions, projected to prevail in the fast-changing global climate.

Abiotic Stress Responses and Microbe-Mediated Mitigation in Plants: The Omics Strategies

Abiotic stresses are the foremost limiting factors for agricultural productivity. Crop plants need to cope up adverse external pressure created by environmental and edaphic conditions with their intrinsic biological mechanisms, failing which their growth, development, and productivity suffer. Microorganisms, the most natural inhabitants of diverse environments exhibit enormous metabolic capabilities to mitigate abiotic stresses. Since microbial interactions with plants are an integral part of the living ecosystem, they are believed to be the natural partners that modulate local and systemic mechanisms in plants to offer defense under adverse external conditions. Plant-microbe interactions comprise complex mechanisms within the plant cellular system. Biochemical, molecular and physiological studies are paving the way in understanding the complex but integrated cellular processes. Under the continuous pressure of increasing climatic alterations, it now becomes more imperative to define and interpret plant-microbe relationships in terms of protection against abiotic stresses. At the same time, it also becomes essential to generate deeper insights into the stress-mitigating mechanisms in crop plants for their translation in higher productivity. Multi-omics approaches comprising genomics, transcriptomics, proteomics, metabolomics and phenomics integrate studies on the interaction of plants with microbes and their external environment and generate multi-layered information that can answer what is happening in real-time within the cells. Integration, analysis and decipherization of the big-data can lead to a massive outcome that has significant chance for implementation in the fields. This review summarizes abiotic stresses responses in plants in-terms of biochemical and molecular mechanisms followed by the microbe-mediated stress mitigation phenomenon. We describe the role of multi-omics approaches in generating multi-pronged information to provide a better understanding of plant-microbe interactions that modulate cellular mechanisms in plants under extreme external conditions and help to optimize abiotic stresses. Vigilant amalgamation of these high-throughput approaches supports a higher level of knowledge generation about root-level mechanisms involved in the alleviation of abiotic stresses in organisms.

Potential Responses of Vascular Plants from the Pristine "Lost World" of the Neotropical Guayana Highlands to Global Warming: Review and New Perspectives

The neotropical Guayana Highlands (GH) are one of the few remaining pristine environments on Earth, and they host amazing biodiversity with a high degree endemism, especially among vascular plants. Despite the lack of direct human disturbance, GH plants and their communities are threatened with extinction from habitat loss due to global warming (GW). Geographic information systems simulations involving the entire known vascular GH flora (>2430 species) predict potential GW-driven extinctions on the order of 80% by the end of this century, including nearly half of the endemic species. These estimates and the assessment of an environmental impact value for each species led to the hierarchization of plants by their risk of habitat loss and the definition of priority conservation categories. However, the predictions assume that all species will respond to GW by migrating upward and at equal rates, which is unlikely, so current estimates should be considered preliminary and incomplete (although they represent the best that can be done with the existing information). Other potential environmental forcings (i.e., precipitation shifts, an increase in the atmospheric CO₂ concentration) and idiosyncratic plant responses (i.e., resistance, phenotypic acclimation, rapid evolution) should also be considered, so detailed eco-physiological studies of the more threatened species are urgently needed. The main obstacles to developing such studies are the remoteness and inaccessibility of the GH and, especially, the difficulty in obtaining official permits for fieldwork.

Selection of Reference Genes for qRT-PCR Analysis of Gene Expression in Stipa grandis during Environmental Stresses

