Elevated carbon dioxide decreases the adverse effects of higher temperature and drought stress by mitigating oxidative stress and improving water status in Arabidopsis thaliana

Appraisal of the Role of Gaseous Signaling Molecules in Thermo-Tolerance Mechanisms in Plants

A significant threat to the ongoing rise in temperature caused by global warming. Plants have many stress-resistance mechanisms, which is responsible for maintaining plant homeostasis. Abiotic stresses largely increase gaseous molecules' synthesis in plants. The study of gaseous signaling molecules has gained attention in recent years. The role of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ethylene, in plants under temperature high-temperature stress are discussed in the current review. Recent studies revealed the critical function that gaseous molecules play in controlling plant growth and development and their ability to respond to various abiotic stresses. Here, we provide a thorough overview of current advancements that prevent heat stress-related plant damage via gaseous molecules. We also explored and discussed the interaction of gaseous molecules. In addition, we provided an overview of the role played by gaseous molecules in high-temperature stress responses, along with a discussion of the knowledge gaps and how this may affect the development of high-temperature-resistant plant species.

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.

Species Survey of Leaf Hyponasty Responses to Warming Plus Elevated CO2

Atmospheric carbon dioxide (CO2) concentrations are increasing and may exceed 800 ppm by 2100. This is increasing global mean temperatures and the frequency and severity of heatwaves. Recently, we showed for the first time that the combination of short-term warming and elevated carbon dioxide (eCO2) caused extreme upward bending (i.e., hyponasty) of leaflets and leaf stems (petioles) in tomato (Solanum lycopersicum), which reduced growth. Here, we examined additional species to test the hypotheses that warming + eCO2-induced hyponasty is restricted to compound-leaved species, and/or limited to the Solanaceae. A 2 × 2 factorial experiment with two temperatures, near-optimal and supra-optimal, and two CO2 concentrations, ambient and elevated (400, 800 ppm), was imposed on similarly aged plants for 7-10 days, after which final petiole angles were measured. Within Solanaceae, compound-leaf, but not simple-leaf, species displayed increased hyponasty with the combination of warming + eCO2 relative to warming or eCO2 alone. In non-solanaceous species, hyponasty, leaf-cupping, and changes in leaf pigmentation as a result of warming + eCO2 were variable across species.

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.

The Impact of Increased CO2 and Drought Stress on the Secondary Metabolites of Cauliflower (Brassica oleracea var. botrytis) and Cabbage (Brassica oleracea var. capitata)

Elevated carbon dioxide and drought are significant stressors in light of climate change. This study explores the interplay between elevated atmospheric CO2, drought stress, and plant physiological responses. Two Brassica oleracea varieties (cauliflowers and cabbage) were utilized as model plants. Our findings indicate that elevated CO2 accelerates assimilation rate decline during drought. The integrity of photosynthetic components influenced electron transport, potentially due to drought-induced nitrate reductase activation changes. While CO2 positively influenced photosynthesis and water-use efficiency during drought, recovery saw decreased stomatal conductance in high-CO2-grown plants. Drought-induced monoterpene emissions varied, influenced by CO2 concentration and species-specific responses. Drought generally increased polyphenols, with an opposing effect under elevated CO2. Flavonoid concentrations fluctuated with drought and CO2 levels, while chlorophyll responses were complex, with high CO2 amplifying drought's effects on chlorophyll content. These findings contribute to a nuanced understanding of CO2-drought interactions and their intricate effects on plant physiology.

Melatonin Alleviates Chromium Toxicity in Maize by Modulation of Cell Wall Polysaccharides Biosynthesis, Glutathione Metabolism, and Antioxidant Capacity

Melatonin, a pleiotropic regulatory molecule, is involved in the defense against heavy metal stress. Here, we used a combined transcriptomic and physiological approach to investigate the underlying mechanism of melatonin in mitigating chromium (Cr) toxicity in Zea mays L. Maize plants were treated with either melatonin (10, 25, 50 and 100 μM) or water and exposed to 100 μM K₂Cr₂O₇ for seven days. We showed that melatonin treatment significantly decreased the Cr content in leaves. However, the Cr content in the roots was not affected by melatonin. Analyses of RNA sequencing, enzyme activities, and metabolite contents showed that melatonin affected cell wall polysaccharide biosynthesis, glutathione (GSH) metabolism, and redox homeostasis. During Cr stress, melatonin treatment increased cell wall polysaccharide contents, thereby retaining more Cr in the cell wall. Meanwhile, melatonin improved the GSH and phytochelatin contents to chelate Cr, and the chelated complexes were then transported to the vacuoles for sequestration. Furthermore, melatonin mitigated Cr-induced oxidative stress by enhancing the capacity of enzymatic and non-enzymatic antioxidants. Moreover, melatonin biosynthesis-defective mutants exhibited decreased Cr stress resistance, which was related to lower pectin, hemicellulose 1, and hemicellulose 2 than wild-type plants. These results suggest that melatonin alleviates Cr toxicity in maize by promoting Cr sequestration, re-establishing redox homeostasis, and inhibiting Cr transport from the root to the shoot.

