Salt-affected Conditions Research Articles

Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands

Agriculture is a main source of nitrous oxide (N2O) emissions. In agricultural systems, direct N2O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N2O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 μg N m−2 h−1) were significant sources for N2O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N2O fluxes in summer, while precipitation events decreased N2O fluxes. Agronomic management strategies, including fertilization (294–-540 kg N hm−2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N2O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N2O fluxes (202–233 μg N m−2 h−1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100–114 μg N m−2 h−1). Furthermore, high available carbon (C) relative to N reduced N2O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N2O emission factor (EF5) across ditches with higher N resulted in lower EF5 and a large coefficient of variation (CV) range. EF5 was 0.0011 for the ditches in this region, while the EF5 (0.0025) currently adopted by the IPCC is relatively high. The EF5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO3−-N, while salinity was also a nonnegligible factor regulating the EF5 variation. The regression model incorporating NO3−-N and the DOC:DIN ratio could greatly enhance the predictions of EF5 for agricultural ditches. Our study filled a key knowledge gap regarding EF5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF5 currently used by the IPCC.

Silicon application mitigated the adverse effects of salt stress and deficit irrigation on drip-irrigated greenhouse tomato

Tomato (Solanum lycopersicum L.) is an important vegetable crop around the world, and water scarcity and soil salinization are two major unfavorable factors limiting the fruit yield and quality of greenhouse tomato in arid and semi-arid regions. The application of exogenous silicon has been reported to enhance tomato yield under salt or drought stress, but its effect under combined salt-drought stress remains unclear. A two-season (autumn 2021 and spring 2022) experiment was conducted with three irrigation levels (I1, 90%−100% θf, where θf is the field capacity; I2, 70%−80% θf; I3, 50%−60% θf), two soil salinity levels (S0, 0.1%, g/g; S1, 0.4%, g/g) and two silicon application rates (T0, 0 mM; T1, 2.4 mM) to explore their combined effects on leaf silicon content, photosynthetic parameters, fruit yield, nutrient yield, crop water productivity (WP), nitrogen uptake and utilization, nutrient water productivity (NWP) and economic benefit of drip-irrigated greenhouse tomato. The results showed that deficit irrigation and salt stress exerted significant inhibiting effects on leaf silicon content, photosynthetic parameters, tomato yield, and water-nitrogen productivity, but the inhibition was alleviated by silicon application, which increased leaf silicon content by 67.0%, net photosynthetic rate (Pn) by 25.3%, fruit yield by 12.3%, WP by 11.9%, nutrient yield by 10.0% (titratable acids) - 27.4% (soluble sugars) and NWP by 9.2% (titratable acids) - 26.1% (soluble sugars) on average. The average fruit yield over the two growing seasons was 66.01, 54.29 and 44.31 t ha−1 under I1, I2 and I3, and the silicon application increased average leaf silicon content by 71.9%, 65.2% and 63.0%, Pn by 11.1%, 12.9% and 7.6%, and fruit yield by 12.8%, 16.2%, and 8.1% at the three irrigation levels, respectively. Plant nitrogen uptake (NU) and nitrogen partial factor productivity (NPFP) increased but nitrogen use efficiency (NUE) tended to decrease with the increasing irrigation level, while silicon application improved NUE by 3.9% and 2.2% and NPFP by 13.8% and 11.7% in the two growing seasons, respectively. Silicon application significantly increased net profit by 24.3% and 17.7% in autumn 2021 and spring 2022, respectively. According to the principal component analysis, I1T1S0 ranked first in both growing seasons. The correlation matrix showed that fruit yield was positively correlated with leaf silicon content, Pn, stomatal conductance, WP, NU, NUE, NPFP, nutrient yield and net profit, while it had negative correlation with NWP. In conclusion, exogenous silicon application can improve the fruit yield and nutrient yield of drip-irrigated greenhouse tomato by enhancing leaf silicon content and photosynthetic capability under drought-stressed and salt-affected conditions.

