Impact of ultraviolet-B radiation on early-season morpho-physiological traits of indica and japonica rice genotypes.

Ultraviolet (UV)-B radiation is considered one of the major detrimental rays coming from the Sun. UV-B radiation has a harmful impact on plant growth and development. The effect of UV-B radiation was studied on 64 rice (Oryza sativa L.) genotypes during the vegetative season. An equal number of genotypes from the japonica (50%) and indica (50%) subspecies were phenotyped using the Soil-Plant-Atmosphere-Research (SPAR) units. The 10 kJ UV-B was imposed 12 days after planting (DAP) and continued for three weeks (21 d). Based on the combined ultraviolet-B radiation response index (CUVBRI) for each genotype, the 64 rice genotypes were classified into sensitive, moderately sensitive, moderately tolerant, and tolerant. Various shoot traits, such as plant height, tiller, and leaf numbers, were measured. We also studied critical root phenological traits like root volume, diameter, tips, and forks. Out of all the studied shoot traits, leaf area showed maximum reduction for both indica (54%) and japonica (48%). Among the root traits, root length decreased by negligible (1%) for indica as compared to japonica (5%), while root crossing and forks showed a maximum decline for japonica (37 and 42%), respectively. This study is timely, meaningful, and required because it will help breeders select a tolerant or sensitive rice line for better yield and production under abiotic stresses.

Chlorophyll fluorescence is a potential indicator to measure photochemical efficiency in early to late soybean maturity groups under changing day lengths and temperatures.

In this study, we employed chlorophyll a fluorescence technique, to indicate plant health and status in response to changing day lengths (photoperiods) and temperatures in soybean early and late maturity groups. Chlorophyll a fluorescence study indicates changes in light reactions in photosystem II. Experiments were performed for 3-day lengths (12.5, 13.5, and 14.5 h) and five temperatures (22/14°C, 26/18°C, 30/22°C, 34/26°C, and 40/32°C), respectively. The I-P phase declined for changing day lengths. Active reaction centers decreased at long day length for maturity group III. We observed that low temperatures impacted the acceptor side of photosystem II and partially impacted electron transport toward the photosystem I end electron acceptor. Results emphasized that higher temperatures (40/32°C) triggered damage at the oxygen-evolving complex and decreased electron transport and photosynthesis. We studied specific leaf areas and aboveground mass. Aboveground parameters were consistent with the fluorescence study. Chlorophyll a fluorescence can be used as a potential technique for high-throughput phenotyping methods. The traits selected in the study proved to be possible indicators to provide information on the health status of various maturity groups under changing temperatures and day lengths. These traits can also be deciding criteria for breeding programs to develop inbreed soybean lines for stress tolerance and sensitivity based on latitudinal variations.

Phytoremediation efficacy of native vegetation for nutrients and heavy metals on soils amended with poultry litter and fertilizer.

Poultry litter on agricultural lands could introduce nitrogen (N), phosphorus (P), heavy metals in soil and ground water. Native vegetations were identified to assess efficacy for phytoremediation of nutrients and metals from soil and water. Objective was to measure capability of multi-year native species to remove metals, nutrients, and prevent Nitrate-N leaching below the rooting zone. Treatments were distributed in four replicates with/without fertilization. Suction lysimeters were installed at 30, 60, and 90-cm depths in 3 of 4 replicates. Species were identified, recorded, five specified cuttings sampled. Plant, soil, water samples were prepared and analyzed by spectroscopy. Nitrate-N extraction, nitrates in water samples were determined using flow injection analysis. Fertilized plots (NVM) had 39% more biomass yield than unfertilized plots (NVN). In plants, nutrient and metal concentrations varied significantly with 14% increase in Zn, 36% and 26% in K and Mg over NVN for first and second year. Uneven between NVM and NVN, topsoil had higher values for most nutrients and metals. Largest P and (NO3-)-N in plant and water were observed from NVM. Cultivation of native vegetation appears to be an effective approach for remediation of excess nitrates-N, P, heavy metals from surface and sub-surface zones of the soil.