Interactions between hydrogen sulphide and rhizobia modulate the physiological and metabolism process during water deficiency-induced oxidative defense in soybean

Hydrogen sulphide (H₂ S), a new gas signal molecule, participates in the regulation of various abiotic stresses in plants. However, how the tandem working of H₂ S and rhizobia affects the adaptation of soybean to water deficiency is still unclear. In this study, we investigated the adaptation mechanism of H₂ S and rhizobia in soybean to water deficiency. Our results revealed that H₂ S and rhizobia jointly enhanced the leaf chlorophyll content and relative water content in plants, and caused an increase in the biomass of soybean seedlings under water deficiency. Besides, in the absence of water, H₂ S enhanced the biomass by affecting the number of nodules and nitrogenase activity during vegetative growth. The expression of nodulation marker genes including early nodulin 40 (GmENOD40), ERF required for nodulation (GmERN) and nodulation inception genes (GmNIN1a, GmNIN2a and GmNIN2b) were upregulated by H₂ S and rhizobia in the nodules. Moreover, the combined effect of H₂ S and rhizobia was proved to affect the enzyme activities and gene expression level of antioxidants, as well as osmotic protective substance content and related gene expression levels under water deficiency in soybean seedlings. In addition, the metabolomic results suggested that the combined effect of H₂ S and rhizobia remarkably promoted the contents of lipids and lipid-like molecules. Our results indicated that H₂ S and rhizobia synergistically reduced the oxidative damage caused by water deficiency through increasing the accumulation of metabolites and strengthening the plant antioxidant capacity.

Physiological and Antioxidant Response to Different Water Deficit Regimes of Flag Leaves and Ears of Wheat Grown under Combined Elevated CO2 and High Temperature

Triticum aestivum L. cv. Gazul is a spring wheat widely cultivated in Castilla y León (Spain). Potted plants were grown in a scenario emulating the climate change environmental conditions expected by the end of this century, i.e., with elevated CO₂ and high temperature under two water deficit regimes: long (LWD) and terminal (TWD). Changes in biomass and morphology, the content of proline (Pro), ascorbate (AsA) and glutathione (GSH), and enzymatic antioxidant activities were analyzed in flag leaves and ears. Additionally, leaf gas exchange was measured. LWD caused a decrease in biomass and AsA content but an increase in Pro content and catalase and GSH reductase activities in flag leaves, whereas TWD produced no significant changes. Photosynthesis was enhanced under both water deficit regimes. Increase in superoxide dismutase activity and Pro content was only observed in ears under TWD. The lack of a more acute effect of LWD and TWD on both organs was attributed to the ROS relieving effect of elevated CO₂. Gazul acted as a drought tolerant variety with anisohydric behavior. A multifactorial analysis showed better adaptation of ears to water deficit than flag leaves, underlining the importance of this finding for breeding programs to improve grain yield under future climate change.

High-resolution shotgun proteomics reveals that increased air [CO2] amplifies the acclimation response of coffea species to drought regarding antioxidative, energy, sugar, and lipid dynamics

As drought threatens crop productivity it is crucial to characterize the defense mechanisms against water deficit and unveil their interaction with the expected rise in the air [CO₂]. For that, plants of Coffea canephora cv. Conilon Clone 153 (CL153) and C. arabica cv. Icatu grown under 380 (aCO₂) or 700 μL L^-1 (eCO₂) were exposed to moderate (MWD) and severe (SWD) water deficits. Responses were characterized through the activity and/or abundance of a selected set of proteins associated with antioxidative (e.g., Violaxanthin de-epoxidase, Superoxide dismutase, Ascorbate peroxidases, Monodehydroascorbate reductase), energy/sugar (e.g., Ferredoxin-NADP reductase, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, sucrose synthase, mannose-6-phosphate isomerase, Enolase), and lipid (Lineolate 13S-lipoxygenase) processes, as well as with other antioxidative (ascorbate) and protective (HSP70) molecules. MWD caused small changes in both genotypes regardless of [CO₂] level while under the single imposition to SWD, only Icatu showed a global reinforcement of most studied proteins supporting its tolerance to drought. eCO₂ alone did not promote remarkable changes but strengthened a robust multi-response under SWD, even supporting the reversion of impacts already observed by CL153 at aCO₂. In the context of climate changes where water constraints and [CO₂] levels are expected to increase, these results highlight why eCO₂ might have an important role in improving drought tolerance in Coffea species.