Exogenous silicon application improves fruit yield and quality of drip-irrigated greenhouse tomato by regulating physiological characteristics and growth under combined drought and salt stress

Improving crop productivity in drought-stressed and salt-affected regions is the key to promoting sustainable agriculture. Silicon is considered to improve the stress tolerance of crops, but how exogenous silicon application affects the fruit yield and quality of tomato under combined drought and salt stress remains unclear. A greenhouse experiment was conducted over two consecutive growing seasons (autumn 2021 and spring 2022) to investigate the effects of exogenous silicon application on leaf greenness, photosynthesis and chlorophyll fluorescence parameters, fruit yield and quality of drip-fertigated greenhouse tomato under various drought and salt stresses. There were three irrigation levels: I1 (90–100% θf, where θf is the field capacity), I2 (70%–80% θf) and I3 (50%–60% θf), two soil salinity levels: S0 (0.1%) and S1 (0.4%), and two silicon application rates: T0 (0 mmol L−1) and T1 (2.4 mmol L−1), resulting in a total of 12 treatments with three replicates. The results showed that the combined drought and salt stress significantly reduced leaf silicon content, plant height, leaf area index, aboveground biomass, net photosynthetic rate, transpiration rate and stomatal conductance of tomato plants, while the exogenous silicon application promoted these parameters. Irrigation level, salinity level, silicon rate and the interaction of irrigation level and salinity level had significant effects on fruit yield. Compared to S0, S1 decreased fruit yield by 18.42%, 17.91% and 20.64% under I1, I2 and I3, respectively. Exogenous silicon application significantly improved fruit yield by 6.52% (I3S0) - 17.73% (I2S1). Irrigation level, salinity level and silicon rate had inconsistent effects on fruit quality. Exogenous silicon application significantly increased total soluble solids, Vitamin C, soluble sugars and lycopene content, without significant effect on titratable acid. The lower irrigation level and higher salinity level resulted in better fruit quality. The path analysis showed that leaf silicon content, net photosynthetic rate, lycopene, average fruit weight had significant direct positive effects on fruit yield. The results are informative for improving drip-fertigated greenhouse tomato productivity and quality under drought-stressed and salt-affected conditions.

Plant growth-promoting bacterial consortia improved the physiology and growth of maize by regulating osmolytes and antioxidants balance under salt-affected field conditions

This study was undertaken to see how microbial consortia influenced maize development and yield under salt-affected conditions. The efficacy of the pre-isolated bacterial strains Burkholderia phytofirmans, Bacillussubtilis, Enterobacter aerogenes, and Pseudomonas syringae and Pseudomonas fluorescens to decrease the detrimental effects of salt on maize was tested in four distinct combinations using Randomized Complete Block Design with three replicates. The results revealed that these strains were compatible and collaborated synergistically, with an 80% co-aggregation percentage under salt-affected conditions. Following that, these strains were tested for their ability to increase maize growth and yield under salt-affected field conditions. The photosynthetic rate (11–50%), relative water content (10–34%), and grain yield (13–21%) of maize were all increased by these various combinations. However, when Burkholderia phytofirmans, Enterobacter aerogenes and Pseudomonas fluorescens were combined, the greatest increase was seen above the un-inoculated control. Furthermore, as compared to the un-inoculated control, the same combination resulted in a 1.5-fold increase in catalase and a 2.0-fold increase in ascorbate concentration. These findings showed that a multi-strain consortium might boost maize's total yield response as a result of better growth under salt stress.

Development of ACCd producer A. brasilense mutant and the effect of inoculation on red pepper plants.

Azospirillum is a plant-growth-promoting bacterium capable of colonizing and promoting growth in numerous crops of agronomic and horticultural significance. The objective of the present study is to develop Azospirillum brasilense CW903-acdS flocculating cells and to test their performance in promoting the growth of red pepper plants grown under salt stress. The flocculating CW903-acdS recorded 12.6, 37.3 and 91.6% higher ACCd activity at 50, 100 and 150mM NaCl concentrations, respectively, compared to non-flocculating (normal) CW903-acdS cells. The flocculating CW903-acdS recorded 29.8 and 24.5% higher specific growth rates compared to non-flocculating CW903-acdS cells at 100 and 150mM NaCl concentration, respectively. The flocculating CW903-acdS recorded 29.7 and 24.5% higher production of IAA-like molecule compared to the non-flocculating CW903-acdS at 100 and 150mM NaCl concentration, respectively. Similarly, 27.5 and 25.7% higher ARA activity was observed with the flocculating CW903-acdS compared to the non-flocculating CW903-acdS type cells at 100 and 150mM NaCl concentration, respectively. In the pot culture experiment at 50 and 100mM NaCl concentration, CW903-acdSinoculated pepper plants recorded 9.4 and 4.7% less ethylene emission, when compared to plants inoculated with non-flocculating CW903-acdS cells. At 100mM NaCl concentration, plants inoculated with flocculating CW903-acdS recorded 27.5% higher dry weight compared to plants inoculated with non-flocculating CW903-acdS cells. This study implied the significance of flocculating CW903-acdS with better stress amelioration and plant growth promotion in red pepper plants grown under salt-affected conditions due to the positive influence of ACCd activity.