Native vegetation has been used for soil fertility, specific reasons like the removal of pesticides or agrochemicals, and other chemical related exposures. Studies on the use of native vegetation for phytoremediation on agricultural lands are uncommon. This research looked at the capability of native vegetation of different species as a viable tool for the remo+val of excess nutrients and heavy metals from agricultural lands. Results indicated native vegetation can take up significant amounts of excess nutrients from soils, proportional to their biomass accumulation. Native Vegetation was therefore found to be a nutrient sink, capable of removing excess nutrients/metals from the soil.

Drought-Induced Responses in Maize under Different Vapor Pressure Deficit Conditions.

Water stress in plants depends on the soil water level and the evaporative demand. In this study, the physiological, biochemical, and molecular response of maize were examined under three evaporative demand conditions (low­1.00 kPa, medium­2.2 kPa, and high­4.00 kPa Vapor pressure deficit (VPD)) at three different soil water content (SWC); well-watered, 45%, and 35% SWC. Plants grown at 35% SWC under high VPD had significant (p < 0.01) lower leaf weight, leaf area, and leaf number than low VPD. Plants under low, medium, and high VPD with drought stress (45% and 35% SWC) showed a 30 to 60% reduction in their leaf area compared to well-watered plants. Gas exchange parameters including photosynthesis, stomatal conductance, and water use efficiency exhibited significant differences (p < 0.01) between treatments, with the highest reduction occuring at 35% SWC and high VPD. Both drought and VPD significantly (p < 0.01) increased C4 enzyme levels and some transcription factors with increased stress levels. Transcription factors primarily related to Abssisic Acid (ABA) synthesis were upregulated under drought, which might be related to high ABA levels. In summary, severe drought levels coupled with high VPD had shown a significant decrease in plant development by modifying enzymes, ABA, and transcription factors.

Morpho-Physiological, Yield, and Transgenerational Seed Germination Responses of Soybean to Temperature.

Temperature is the primary factor affecting the morpho-physiological, developmental, and yield attributes of soybean. Despite several temperature and soybean studies, functional relationships between temperature and soybean physiology and yield components are limited. An experiment was conducted to determine the optimum temperature for soybean gas exchange and yield components using indeterminate (Asgrow AG5332, AG) and determinate (Progeny P5333 RY, PR) growth habit cultivars. Plants grown outdoors were exposed to 5 day/night temperature treatments, 21/13, 25/17, 29/21, 33/25, and 37°C/29°C, from flowering to maturity using the sunlit plant growth chambers. Significant temperature and cultivar differences were recorded among all measured parameters. Gas exchange parameters declined with increasing temperature treatments during the mid-pod filling stage, and quadratic functions best described the response. The optimum temperature for soybean pod weight, number, and seed number was higher for AG than PR, indicating greater high-temperature tolerance. Soybean exposed to warmer parental temperature (37°C/29°C) during pod filling decreased significantly the transgenerational seed germination when incubated at 18, 28, and 38°C. Our findings suggest that the impact of temperature during soybean development is transferable. The warmer temperature has adverse transgenerational effects on seed germination ability. Thus, developing soybean genotypes tolerant to high temperatures will help growers to produce high-yielding and quality beans. The quantified temperature, soybean physiology, and yield components-dependent functional algorithms would be helpful to develop adaptation strategies to offset the impacts of extreme temperature events associated with future climate change.

Effect of High-Temperature Stress on Plant Physiological Traits and Mycorrhizal Symbiosis in Maize Plants.

Increasing high temperature (HT) has a deleterious effect on plant growth. Earlier works reported the protective role of arbuscular mycorrhizal fungi (AMF) under stress conditions, particularly influencing the physiological parameters. However, the protective role of AMF under high-temperature stress examining physiological parameters with characteristic phospholipid fatty acids (PLFA) of soil microbial communities including AMF has not been studied. This work aims to study how high-temperature stress affects photosynthetic and below-ground traits in maize plants with and without AMF. Photosynthetic parameters like quantum yield of photosystem (PS) II, PSI, electron transport, and fractions of open reaction centers decreased in HT exposed plants, but recovered in AMF + HT plants. AMF + HT plants had significantly higher AM-signature 16:1ω5cis neutral lipid fatty acid (NLFA), spore density in soil, and root colonization with lower lipid peroxidation than non-mycorrhizal HT plants. As a result, enriched plants had more active living biomass, which improved photosynthetic efficiency when exposed to heat. This study provides an understanding of how AM-mediated plants can tolerate high temperatures while maintaining the stability of their photosynthetic apparatus. This is the first study to combine above- and below-ground traits, which could lead to a new understanding of plant and rhizosphere stress.

