Elevated CO2 concentration enhances drought resistance of soybean by regulating cell structure, cuticular wax synthesis, photosynthesis, and oxidative stress response.

The atmospheric [CO2] and the frequency and intensity of extreme weather events such as drought are increased, leading to uncertainty to soybean production. Elevated [CO2] (eCO2) partially mitigates the adverse effects of drought stress on crop growth and photosynthetic performance, but the mitigative mechanism is not well understood. In this study, soybean seedlings under drought stress simulated by PEG-6000 were grown in climate chambers with different [CO2] (400 µmol mol-1 and 700 µmol mol-1). The changes in anatomical structure, wax content, photosynthesis, and antioxidant enzyme were investigated by the analysis of physiology and transcriptome sequencing (RNA-seq). The results showed that eCO2 increased the thickness of mesophyll cells and decreased the thickness of epidermal cells accompanied by reduced stomatal conductance, thus reducing water loss in soybean grown under drought stress. Meanwhile, eCO2 up-regulated genes related to wax anabolism, thus producing more epidermal wax. Under drought stress, eCO2 increased net photosynthetic rate (PN), ribulose-1,5-bisphosphate carboxylase/oxygenase activity, and alerted the gene expressions in photosynthesis. The increased sucrose synthesis and decreased sucrose decomposition contributed to the progressive increase in the soluble saccharide contents under drought stress with or without eCO2. In addition, eCO2 increased the expressions of genes associated with peroxidase (POD) and proline (Pro), thus enhancing POD activity and Pro content and improving the drought resistance in soybean. Taken together, these findings deepen our understanding of the effects of eCO2 on alleviating drought stress in soybean and provide potential target genes for the genetic improvement of drought tolerance in soybean.

Physiological and transcriptome analysis of response of soybean (Glycine max) to cadmium stress under elevated CO2 concentration.

The continuous accumulation of Cd has long-lasting detrimental effects on plant growth and food safety. Although elevated CO2 concentration (EC) has been reported to reduce Cd accumulation and toxicity in plants, evidence on the functions of elevated CO2 concentration and its mechanisms in the possible alleviation of Cd toxicity in soybean are limited. Here, we used physiological and biochemical methods together with transcriptomic comparison to explore the effects of EC on Cd-stressed soybean. Under Cd stress, EC significantly increased the weight of roots and leaves, promoted the accumulations of proline, soluble sugars, and flavonoid. In addition, the enhancement of GSH activity and GST gene expressions promoted Cd detoxification. These defensive mechanisms reduced the contents of Cd2+, MDA, and H2O2 in soybean leaves. The up-regulation of genes encoding phytochelatin synthase, MTPs, NRAMP, and vacuoles protein storage might play vital roles in the transportation and compartmentalization process of Cd. The MAPK and some transcription factors such as bHLH, AP2/ERF, and WRKY showed changed expressions and might be engaged in mediation of stress response. These findings provide a boarder view on the regulatory mechanism of EC on Cd stress and provide numerous potential target genes for future engineering of Cd-tolerant cultivars in soybean breeding programs under climate changes scenarios.

Trade-offs between wheat soil N2O emissions and C sequestration under straw return, elevated CO2 concentration, and elevated temperature.

The feedback between nitrous oxide (N2O) emissions, straw management and future climate scenarios is not well understood, especially in wheat ecosystems. In this study, the changes in N2O emissions, soil properties, enzymes, and functional genes involved in N cycling were measured with straw return (incorporation and mulching) and straw removal, under elevated [CO2] (+200 µmol mol-1 above ambient [CO2]), elevated temperature (+2 °C above ambient temperature), and their combination. The net global warming potential (NGWP) and greenhouse gas intensity (GHGI) were evaluated in combination with greenhouse gas emissions, yield, and soil organic carbon (C) sequestration. Compared with the ambient condition, elevated [CO2] and elevated temperature suppressed N2O emission by 41 %-46 %. Straw return significantly increased N2O emission by 31 %-109 % through increasing soil C and N substrates and denitrifying genes abundance, compared with straw removal. In addition, the impact of straw return on N2O emission was greater than that of elevated [CO2] or temperature. Straw return generally reduced NGWP by 166.2-3353.3 kg CO2-eq ha-1 and GHGI by 0.4-1.1 kg CO2-eq kg-1 through increasing soil organic C sequestration by 0.1-1.1 t C ha-1 and grain yield by 280.8 kg ha-1-1595.4 kg ha-1. Straw return would stimulate N2O emissions from this wheat cropping system under future warmer, elevated [CO2] climates, but simultaneously increase grain yield and soil organic C sequestration to a greater extent. Overall, straw return is beneficial to climate change mitigation; in particular, straw incorporation would be more effective than straw mulching.

Effects of elevated CO2 concentration on cell structure and stress resistance physiology of Setaria italica under drought stress.