Transcriptome Profiling to the Effects of Drought Stress on Different Propagation Modes of Tea Plant (Camellia sinensis)

Tea plant (Camellia sinensis) is an important economic beverage crop. Drought stress seriously affects the growth and development of tea plant and the accumulation of metabolites, as well as the production, processing, yield and quality of tea. Therefore, it is necessary to understand the reaction mechanism of tea plant under drought conditions and find efficient control methods. Based on transcriptome sequencing technology, this study studied the difference of metabolic level between sexual and asexual tea plants under drought stress. In this study, there were multiple levels of up-regulation and down-regulation of differential genes related to cell composition, molecular function and biological processes. Transcriptomic data show that the metabolism of tea plants with different propagation modes of QC and ZZ is different under drought conditions. In the expression difference statistics, it can be seen that the differential genes of QC are significantly more than ZZ; GO enrichment analysis also found that although differential genes in biological process are mainly enriched in the three pathways of metabolic, single organism process and cellular process, cellular component is mainly enriched in cell, cell part, membrane, and molecular function, and binding, catalytic activity, and transporter activity; the enrichment order of differential genes in these pathways is different in QC and ZZ. This difference is caused by the way of reproduction. The further study of these differential genes will lay a foundation for the cultivation methods and biotechnology breeding to improve the quality of tea.

Mitochondrial and peroxisomal NAD+ uptake are important for improved photosynthesis and seed yield under elevated CO2 concentrations

As sessile organisms, plants must adapt their physiology and developmental processes to cope with challenging environmental circumstances, such as the ongoing elevation in atmospheric carbon dioxide (CO₂ ) levels. Nicotinamide adenine dinucleotide (NAD⁺ ) is a cornerstone of plant metabolism and plays an essential role in redox homeostasis. Given that plants impaired in NAD metabolism and transport often display growth defects, low seed production and disturbed stomatal development/movement, we hypothesized that subcellular NAD distribution could be a candidate for plants to exploit the effects of CO₂ fertilization. We report that an efficient subcellular NAD⁺ distribution is required for the fecundity-promoting effects of elevated CO₂ levels. Plants with reduced expression of either mitochondrial (NDT1 or NDT2) or peroxisomal (PXN) NAD⁺ transporter genes grown under elevated CO₂ exhibited reduced total leaf area compared with the wild-type while PXN mutants also displayed reduced leaf number. NDT2 and PXN lines grown under elevated CO₂ conditions displayed reduced rosette dry weight and lower photosynthetic rates coupled with reduced stomatal conductance. Interestingly, high CO₂ doubled seed production and seed weight in the wild-type, whereas the mutants were less responsive to increases in CO₂ levels during reproduction, producing far fewer seeds than the wild-type under both CO₂ conditions. These data highlight the importance of mitochondrial and peroxisomal NAD⁺ uptake mediated by distinct NAD transporter proteins to modulate photosynthesis and seed production under high CO₂ levels.

The Morpho-Physio-Biochemical Attributes of Urban Trees for Resilience in Regional Ecosystems in Cities: A Mini-Review

Increased urbanization means human beings become the dominant species and reduction in canopy cover. Globally, urban trees grow under challenging and complex circumstances with urbanization trends of increasing anthropogenic carbon dioxide (CO2) emissions, high temperature and drought stress. This study aims to provide a better understanding of urban trees’ morpho-physio-biochemical attributes that can support sustainable urban greening programs and urban climate change mitigation policies. Globally, urban dwellers’ population is on the rise and spreading to suburban areas over time with an increase in domestic CO2 emissions. Uncertainty and less information on urban tree diversification and resistance to abiotic stress may create deterioration of ecosystem resilience over time. This review uses general parameters for urban tree physiology studies and employs three approaches for evaluating ecosystem resilience based on urban stress resistance in relation to trees’ morphological, physiological and biochemical attributes. Due to the lack of a research model of ecosystem resilience and urban stress resistance of trees, this review demonstrates that the model concept supports future urban tree physiology research needs. In particular, it is necessary to develop integral methodologies and an urban tree research concept to assess how main and combined effects of drought and/or climate changes affect indigenous and exotic trees that are commonly grown in cities.