Impact of water stress under ambient and elevated carbon dioxide across three temperature regimes on soybean canopy gas exchange and productivity.

The present study investigated the interactive effects of three environmental stress factors elevated CO2, temperature, and drought stress on soybean growth and yield. Experiments were conducted in the sunlit, controlled environment Soil-Plant-Atmosphere-Research chambers under two-level of irrigation (WW-well water and WS-water stress-35%WW) and CO2 (aCO2-ambient 400 µmol mol-1 and eCO2-elevated 800 µmol mol-1) and each at the three day/night temperature regimes of 24/18 °C (MLT-moderately low), 28/22 °C (OT-optimum), and 32/26 °C (MHT-moderately high). Results showed the greatest negative impact of WS on plant traits such as canopy photosynthesis (PCnet), total dry weight (TDwt), and seed yield. The decreases in these traits under WS ranged between 40 and 70% averaged across temperature regimes with a greater detrimental impact in plants grown under aCO2 than eCO2. The MHT had an increased PCnet, TDwt, and seed yield primarily under eCO2, with a greater increase under WW than WS conditions. The eCO2 stimulated PCnet, TDwt, and seed yield more under WS than WW. For instance, on average across T regimes, eCO2 stimulated around 25% and 90% dry mass under WW and WS, respectively, relative to aCO2. Overall, eCO2 appears to benefit soybean productivity, at least partially, under WS and the moderately warmer temperature of this study.

Regulation of Photosystem II Heterogeneity and Photochemistry in Two Cultivars of C4 Crop Sugarcane Under Chilling Stress.

In subtropical regions, chilling stress is one of the major constraints for sugarcane cultivation, which hampers yield and sugar production. Two recently released sugarcane cultivars, moderately chilling tolerant Guitang 49 and chilling tolerant Guitang 28, were selected. The experiments were conducted in the controlled environment, and seedlings were exposed to optimum (25°C/15°C), chilling (10°C/5°C), and recovery (25°C/15°C) temperature conditions. PSII heterogeneity was studied in terms of reducing side and antenna size heterogeneity. Under chilling, reducing side heterogeneity resulted in increased number of QB non-reducing centers, whereas antenna side heterogeneity resulted in enhanced number of inactive ß centers in both cultivars, but the magnitude of change was higher in Guitang 49 than Guitang 28. Furthermore, in both cultivars, quantum efficiency of PSII, status of water splitting complex, and performance index were adversely affected by chilling, along with reduction in net photosynthesis rate and nighttime respiration and alterations in leaf optical properties. The extents of negative effect on these parameters were larger in Guitang 49 than in Guitang 28. These results reveal a clear differentiation in PSII heterogeneity between differentially chilling tolerant cultivars. Based on our studies, it is concluded that PSII heterogeneity can be used as an additional non-invasive and novel technique for evaluating any type of environmental stress in plants.

Varying Atmospheric CO2 Mediates the Cold-Induced CBF-Dependent Signaling Pathway and Freezing Tolerance in Arabidopsis.