Stipa grandis P. Smirn. is a dominant plant species in the typical steppe of the Xilingole Plateau of Inner Mongolia. Selection of suitable reference genes for the quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) is important for gene expression analysis and research into the molecular mechanisms underlying the stress responses of S. grandis. In the present study, 15 candidate reference genes (EF1 beta, ACT, GAPDH, SamDC, CUL4, CAP, SNF2, SKIP1, SKIP5, SKIP11, UBC2, UBC15, UBC17, UCH, and HERC2) were evaluated for their stability as potential reference genes for qRT-PCR under different stresses. Four algorithms were used: GeNorm, NormFinder, BestKeeper, and RefFinder. The results showed that the most stable reference genes were different under different stress conditions: EF1beta and UBC15 during drought and salt stresses; ACT and GAPDH under heat stress; SKIP5 and UBC17 under cold stress; UBC15 and HERC2 under high pH stress; UBC2 and UBC15 under wounding stress; EF1beta and UBC17 under jasmonic acid treatment; UBC15 and CUL4 under abscisic acid treatment; and HERC2 and UBC17 under salicylic acid treatment. EF1beta and HERC2 were the most suitable genes for the global analysis of all samples. Furthermore, six target genes, SgPOD, SgPAL, SgLEA, SgLOX, SgHSP90 and SgPR1, were selected to validate the most and least stable reference genes under different treatments. Our results provide guidelines for reference gene selection for more accurate qRT-PCR quantification and will promote studies of gene expression in S. grandis subjected to environmental stress.

Climate Influences the Content and Chemical Composition of Foliar Tannins in Green and Senesced Tissues of Quercus rubra

Environmental stresses not only influence production of plant metabolites but could also modify their resorption during leaf senescence. The production-resorption dynamics of polyphenolic tannins, a class of defense compound whose ecological role extends beyond tissue senescence, could amplify the influence of climate on ecosystem processes. We studied the quantity, chemical composition, and tissue-association of tannins in green and freshly-senesced leaves of Quercus rubra exposed to different temperature (Warming and No Warming) and precipitation treatments (Dry, Ambient, Wet) at the Boston-Area Climate Experiment (BACE) in Massachusetts, USA. Climate influenced not only the quantity of tannins, but also their molecular composition and cell-wall associations. Irrespective of climatic treatments, tannin composition in Q. rubra was dominated by condensed tannins (CTs, proanthocyanidins). When exposed to Dry and Ambient^*Warm conditions, Q. rubra produced higher quantities of tannins that were less polymerized. In contrast, under favorable conditions (Wet), tannins were produced in lower quantities, but the CTs were more polymerized. Further, even as the overall tissue tannin content declined, the content of hydrolysable tannins (HTs) increased under Wet treatments. The molecular composition of tannins influenced their content in senesced litter. Compared to the green leaves, the content of HTs decreased in senesced leaves across treatments, whereas the CT content was similar between green and senesced leaves in Wet treatments that produced more polymerized tannins. The content of total tannins in senesced leaves was higher in Warming treatments under both dry and ambient precipitation treatments. Our results suggest that, though climate directly influenced the production of tannins in green tissues (and similar patterns were observed in the senesced tissue), the influence of climate on tannin content of senesced tissue was partly mediated by the effect on the chemical composition of tannins. These different climatic impacts on leaves over the course of a growing season may alter forest dynamics, not only in decomposition and nutrient cycling dynamics, but also in herbivory dynamics.

Ecosystem responses to warming and watering in typical and desert steppes

Global warming is projected to continue, leading to intense fluctuations in precipitation and heat waves and thereby affecting the productivity and the relevant biological processes of grassland ecosystems. Here, we determined the functional responses to warming and altered precipitation in both typical and desert steppes. The results showed that watering markedly increased the aboveground net primary productivity (ANPP) in a typical steppe during a drier year and in a desert steppe over two years, whereas warming manipulation had no significant effect. The soil microbial biomass carbon (MBC) and the soil respiration (SR) were increased by watering in both steppes, but the SR was significantly decreased by warming in the desert steppe only. The inorganic nitrogen components varied irregularly, with generally lower levels in the desert steppe. The belowground traits of soil total organic carbon (TOC) and the MBC were more closely associated with the ANPP in the desert than in the typical steppes. The results showed that the desert steppe with lower productivity may respond strongly to precipitation changes, particularly with warming, highlighting the positive effect of adding water with warming. Our study implies that the habitat- and year-specific responses to warming and watering should be considered when predicting an ecosystem's functional responses under climate change scenarios.