Impacts of water deficit and post-drought irrigation on transpiration rate, root activity, and biomass yield of Festuca arundinacea during phytoextraction

Water deficit is a hazardous threat to phytoremediation, while the photosynthetic efficiency of plant leaves can rapidly recover after post-drought irrigation, thereby enhancing the root activity, transpiration rate, and metal accumulation capacity of plants. This study was designed to test whether the phytoextraction effect of drought-stressed Festuca arundinacea could recover to normal levels after post-drought irrigation. Two drought stress levels (D₁, slight stress and D₂, moderate stress) were carried out at one of five plant growth stages (G₁, germinating; G₂, tillering; G₃, jointing; G₄, booting; and G₅, flowering). The results showed that drought stress, regardless of level, significantly decreased the transpiration rate of F. arundinacea by 38.9%-85.7%. The degree of reduction of this physiological index was significantly higher in D₁G₁ and D₂G₁ than in other treatments. The biomass yield and root activity in D₁G₃, D₁G₄, D₁G₅, D₂G₃, and D₂G₄ recovered and even surpassed the normal values after rewatering, suggesting that the detrimental effects of drought stress on F. arundinacea at certain growth stages can be compensated by post-drought irrigation. Drought stress also decreased the Cd uptake capacity of F. arundinacea, and the degree of reduction depended on the stress level and growth stage. Overcompensation for Cd accumulation was observed in D₁G₃, D₁G₄, D₂G₃, and D₂G₄ after post-drought irrigation. The results indicated that suitable irrigation strategies can improve the phytoextraction effect of F. arundinacea and conserve water resources in practice.

Effect of using Celosia argentea grown from seeds treated with a magnetic field to conduct Cd phytoremediation in drought stress conditions

The mechanisms of the stimulatory effect of external magnetic fields on plant growth have been revealed; however, the role of magnetic fields in the efficiency of phytoremediation with Celosia argentea grown under drought stress which results in detrimental influences on food security has not been reported. Therefore, this study evaluated the physiological responses of C. argentea to the interactions between exposure to a magnetic field and drought stress. Compared with a control, a drought treatment negatively affected the dry weight, transpiration rate, and Cd extraction efficiency of the species and caused oxidative damage in plant cells, as manifested by the increase in malondialdehyde levels and antioxidant enzyme activities. The biomass production, pigment levels, Cd content, and phytoremediation efficiency of the plant were positively affected by all magnetic field treatments compared to the control. All magnetic treatments, except those at 30 mT, alleviated the detrimental effects induced by a 10-day irrigation regime by enhancing the dry weight, chlorophyll content, and activities of antioxidant enzymes in the leaves of the plant. In terms of the interaction between pre-sowing magnetic field seed treatment and drought stress, a 100 mT treatment increased most of the measured parameters, particularly under a 3-day irrigation regime; this corresponded to the optimal phytoremediation efficiency. The results suggest that magnetic field treatment is a novel, economical, and practicable strategy by which to increase the efficiency of phytoremediation using C. argentea under drought stress.

Perfluorooctanoic acid (PFOA) changes nutritional compositions in lettuce (Lactuca sativa) leaves by activating oxidative stress

Perfluorooctanoic acid (PFOA) is a typical persistent organic pollutant commonly detected in ecosystem. Insights into the risks of PFOA in crops, from the perspectives of food nutritional compositions, are sparse. In this study, the physiological responses to PFOA induced oxidative stress were investigated in lettuce (Lactuca sativa) leaves hydroponically exposed to 5 and 50 μg/L PFOA. The effects on photosynthesis and nutritional compositions were characterized. 35.1 and 316.7 ng/g dry weight PFOA were bio-accumulated in lettuce leaves under exposure to 5 and 50 μg/L PFOA, respectively. PFOA led to exposure-dependent over-generation of reactive oxidative species (ROS; H2O2, 8.1%-38.7%; OH, 11.3%-26.4%; O2-, 3.1%-22.8%) in leaves. Both non-enzymatic and enzymatic antioxidants were activated to scavenge ROS. Nevertheless, metabolomics results indicated some nutritional compositions in lettuce leaves were elevated by environmentally relevant concentrations of PFOA. Both primary metabolites, such as carbohydrates in the tricarboxylic acid cycle and amino acids, and secondary metabolites, such as bioactive (poly)phenol and alkaloid compounds, were significantly up-regulated. Leaf net photosynthetic rates were stimulated and intercellular CO2 concentration was decreased. A thorough scheme on the interaction between PFOA and lettuce leaves was proposed as well, to enhance the understanding of PFOA risks in crops.