Changes in the stomatal aperture in response to CO2 levels allow plants to manage water usage, optimize CO2 uptake and adjust to environmental stimuli. The current study reports that sub-ambient CO2 up-regulated the low temperature induction of the C-repeat Binding Factor (CBF)-dependent cold signaling pathway in Arabidopsis (Arabidopsis thaliana) and the opposite occurred in response to supra-ambient CO2. Accordingly, cold induction of various downstream cold-responsive genes was modified by CO2 treatments and expression changes were either partially or fully CBF-dependent. Changes in electrolyte leakage during freezing tests were correlated with CO2's effects on CBF expression. Cold treatments were also performed on Arabidopsis mutants with altered stomatal responses to CO2, i.e., high leaf temperature 1-2 (ht1-2, CO2 hypersensitive) and ß-carbonic anhydrase 1 and 4 (ca1ca4, CO2 insensitive). The cold-induced expression of CBF and downstream CBF target genes plus freezing tolerance of ht1-2 was consistently less than that for Col-0, suggesting that HT1 is a positive modulator of cold signaling. The ca1ca4 mutant had diminished CBF expression during cold treatment but the downstream expression of cold-responsive genes was either similar to or greater than that of Col-0. This finding suggested that ßCA1/4 modulates the expression of certain cold-responsive genes in a CBF-independent manner. Stomatal conductance measurements demonstrated that low temperatures overrode low CO2-induced stomatal opening and this process was delayed in the cold tolerant mutant, ca1ca4, compared to the cold sensitive mutant, ht1-2. The similar stomatal responses were evident from freezing tolerant line, Ox-CBF, overexpression of CBF3, compared to wild-type ecotype Ws-2. Together, these results indicate that CO2 signaling in stomata and CBF-mediated cold signaling work coordinately in Arabidopsis to manage abiotic stress.

Stomatal closure response to soil drying at different vapor pressure deficit conditions in maize.

A plant transpiration rate under progressive soil drying remains constant until a threshold fraction of transpirable soil water (FTSW) is reached, and subsequently decreases linearly. The sensitivity of this function and the involvement of abscisic acid (ABA) and aquaporins in such responses have not been compared at various levels of atmospheric evaporative demand conditions. This study was conducted in controlled environment chambers with a drought-tolerant maize hybrid imposing progressive drought stress under three levels of vapor pressure deficit (VPD- 1.2, 2.3, and 3.5 kPa). A shift in threshold-FTSW from 1.2 kPa (FTSW-0.42) VPD to 3.5 kPa(FTSW-0.51) VPD was observed, showing an effect of VPD on stomatal closure response under soil drought conditions. Foliar ABA showed a substantial rise approximately at the same time as of stomatal closure initiated (FTSW-threshold), indicating ABA involvement. As the drought progressed, an increase in plasma membrane intrinsic protein and a decrease in tonoplast intrinsic protein expression levels were observed. Overall, this study suggests the influence of evaporative demand on the initiation of stomatal closure of drought-tolerant maize subjected to soil drying. The sensitivity of stomatal closure was associated with foliar ABA under drought stress but not under high evaporative demand conditions, indicating alternative water conservative mechanisms.

Grain Inorganic Arsenic Content in Rice Managed Through Targeted Introgressions and Irrigation Management.

Arsenic (As) accumulation in rice grain is a significant public health concern. Inorganic As (iAs) is of particular concern because it has increased toxicity as compared to organic As. Irrigation management practices, such as alternate wetting and drying (AWD), as well as genotypic differences between cultivars, have been shown to influence As accumulation in rice grain. A 2 year field study using a Lemont × TeQing backcross introgression line (TIL) mapping population examined the impact of genotype and AWD severity on iAs grain concentrations. The "Safe"-AWD [35-40% soil volumetric water content (VWC)] treatment did not reduce grain iAs levels, whereas the more severe AWD30 (25-30% VWC) consistently reduced iAs concentrations across all genotypes. The TILs displayed a range of iAs concentrations by genotype, from less than 10 to up to 46 µg kg-1 under AWD30 and from 28 to 104 µg kg-1 under Safe-AWD. TIL grain iAs concentrations for flood treatments across both years ranged from 26 to 127 µg kg-1. Additionally, seven quantitative trait loci (QTLs) were identified in the mapping population associated with grain iAs. A subset of eight TILs and their parents were grown to confirm field-identified grain iAs QTLs in a controlled greenhouse environment. Greenhouse results confirmed the genotypic grain iAs patterns observed in the field; however, iAs concentrations were higher under greenhouse conditions as compared to the field. In the greenhouse, the number of days under AWD was negatively correlated with grain iAs concentrations. Thus, longer drying periods to meet the same soil VWC resulted in lower grain iAs levels. Both the number and combinations of iAs-affecting QTLs significantly impacted grain iAs concentrations. Therefore, identifying more grain iAs-affecting QTLs could be important to inform future breeding efforts for low iAs rice varieties. Our study suggests that coupling AWD practices targeting a soil VWC of less than or equal to 30% coupled with the use of cultivars developed to possess multiple QTLs that negatively regulate grain iAs concentrations will be helpful in mitigating exposure of iAs from rice consumption.