Soil Aggregate Stability in Salt-Affected Vineyards: Depth-Wise Variability Analysis

Soil aggregate stability is an ideal integrative soil quality indicator, but little is known about the relevance of such an indicator with soil depth for salt-affected soils. The objective of this study was to determine soil aggregate stability and identify preponderant aggregation factors, both in topsoil and subsoil horizons in salt-affected conditions. We conducted field investigations by describing soil profiles in pedological pits and by collecting soil samples from different field units. Soils were sampled within different soil horizon types, from superficial tilled organo-mineral horizons to mineral horizons. For all soil samples, we determined the mean weight diameter (MWD) as an indicator of soil aggregate stability and also determined associated physical and chemical properties in some samples. The measured MWD value from 0.28 mm to 1.10 mm could be categorised as unstable, with MWD values and variability decreasing drastically from the topsoil to the deepest mineral horizons. Analysis of MWD in relation to physical and chemical properties suggested that the variability in the MWD value of A-horizons was influenced by both clay fraction abundance and soil organic carbon (SOC) content and the nature of the agricultural practices, while at deeper B-horizons, the decrease in SOC content and the variability in other soil properties with soil depth could be used to explain the overall low aggregate stability. In this study, investigations of soil pits coupled with measurements of soil aggregate stability indicated that it could be possible to restore soil structure quality by limiting deep soil profile compaction in order to improve salt leaching and exportation.

Exogenous glutathione-mediated tolerance to deficit irrigation in salt-affected Capsicum frutescence (L.) plants is connected with higher antioxidant content and ionic homeostasis

As an important medicinal plant used in traditional and modern medicine, chili peppers are sensitive or moderately sensitive to drought or salt stress, respectively. Therefore, potential changes due to foliar-applied glutathione (GSH; 0, 0.4 and 0.8 mM) response on growth, yield, and physio-biochemical attributes, as well as water use efficiency (WUE) and fruit alkaloid capsaicin of chili pepper plants were investigated when grown under deficit irrigation in salt-affected soil (EC = 6.74 dS m–1). Two deficit irrigation water (DiW) regimes (80% and 60% of soil field capacity; FC) were used versus 100% of FC as a control. Both DiW treatments negatively affected growth and yield parameters, SPAD chlorophyll index, nutrient status, K+/Na+ ratio, and plant anatomical features. In contrast, osmoprotectants, ascorbate, glutathione, capsaicin, and phenolic contents, as well as WUE were increased in association with higher Na+ and Cl– contents. However, exogenously-applied GSH caused significant increases in the above-mentioned parameters along with an additional increase in osmoprotectants, antioxidants, and capsaicin contents, and a decrease in Na+ and Cl– levels compared to corresponding controls. The highest WUE, growth, and fruit yield responses were recorded at 0.8 mM GSH applied to plants under DiW at 80% FC + salinity (6.74 dS m–1). Therefore, this study suggested the use of leafy-applied GSH at 0.8 mM for satisfactory growth and yield with the highest WUE of chili pepper plants grown under salt-affected conditions with deficit irrigation.

Halotolerant potassium solubilizing plant growth promoting rhizobacteria may improve potassium availability under saline conditions.

Environmental change is one of the primary issues faced by the farming community. Low rainfall and high temperature in arid and semiarid regions lead to the development of secondary salinisation, thus making the problem more severe. Under saline conditions, sodium is the most crucial cation that competes with potassium (K) and adversely affects plant metabolism by inhibiting plant enzymatic activities. Potassium-solubilising bacteria (KSB) play a vital role in solubilising fixed potassium and making it accessible to plants. In the current study, 42 KSB strains were isolated from paddy rhizosphere soil grown under salt-affected conditions. The plant-growth-promoting (PGP) properties of these rhizobacteria were also evaluated. Thirteen KSB strains, positive for all tested PGP traits, were evaluated for potassium solubilisation under sodium stress, namely, 0%, 3%, 5% and 7% NaCl stress. The five best strains (Acinetobacter pittii strain L1/4, A. pittii strain L3/3, Rhizobium pusense strain L3/4, Cupriavidus oxalaticus strain L4/12 and Ochrobactrum ciceri strain L5/1) based on the K-solubilising potential were identified by amplification, sequencing and bioinformatic analysis of the 16S rDNA sequences. The maximum potassium solubilisation was measured at 30°C and pH7 with glucose as carbon source. The application of these KSB strains significantly improved the shoot length, fresh weight, dry weight and chlorophyll contents of paddy plants grown under saline conditions. Hence, these strains could be halotolerant KSB bioinoculants that can be used to protect plants against salt stress.