The frequency of drought will increase under further warming. The increase in atmospheric CO2 concentration, along with more frequent drought, will affect crop growth. We examined the changes of cell structure, photosynthetic physiology, antioxidant enzymes, osmotic regulatory substances, and yield of foxtail millet (Setaria ita-lica) leaves under different CO2 concentrations (ambient air CO2 concentration and ambient atmospheric CO2 concentration + 200 µmol·mol-1) and water treatment (soil moisture content maintained at 45%-55%, and 70%-80% of field capacity, representing mild drought and normal water condition, respectively). The results showed that elevated CO2 concentration increased the number of starch grains, the area of single starch grains, and the total area of starch grains in the chloroplast of millet mesophyll cells. Under mild drought condition, elevated CO2 concentration increased net photosynthetic rate of millet leaves at the booting stage by 37.9%, but did not affect water use efficiency at this stage. Elevated CO2 concentration increased net photosynthetic rate and water use efficiency of millet leaves under mild drought condition at the filling stage by 15.0% and 44.2%, respectively. Under mild drought condition, elevated CO2 concentration increased the content of peroxidase (POD) and soluble sugar in millet leaves at the booting stage by 39.3% and 8.0%, respectively, but decreased proline content by 31.5%. It increased the content of POD in millet leaves at the filling stage by 26.5% but decreased the content of MDA and proline by 37.2% and 39.3%, respectively. Under mild drought condition, elevated CO2 concentration significantly increased the number of grain spikes by 44.7% and yield by 52.3% in both years compared with normal water condition. The effect of elevated CO2 concentration on grain yield under mild drought conditions was higher than that under normal water condition. Under mild drought conditions, elevated CO2 concentration increased leaf thickness, vascular bundle sheath cross-sectional area, net photosynthetic rate, and water use efficiency of millet, improved the antioxidant oxidase activity, and changed the concentration of osmotic regulatory substances, alleviated the nega-tive effect of drought on foxtail millet, and finally increased the number of grains per ear and yield of foxtail millet. This study would provide a theoretical basis for millet production and sustainable agricultural development in arid areas under future climate change.

An optimistic future of C4 crop broomcorn millet (Panicum miliaceum L.) for food security under increasing atmospheric CO2 concentrations.

Broomcorn millet, a C4 cereal, has better tolerance to environmental stresses. Although elevated atmospheric CO2 concentration has led to grain nutrition reduction in most staple crops, studies evaluating its effects on broomcorn millet are still scarce. The yield, nutritional quality and metabolites of broomcorn millet were investigated under ambient CO2 (aCO2, 400 µmol mol-1) and elevated CO2 (eCO2, aCO2+ 200 µmol mol-1) for three years using open-top chambers (OTC). The results showed that the yield of broomcorn millet was markedly increased under eCO2 compared with aCO2. On average, eCO2 significantly increased the concentration of Mg (27.3%), Mn (14.6%), and B (21.2%) over three years, whereas it did not affect the concentration of P, K, Fe, Ca, Cu or Zn. Protein content was significantly decreased, whereas starch and oil concentrations were not changed by eCO2. With the greater increase in grain yield, eCO2 induced increase in the grain accumulations of P (23.87%), K (29.5%), Mn (40.08%), Ca (22.58%), Mg (51.31%), Zn (40.95%), B (48.54%), starch (16.96%) and oil (28.37%) on average for three years. Flavonoids such as kaempferol, apigenin, eriodictyol, luteolin, and chrysoeriol were accumulated under eCO2. The reduction in L-glutamine and L-lysine metabolites, which were the most representative amino acid in grain proteins, led to a reduction of protein concentration under eCO2. Broomcorn millet has more desirable nutritional traits for combating hidden hunger. This may potentially be useful for breeding more nutritious plants in the era of climate change.

[Effects of elevated CO2 concentration on photosynthetic acclimation of winter wheat under drought condition]. / 大气CO2æµ“åº¦å‡é«˜å¯¹å¹²æ—±æ¡ä»¶ä¸‹å†¬å°éº¦å¶ç‰‡å ‰åˆé€‚åº”çš„å½±å“.

Mechanisms underlying leaf photosynthetic acclimation in winter wheat under elevation of CO2 concentration ([CO2]) remain unclear. The aim of the study was to investigate the effects of source-sink variation on photosynthetic acclimation induced by drought under elevated [CO2]. A winter wheat (Triticum aestivum L. 'Zhengmai 9023') pot experiment was conducted in open top climate chambers with [CO2] of 400µmol·mol-1 or 600 µmol·mol-1 and soil water content at 80%±5% or 55%±5% of field capacity. The parameters of chlorophyll fluorescence, electron transport rate, photosynthetic curve, leaf nitrogen content, and grain yield were measured at the elongation and heading stages. Under drought condition, leaf PSⅡ photochemical efficiency was not affected by elevated [CO2], but the maximum electron transport rate and the ratio of electron partitioned to carboxylation reaction in Calvin cycle was increased at the elongation stage, and thus the Rubisco carboxylation rate and maximum photosynthetic rate were increased. Although the maximum electron transportation rate and partitioning ratio of electron to carboxylation reaction in Calvin cycle remained high at the heading stage, the PSⅡ photochemical efficiency, Rubisco carboxylation rate, and triose phosphate utilization rate were decreased by elevated [CO2], which consequently reduced the maximum photosynthetic rate for plant under drought stress. Under drought condition, elevated [CO2] increased wheat tiller biomass, kernel number, and kernel weight per ear, but decreased infertile kernel number, resulting in an overall increase in grain weight. In conclusion, the elevated [CO2]-induced increase in wheat grain yield per tiller under drought condition was mainly caused by enhanced photosynthetic performance at the elongation stage. The photosynthetic acclimation in source leaves during the heading stage under elevated [CO2] was mainly attributed to the reduction in PSⅡ photochemical efficiency and triose phosphate utilization rate, but not to the maximum electron transportation rate, ratio of electron partitioned to carboxylation in Calvin cycle or sink leaf strength.