De novo characterization of the Goji berry (Lycium barbarium L.) fruit transcriptome and analysis of candidate genes involved in sugar metabolism under different CO2 concentrations.

Goji berry (Lycium barbarum L.) is one of the important economic crops due to its exceptional nutritional value and medicinal benefits. Although reduced sugar levels in goji berry exposed to long-term elevated carbon dioxide (CO2) have been documented, the underlying molecular mechanisms remain unknown. The objective of this study was to explore the transcriptome of goji berry fruit under ambient and elevated CO2 concentrations and further to screen the differentially expressed genes (DEGs) for functions related to sugar metabolism. Fruit samples from goji berry exposed to ambient (400 µmol mol-1) and elevated (700 µmol mol-1) levels of CO2 for 120 days were analyzed for total sugar, carotenoid and flavone analysis. In this study, a reduction in total sugar and carotenoid levels in the fruits grown under elevated CO2 levels were observed. Fruit samples were also used to construct cDNA libraries using a HiSeqTM2500 platform. Consequently, 81,100 unigenes were assembled, of which 35,111 (43.3%) were annotated using various databases. Through DEGs analysis, it was found that 55 genes were upregulated and 18 were down-regulated in response to elevated CO2 treatment. Genes involved in the sugar metabolism and the related pathways were identified by Gene Ontology and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis. Furthermore, three genes, LBGAE (Lycium barbarum UDP-glucuronate 4-epimerase), LBGALA (Lycium barbarum alpha-galactosidase) and LBMS (Lycium barbarum malate synthase), associated with sugar metabolism were identified and discussed with respect to the reduction in the total sugar levels along with the enzymes acid invertase (AI), sucrose synthase (SS) and sucrose phosphate synthase (SPS) of the sucrose metabolism. This study can provide gene sources for elucidating the molecular mechanisms of sugar metabolism in the fruit of goji berry under elevated CO2.

Transpiration Response of Cotton to Vapor Pressure Deficit and Its Relationship With Stomatal Traits.

Many studies have demonstrated that the cotton in warm environments is vulnerable to water-limitations thus reducing the yield. A number of plant traits have been recommended to ameliorate the effects of water deficits on plant growth and yield. Limitation on maximum transpiration rate (TR) under high vapor pressure deficit (VPD), usually occurs during midday, is often considered as a water conservation trait. The genotypes with this trait are desirable in high VPD environments where water deficits commonly develop in the later part of the growing season. Our objective of the study was to find the genotypic variation for the trait limited TR under high VPD and also to study leaf temperature, water potential, photosynthesis, and stomatal conductance responses. Also, our objective was also to study the structural changes in the stomatal traits when exposed to long term high VPD conditions and involvement in such responses. In the present study, 17 cotton genotypes were studied for their (TR) response to various VPD environments under well irrigated conditions. Out of 17, eight genotypes limited TR after approximately 2 kPa VPD and rest of them increased their TR with increased VPD. Five selected genotypes with different TR response to increasing VPD were further studied for gas exchange and stomatal properties. All genotypes, irrespective of exhibiting limited TR at high VPD, reduced stomatal conductance, photosynthesis and water potential at high VPD of 3.3 kPa. The genotypes with limited TR modified their stomatal traits mostly on the adaxial surface with frequent and small stomata under high VPD. The genotypes with limited TR also exhibited an increase in epidermal cell expansion and stomatal index at contrasting VPD gradients to effectively balance the liquid and vapor phase conductance to limit TR at high VPD.

Phosphorus Nutrition Affects Temperature Response of Soybean Growth and Canopy Photosynthesis.