Elevated CO2 can modify the response to a water status gradient in a steppe grass: from cell organelles to photosynthetic capacity to plant growth

BACKGROUND

The atmospheric CO2 concentration is rising continuously, and abnormal precipitation may occur more frequently in the future. Although the effects of elevated CO2 and drought on plants have been well reported individually, little is known about their interaction, particularly over a water status gradient. Here, we aimed to characterize the effects of elevated CO2 and a water status gradient on the growth, photosynthetic capacity, and mesophyll cell ultrastructure of a dominant grass from a degraded grassland.

RESULTS

Elevated CO2 stimulated plant biomass to a greater extent under moderate changes in water status than under either extreme drought or over-watering conditions. Photosynthetic capacity and stomatal conductance were also enhanced by elevated CO2 under moderate drought, but inhibited with over-watering. Severe drought distorted mesophyll cell organelles, but CO2 enrichment partly alleviated this effect. Intrinsic water use efficiency (WUEi) and total biomass water use efficiency (WUEt) were increased by elevated CO2, regardless of water status. Plant structural traits were also found to be tightly associated with photosynthetic potentials.

CONCLUSION

The results indicated that CO2 enrichment alleviated severe and moderate drought stress, and highlighted that CO2 fertilization's dependency on water status should be considered when projecting key species' responses to climate change in dry ecosystems.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Trait Acclimation Mitigates Mortality Risks of Tropical Canopy Trees under Global Warming

There is a heated debate about the effect of global change on tropical forests. Many scientists predict large-scale tree mortality while others point to mitigating roles of CO2 fertilization and - the notoriously unknown - physiological trait acclimation of trees. In this opinion article we provided a first quantification of the potential of trait acclimation to mitigate the negative effects of warming on tropical canopy tree growth and survival. We applied a physiological tree growth model that incorporates trait acclimation through an optimization approach. Our model estimated the maximum effect of acclimation when trees optimize traits that are strongly plastic on a week to annual time scale (leaf photosynthetic capacity, total leaf area, stem sapwood area) to maximize carbon gain. We simulated tree carbon gain for temperatures (25-35°C) and ambient CO2 concentrations (390-800 ppm) predicted for the 21st century. Full trait acclimation increased simulated carbon gain by up to 10-20% and the maximum tolerated temperature by up to 2°C, thus reducing risks of tree death under predicted warming. Functional trait acclimation may thus increase the resilience of tropical trees to warming, but cannot prevent tree death during extremely hot and dry years at current CO2 levels. We call for incorporating trait acclimation in field and experimental studies of plant functional traits, and in models that predict responses of tropical forests to climate change.

Future Climate CO2 Levels Mitigate Stress Impact on Plants: Increased Defense or Decreased Challenge?

Elevated atmospheric CO2 can stimulate plant growth by providing additional C (fertilization effect), and is observed to mitigate abiotic stress impact. Although, the mechanisms underlying the stress mitigating effect are not yet clear, increased antioxidant defenses, have been held primarily responsible (antioxidant hypothesis). A systematic literature analysis, including "all" papers [Web of Science (WoS)-cited], addressing elevated CO2 effects on abiotic stress responses and antioxidants (105 papers), confirms the frequent occurrence of the stress mitigation effect. However, it also demonstrates that, in stress conditions, elevated CO2 is reported to increase antioxidants, only in about 22% of the observations (e.g., for polyphenols, peroxidases, superoxide dismutase, monodehydroascorbate reductase). In most observations, under stress and elevated CO2 the levels of key antioxidants and antioxidant enzymes are reported to remain unchanged (50%, e.g., ascorbate peroxidase, catalase, ascorbate), or even decreased (28%, e.g., glutathione peroxidase). Moreover, increases in antioxidants are not specific for a species group, growth facility, or stress type. It seems therefore unlikely that increased antioxidant defense is the major mechanism underlying CO2-mediated stress impact mitigation. Alternative processes, probably decreasing the oxidative challenge by reducing ROS production (e.g., photorespiration), are therefore likely to play important roles in elevated CO2 (relaxation hypothesis). Such parameters are however rarely investigated in connection with abiotic stress relief. Understanding the effect of elevated CO2 on plant growth and stress responses is imperative to understand the impact of climate changes on plant productivity.