Primary Metabolite Profile Changes in Coffea spp. Promoted by Single and Combined Exposure to Drought and Elevated CO2 Concentration

Climate change scenarios pose major threats to many crops worldwide, including coffee. We explored the primary metabolite responses in two Coffea genotypes, C. canephora cv. Conilon Clone 153 and C. arabica cv. Icatu, grown at normal (aCO₂) or elevated (eCO₂) CO₂ concentrations of 380 or 700 ppm, respectively, under well-watered (WW), moderate (MWD), or severe (SWD) water deficit conditions, in order to assess coffee responses to drought and how eCO₂ can influence such responses. Primary metabolites were analyzed with a gas chromatography time-of-flight mass spectrometry metabolomics platform (GC-TOF-MS). A total of 48 primary metabolites were identified in both genotypes (23 amino acids and derivatives, 10 organic acids, 11 sugars, and 4 other metabolites), with differences recorded in both genotypes. Increased metabolite levels were observed in CL153 plants under single and combined conditions of aCO₂ and drought (MWD and SWD), as opposed to the observed decreased levels under eCO₂ in both drought conditions. In contrast, Icatu showed minor differences under MWD, and increased levels (especially amino acids) only under SWD at both CO₂ concentration conditions, although with a tendency towards greater increases under eCO₂. Altogether, CL153 demonstrated large impact under MWD, and seemed not to benefit from eCO₂ in either MWD and SWD, in contrast with Icatu.

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.

An Oxygen Delivery Polymer Enhances Seed Germination in a Martian-like Environment

Critical to the success of establishing a sustainable human presence on Mars is the ability to economically grow crop plants. Several environmental factors make it difficult to fully rely on local resources for agriculture. These include nutrient sparse regolith, low and fluctuating temperatures, a high amount of ultraviolet radiation, and water trapped locally in the form of ice or metal oxides. While the 96% CO2 martian atmosphere is ideal to support photosynthesis, high CO2 concentrations inhibit germination. An added difficulty is the fact that a vast majority of crop plants require oxygen for germination. Here, we report the production of a polymer-based oxygen delivery system that supports the germination and growth of cress seeds (Lepidium sativum) in a martian regolith simulant under a martian atmosphere at 101 kPa. The oxygen-donating system is based on a low-density lightly cross-linked polyacrylate that is foamed and converted into a dry powder. It is lightweight, added in low amounts to regolith simulant, and efficiently donates enough oxygen throughout the volume of hydrated regolith simulant to fully support seed germination and plant growth. Germination rates, plant development, and plant mass are nearly identical for L. sativum grown in 100% CO2 in the presence of the oxygen-donating lightly cross-linked polyacrylate compared with plants grown in air. The polymer system also serves to protect root structures and better anchors plants in the regolith simulant.

CO2 enrichment enhanced drought resistance by regulating growth, hydraulic conductivity and phytohormone contents in the root of cucumber seedlings

The coordinated effects of CO2 enrichment and drought stress on cucumber leaves have attracted increasing research attention, but few studies have investigated the effects of CO2 enrichment on the root system under drought stress. So we analyzed the morphological parameters, hydraulic conductivity, aquaporin-related gene expression, and endogenous phytohormone contents in roots of cucumber seedlings cultured under different CO2 concentrations (approximately 400 and 800 ± 40 μmol mol-1) and drought stresses simulated by polyethylene glycol 6000 (0%, 5%, and 10%). The results showed that under drought stress, regardless of the CO2 concentration, the root biomass and hydraulic conductivity decreased, the contents of auxin (IAA), zeatin nucleoside (ZR), and gibberellin (GA) decreased, the abscisic acid (ABA) content and the transcript levels of the aquaporin-related genes CsPIP2-4 increased, and the transcript levels of the aquaporin-related genes CsPIP2-5 and CsPIP2-7 decreased compared with no drought stress. Under moderate drought stress, CO2 enrichment decreased ABA content and the transcript level of CsPIP2-4, increased root biomass and GA content and the transcript level of CsPIP2-7, improved contribution rate of cell-to-cell water transport (mediated by aquaporins) and roots hydraulic conductivity. In summary, drought stress changed the water transport capacity of the roots and inhibited the growth of cucumber seedlings. CO2 enrichment regulated phytohormone contents and aquaporin-related gene expression, maintained the normal contribution rate of cell-to-cell water transport, and improved the root biomass and hydraulic conductivity, thereby alleviated the negative effects of drought stress on cucumber seedlings.