In nature, crops such as soybean are concurrently exposed to temperature (T) stress and phosphorus (P) deficiency. However, there is a lack of reports regarding soybean response to T × P interaction. To fill in this knowledge-gap, soybean was grown at four daily mean T of 22, 26, 30, and 34°C (moderately low, optimum, moderately high, and high temperature, respectively) each under sufficient (0.5 mM) and deficient (0.08 mM) P nutrition for the entire season. Phosphorus deficiency exacerbated the low temperature stress, with further restrictions on growth and net photosynthesis. For P deficient soybean at above optimum temperature (OT) regimes, growth, and photosynthesis was maintained at levels close to those of P sufficient plants, despite a lower tissue P concentration. P deficiency consistently decreased plant tissue P concentration ≈55% across temperatures while increasing intrinsic P utilization efficiency of canopy photosynthesis up to 147%, indicating a better utilization of tissue P. Warmer than OTs delayed the time to anthesis by 8-14 days and pod development similarly across P levels. However, biomass partitioning to pods was greater under P deficiency. There were significant T × P interactions for traits such as plant growth rates, total leaf area, biomass partitioning, and dry matter production, which resulted a distinct T response of soybean growth between sufficient and deficient P nutrition. Under sufficient P level, both lower and higher than optimum T tended to decrease total dry matter production and canopy photosynthesis. However, under P-deficient condition, this decrease was primarily observed at the low T. Thus, warmer than optimum T of this study appeared to compensate for decreases in soybean canopy photosynthesis and dry matter accumulation resulting from P deficiency. However, warmer than OT appeared to adversely affect reproductive structures, such as pod development, across P fertilization. This occurred despite adaptations, especially the increased P utilization efficiency and biomass partitioning to pods, shown by soybean under P deficiency.

Co-regulation of photosynthetic processes under potassium deficiency across CO2 levels in soybean: mechanisms of limitations and adaptations.

Plants photosynthesis-related traits are co-regulated to capture light and CO2 to optimize the rate of CO2 assimilation (A). The rising CO2 often benefits, but potassium (K) deficiency adversely affects A that contributes to the majority of plant biomass. To evaluate mechanisms of photosynthetic limitations and adaptations, soybean was grown under controlled conditions with an adequate (control, 5.0 mM) and two K-deficient (moderate, 0.50 and severe, 0.02 mM) levels under ambient (aCO2; 400 µmol mol-1) and elevated CO2 (eCO2; 800 µmol mol-1). Results showed that under severe K deficiency, pigments, leaf absorption, processes of light and dark reactions, and CO2 diffusion through stomata and mesophyll were down co-regulated with A while light compensation point increased and photorespiration, alternative electron fluxes, and respiration were up-regulated. However, under moderate K deficiency, these traits were well co-regulated with the sustained A without any obvious limitations amid ≈ 50% reduction in leaf K level. Primary mechanism of K limitation to A was either biochemical processes (Lb ≈ 60%) under control and moderate K deficiency or the CO2 diffusion limitations (DL ≈ 70%) with greater impacts of mesophyll than stomatal pathways under severe K deficiency. The eCO2 increased DL while lessened the Lb under K deficiency. Adaptation strategies to severe K deficiency included an enhanced K utilization efficiency (KUE), and reduction of photosystem II excitation pressure by decreasing photosynthetic pigments, light absorption, and photochemical quenching while increasing photorespiration and alternative electron fluxes. The eCO2 also stimulated A and KUE when K deficiency was not severe. Thus, plants responded to K deficiency by a coordinated regulation of photosynthetic processes to optimize A, and eCO2 failed to alleviate the DL in severely K-deficient plants.

Potassium Starvation Limits Soybean Growth More than the Photosynthetic Processes across CO2 Levels.