Elevated CO2-induced changes in photosynthesis, antioxidant enzymes and signal transduction enzyme of soybean under drought stress.

Rising atmospheric [CO2] influences plant growth, development, productivity and stress responses. Soybean is a major oil crop. At present, it is unclear how elevated [CO2] affects the physiological and biochemical pathways of soybean under drought stress. In this study, changes in the photosynthetic capacity, photosynthetic pigment and antioxidant level were evaluated in soybean at flowering stages under different [CO2] (400 µmol mol-1 and 600 µmol mol-1) and water level (the relative water content of the soil was 75-85% soil capacity, and the relative water content of the soil was 35-45% soil capacity under drought stress). Changes in levels of osmolytes, hormones and signal transduction enzymes were also determined. The results showed that under drought stress, increasing [CO2] significantly reduced leaf transpiration rate (E), net photosynthetic rate (PN) and chlorophyll b content. Elevated [CO2] significantly decreased the content of malondialdehyde (MDA) and proline (PRO), while significantly increased superoxide dismutase (SOD) and abscisic acid (ABA) under drought stress. Elevated [CO2] significantly increased the transcript and protein levels of calcium-dependent protein kinase (CDPK), and Glutathione S- transferase (GST). The content of HSP-70 and the corresponding gene expression level were significantly reduced by elevated [CO2], irrespective of water treatments. Taken together, these results suggest that elevated [CO2] does not alleviate the negative impacts of drought stress on photosynthesis. ABA, CDPK and GST may play an important role in elevated CO2-induced drought stress responses.

Photosynthesis and yield response to elevated CO2, C4 plant foxtail millet behaves similarly to C3 species.

Foxtail millet (Setaria italica) is a nutrient-rich food source traditionally grown in arid and semi-arid areas, as it is well adapted to drought climate. Yet there is limited information as how the crop responses to the changing climate. In order to investigate the response of foxtail millet to elevated [CO2] and the underlying mechanism, the crop was grown at ambient [CO2] (400 µmol mol-1) and elevated [CO2] (600 µmol mol-1) in an open-top chamber (OTC) experimental facility in North China. The changes in leaf photosynthesis, chlorophyll fluorescence, biomass, yield and global gene expression in response to elevated [CO2] were determined. Despite foxtail millet being a C4 photosynthetic crop, photosynthetic rates (PN) and intrinsic water-use efficiency (WUEi), were increased under elevated [CO2]. Similarly, grain yield and above-ground biomass also significantly increased (P <  0.05) for the two years of experimentation under elevated [CO2]. Increases in seeds and tiller number, spike and stem weight were the main contributors to the increased grain yield and biomass. Using transcriptomic analyses, this study further identified some genes which play a role in cell wall reinforcement, shoot initiation, stomatal conductance, carbon fixation, glycolysis / gluconeogenesis responsive to elevated [CO2]. Changes in these genes reduced plant height, increased stem diameters, and promote CO2 fixation. Higher photosynthetic rates at elevated [CO2] demonstrated that foxtail millet was not photosynthetically saturated at elevated [CO2] and its photosynthesis response to elevated [CO2] were analogous to C3 plants.

Elevated CO2 reduces the adverse effects of drought stress on a high-yielding soybean (Glycine max (L.) Merr.) cultivar by increasing water use efficiency.

Soybean (Glycine max (L.) Merr.) is the world's most important grain legume. The impacts of climate change such as elevated CO2 and drought on soybean physiological and morphological responses are not well understood. This study evaluated the effects of elevated CO2 (ambient concentration + 200 mmol mol-1) and drought stress (35-45% of relative water content) on soybean leaf photosynthesis, chlorophyll fluorescence, stress physiological indexes, morphological parameters, biomass and yield over 2 years at the open-top chamber (OTC) experimental facility in North China. We found that drought decreased intrinsic efficiency of PSII (Fv'/Fm'), effective quantum yield of PSII photochemistry (ΦPSII), photochemical quenching coefficient (qP), and yield of soybean, increased nonphotochemical quenching (NPQ), peroxidase (POD), and malondialdehyde (MDA), but had no effect on superoxide dismutase (SOD) or soluble sugar content. Elevated [CO2] increased net photosynthetic rate (PN), water-use efficiency (WUE), ΦPSII, qP, SOD, soluble sugar content and yield of soybean. Elevated [CO2] enhanced the positive effects of drought on WUE, but reduced the negative effects of drought on ΦPSII and qP. Elevated [CO2] enhanced the resistance to drought by improving the capacity of photosynthesis and WUE in soybean leaves.