Warming effects on photosynthesis of subtropical tree species: a translocation experiment along an altitudinal gradient

Ongoing climate warming induced by human activities may have great impacts on trees, yet it remains unresolved how subtropical tree species respond to rising temperature in the field. Here, we used downward translocation to investigate the effects of climate warming on leaf photosynthesis of six common tree species in subtropical China. During the experimental period between 2012 and 2014, the mean average photosynthetic rates (Asat) under saturating light for Schima superba, Machilus breviflora, Pinus massoniana and Ardisia lindleyana in the warm site were7%, 19%, 20% and 29% higher than those in the control site. In contrast, seasonal Asat for Castanopsis hystrix in the warm site were lower compared to the control site. Changes in Asat in response to translocation were mainly associated with those in leaf stomatal conductance (gs) and photosynthetic capacity (RuBP carboxylation, RuBP regeneration capacity). Our results imply that climate warming could have potential impacts on species composition and community structure in subtropical forests.

Responses of plant biomass, photosynthesis and lipid peroxidation to warming and precipitation change in two dominant species (Stipa grandis and Leymus chinensis) from North China Grasslands

Influential factors of global change affect plant carbon uptake and biomass simultaneously. Although the effects from warming and precipitation change have been extensive studied separately, the responses of plant biomass, photosynthesis, and lipid peroxidation to the interaction of these factors are still not fully understood. In this study, we examined the physiological responses of two dominant plant species from grasslands of northern China with different functional traits to combinations of five simulated warming patterns and five simulated precipitation patterns in environment-controlled chambers. Our results showed that the biomass, net CO 2 assimilation rate (P n), maximal efficiency of photosystem II photochemistry (F v/F m), and chlorophyll content (Chl) of Stipa grandis and Leymus chinensis were enhanced by moderate warming and plus precipitation, but they declined drastically with high temperature and drought. High temperature and drought also led to significant malondialdehyde (MDA) accumulation, which had a negative correlation with leaf biomass. The lower level of lipid peroxidation in leaves of S. grandis suggests that this species is better protected from oxidative damage under heat stress, drought stress and their interactive conditions than L. chinensis. Using the subordinate function values method, we found S. grandis to be more sensitive to climate change than L. chinensis and the gross biomass and root biomass of S. grandis and the leaf biomass of L. chinensis were most sensitive to climate change. Furthermore, the P n of both S. grandis and L. chinensis had a significant linear relationship with F v/F m and Chl, indicating that carbon assimilation may be caused by nonstomatal limitations.

Sensitive Indicators of Zonal Stipa Species to Changing Temperature and Precipitation in Inner Mongolia Grassland, China

Climate change often induces shifts in plant functional traits. However, knowledge related to sensitivity of different functional traits and sensitive indicator representing plant growth under hydrothermal change remains unclear. Inner Mongolia grassland is predicted to be one of the terrestrial ecosystems which are most vulnerable to climate change. In this study, we analyzed the response of four zonal Stipa species (S. baicalensis, S. grandis, S. breviflora, and S. bungeana) from Inner Mongolia grassland to changing temperature (control, increased 1.5, 2, 4, and 6°C), precipitation (decreased 30 and 15%, control, increased 15 and 30%) and their combined effects via climate control chambers. The relative change of functional traits in the unit of temperature and precipitation change was regarded as sensitivity coefficient and sensitive indicators were examined by pathway analysis. We found that sensitivity of the four Stipa species to changing temperature and precipitation could be ranked as follows: S. bungeana > S. grandis > S. breviflora > S. baicalensis. In particular, changes in leaf area, specific leaf area and root/shoot ratio could account for 86% of the changes in plant biomass in the four Stipa species. Also these three measurements were more sensitive to hydrothermal changes than the other functional traits. These three functional indicators reflected the combination of plant production capacity (leaf area), adaptive strategy (root/shoot ratio), instantaneous environmental effects (specific leaf area), and cumulative environmental effects (leaf area and root/shoot ratio). Thus, leaf area, specific leaf area and root/shoot ratio were chosen as sensitive indicators in response to changing temperature and precipitation for Stipa species. These results could provide the basis for predicting the influence of climate change on Inner Mongolia grassland based on the magnitude of changes in sensitive indicators.