Elevated carbon dioxide (eCO2) often enhances plant photosynthesis, growth, and productivity. However, under nutrient-limited conditions the beneficial effects of high CO2 are often diminished. To evaluate the combined effects of potassium (K) deficiency and eCO2 on soybean photosynthesis, growth, biomass partitioning, and yields, plants were grown under controlled environment conditions with an adequate (control, 5.0 mM) and two deficient (0.50 and 0.02 mM) levels of K under ambient CO2 (aCO2; 400 µmol mol-1) and eCO2 (800 µmol mol-1). Results showed that K deficiency limited soybean growth traits more than photosynthetic processes. An ~54% reduction in leaf K concentration under 0.5 mM K vs. the control caused about 45% less leaf area, biomass, and yield without decreasing photosynthetic rate (Pnet). In fact, the steady photochemical quenching, efficiency, and quantum yield of photosystem II, chlorophyll concentration (TChl), and stomatal conductance under 0.5 mM K supported the stable Pnet. Biomass decline was primarily attributed to the reduced plant size and leaf area, and decreased pod numbers and seed yield in K-deficient plants. Under severe K deficiency (0.02 mM K), photosynthetic processes declined concomitantly with growth and productivity. Increased specific leaf weight, biomass partitioning to the leaves, decreased photochemical quenching and TChl, and smaller plant size to reduce the nutrient demands appeared to be the means by which plants adjusted to the severe K starvation. Increased K utilization efficiency indicated the ability of K-deficient plants to better utilize the tissue-available K for biomass accumulation, except under severe K starvation. The enhancement of soybean growth by eCO2 was dependent on the levels of K, leading to a K × CO2 interaction for traits such as leaf area, biomass, and yield. A lack of eCO2-mediated growth and photosynthesis stimulation under severe K deficiency underscored the importance of optimum K fertilization for maximum crop productivity under eCO2. Thus, eCO2 compensated, at least partially, for the reduced soybean growth and seed yield under 0.5 mM K supply, but severe K deficiency completely suppressed the eCO2-enhanced seed yield.

Soybean grown under elevated CO2 benefits more under low temperature than high temperature stress: Varying response of photosynthetic limitations, leaf metabolites, growth, and seed yield.

To evaluate the combined effect of temperature and CO2 on photosynthetic processes, leaf metabolites and growth, soybean was grown under a controlled environment at low (22/18°C, LT), optimum (28/24°C, OT) and high (36/32°C HT) temperatures under ambient (400µmolmol-1; aCO2) or elevated (800µmolmol-1; eCO2) CO2 concentrations during the reproductive stage. In general, the rate of photosynthesis (A), stomatal (gs) and mesophyll (gm) conductance, quantum yield of photosystem II, rates of maximum carboxylation (VCmax), and electron transport (J) increased with temperature across CO2 levels. However, compared with OT, the percentage increases in these parameters at HT were lower than the observed decline at LT. The photosynthetic limitation at LT and OT was primarily caused by photo-biochemical processes (49-58%, Lb) followed by stomatal (27-32%, Ls) and mesophyll (15-19%, Lm) limitations. However, at HT, it was primarily caused by Ls (41%) followed by Lb (33%) and Lm (26%). The dominance of Lb at LT and OT was associated with the accumulation of non-structural carbohydrates (e.g., starch) and several organic acids, whereas this accumulation did not occur at HT, indicating increased metabolic activities. Compared with OT, biomass and seed yield declined more at HT than at LT. The eCO2 treatment compensated for the temperature-stress effects on biomass but only partially compensated for the effects on seed yield, especially at HT. Photosynthetic downregulation at eCO2 was possibly due to the accumulation of non-structural carbohydrates and the decrease in gs and Astd (standard A measured at 400µmolmol-1 sub-stomatal CO2 concentration), as well as the lack of CO2 effect on gm, VCmax, and J, and photosynthetic limitation. Thus, the photosynthetic limitation was temperature-dependent and was primarily influenced by the alteration in photo-biochemical processes and metabolic activities. Despite the inconsistent response of photosynthesis (or biomass accumulation) and seed yield, eCO2 tended to fully or partially compensate for the adverse effect of the respective LT and HT stresses under well-watered and sufficient nutrient conditions.

Random Forests for Global and Regional Crop Yield Predictions.

Accurate predictions of crop yield are critical for developing effective agricultural and food policies at the regional and global scales. We evaluated a machine-learning method, Random Forests (RF), for its ability to predict crop yield responses to climate and biophysical variables at global and regional scales in wheat, maize, and potato in comparison with multiple linear regressions (MLR) serving as a benchmark. We used crop yield data from various sources and regions for model training and testing: 1) gridded global wheat grain yield, 2) maize grain yield from US counties over thirty years, and 3) potato tuber and maize silage yield from the northeastern seaboard region. RF was found highly capable of predicting crop yields and outperformed MLR benchmarks in all performance statistics that were compared. For example, the root mean square errors (RMSE) ranged between 6 and 14% of the average observed yield with RF models in all test cases whereas these values ranged from 14% to 49% for MLR models. Our results show that RF is an effective and versatile machine-learning method for crop yield predictions at regional and global scales for its high accuracy and precision, ease of use, and utility in data analysis. RF may result in a loss of accuracy when predicting the extreme ends or responses beyond the boundaries of the training data.

Methods of mesophyll conductance estimation: its impact on key biochemical parameters and photosynthetic limitations in phosphorus-stressed soybean across CO2.

Despite the development of various methods, the rapid estimation of mesophyll conductance (gm ) for a large number of samples is still a daunting challenge. Although the accurate estimation of gm is critical to partition photosynthetic limitations by stomatal (Ls ) and mesophyll (Lm ) conductance and by photo-biochemical (Lb ) processes, the impact of various gm estimation methods on this is ambiguous. As phosphorus (P) starvation and elevated CO2 (eCO2 ) strongly affect photosynthetic processes, their combined effect on the proportional changes in these limitations are not well understood. To investigate this, while also evaluating distinct recent methods of gm estimation sharing few common theories and assumptions, soybean was grown under a range of P nutrition at ambient and eCO2 . Methods significantly affected gm and carboxylation efficiency (VCmax ) but not other photosynthetic parameters. In all the methods, all photosynthetic parameters responded similarly to treatments. However, the percentage difference between VCmax assuming finite and infinite gm was highly inconsistent among methods. The primary mechanism responsible for P limitation to soybean photosynthesis was not CO2 diffusion limitations but Lb comprised of reduced chlorophyll, photochemistry and biochemical processes. The eCO2 decreased Lb but increased Lm without affecting Ls across leaf P concentration. Although each method explored advances of our understanding about gm variability, they all require assumptions of varying degrees, which lead to the discrepancy in the gm values. Among the methods, the oxygen sensitivity-based gm estimation appeared to be suitable for the quick assessment of a large number of samples or genotypes. Digital tools are provided for the easy estimation of gm for some methods.

Varying Response of the Concentration and Yield of Soybean Seed Mineral Elements, Carbohydrates, Organic Acids, Amino Acids, Protein, and Oil to Phosphorus Starvation and CO2 Enrichment.

A detailed investigation of the concentration (e.g., mg g-1 seed) and total yield (e.g., g plant-1) of seed mineral elements and metabolic profile under phosphorus (P) starvation at ambient (aCO2) and elevated carbon dioxide (eCO2) in soybean is limited. Soybean plants were grown in a controlled environment at either sufficient (0.50 mM P, control) or deficient (0.10 and 0.01 mM, P-stress) levels of P under aCO2 and eCO2 (400 and 800 µmol mol-1, respectively). Both the concentration and yield of 36 out of 38 seed components responded to P treatment and on average 25 and 11 components increased and decreased, respectively, in response to P starvation. Concentrations of carbohydrates (e.g., glucose, sugar alcohols), organic acids (e.g., succinate, glycerate) and amino acids increased while oil, and several minerals declined under P deficiency. However, the yield of the majority of seed components declined except several amino acids (e.g., phenylalanine, serine) under P deficiency. The concentration-based relationship between seed protein and oil was negative (r2 = 0.96), whereas yield-based relationship was positive (r2 = 0.99) across treatments. The CO2 treatment also altered the concentration of 28 out of 38 seed components, of which 23 showed decreasing (e.g., sucrose, glucose, citrate, aconitate, several minerals, and amino acids) while C, iron, Mn, glycerate, and oil showed increasing trends at eCO2. Despite a decreased concentration, yields of the majority of seed components were increased in response to eCO2, which was attributable to the increased seed production especially near sufficient P nutrition. The P × CO2 interactions for the concentration of amino acids and the yield of several components were due to the lack of their response to eCO2 under control or the severe P starvation, respectively. Thus, P deficiency primarily reduced the concentration of oil and mineral elements but enhanced a majority of other components. However, seed components yield consistently declined under P starvation except for several amino acids. The study highlighted a P nutritional-status dependent response of soybean seed components to eCO2 suggesting the requirement of an adequate P supply to obtain the beneficial effects of eCO2 on the overall yield of various seed components.