Ca2+/Na+ Ratio as a Critical Marker for Field Evaluation of Saline-Alkaline Tolerance in Alfalfa (Medicago sativa L.)

Current indices of saline-alkaline (SA) tolerance are mainly based on the traditional growth and physiological indices for salinity tolerance and likely affect the accuracy of alfalfa tolerance predictions. We determined whether the inclusion of soil alkalinity-affected indices, particularly Ca2+, Mg2+, and their ratios to Na+ in plants, based on the traditional method could improve the prediction accuracy of SA tolerance in alfalfa, determine important indices for SA tolerance, and identify suitable alfalfa cultivars in alkaline salt-affected soils. Fifty alfalfa cultivars were evaluated for their SA tolerance under SA and non-SA field conditions. The SA-tolerance coefficient (SATC) for each investigated index of the alfalfa shoot was calculated as the ratio of SA to non-SA field conditions, and the contribution of SATC under different growth and physiological indices to SA tolerance was quantified based on the inclusion/exclusion of special alkalinity-affected indices. The traditional method, excluding the special alkalinity-affected indices, explained nearly all of the variation in alfalfa SA tolerance, and the most important predictor was the SATC of stem length. The new method, which included these special alkalinity-affected indices, had similar explanatory power but instead identified the SATC of shoot Ca2+/Na+ ratio, followed by that of stem length, as key markers for the field evaluation of SA tolerance. Ca2+, Mg2+, and their ratios to Na+ hold promise for enhancing the robustness of SA-tolerance predictions in alfalfa. These results encourage further investigation into the involvement of Ca2+ in such predictions in other plant species and soil types under more alkaline salt-affected conditions.

Hanoi Cohesive Soil in Salt-Affected Conditions: Soil Properties and 2D Consolidation Analysis

The paper focuses on the geotechnical experiments and numerical simulation of 2D consolidation analysis for the cohesive soils fully saturated by solutions of four salt concentrations, i.e. 0.0, 9.9, 19.8, and 33.0 g/L. The experiments were carried out on undisturbed soil samples taken from the boreholes in Hanoi area where the subsoil has not been affected by saline intrusion yet. The composition and properties of the cohesive soil in salt-affected conditions were investigated with transmission electron microscopic method and other geotechnical experiments. Consolidation analysis with finite element method was applied to an assumed embankment on saline subsoil. The analytical results showed that swelling mineral of montlorillonite is present in the Hanoi cohesive soils. The water content of the studied soils decreases as the salt concentration increases. Meanwhile, the bulk density unclearly changes in the same testing conditions. The coefficient of consolidation reduces, respectively, with the salinities, from 22 to 50% as salinities rise up to 33.0 g/L. Compressibility index increases linearly and proportionally with the salinities. In 2D analytical model, the settlement or vertical displacement of the subsoil in HD1 site reaches 14.8% and that of GL2 site is 18.7% at saline solution of 33.0 g/L. The horizontal displacement of the subsoil reaches 13.90% in HD1 site and 17.55% in GL2 site from the baseline. The time at primary consolidation completion is longer as saturated solution is saltier. The prolongation of primary consolidation time gets over 30% if the subsoil is at the most salty. These changes illustrate the degradation of soils in the saline media.

The ACC deaminase expressing endophyte Pseudomonas spp. Enhances NaCl stress tolerance by reducing stress-related ethylene production, resulting in improved growth, photosynthetic performance, and ionic balance in tomato plants

Plant growth promoting bacteria (PGPB) endophytes that express 1-aminocyclopropane-1-carboxylate (ACC) deaminase reportedly confer plant tolerance to abiotic stresses such as salinity by lowering stress-related ethylene levels. Two preselected ACC deaminase expressing endophytic Pseudomonas spp. strains, OFT2 and OFT5, were compared in terms of their potential to promote plant growth, leaf water contents, photosynthetic performance, and ionic balance of tomato plants under conditions of moderate NaCl stress (75 mM). Salinity stress strongly affected growth, leaf water contents, and photosynthetic performance of tomato seedlings, and inoculation with either OFT2 or OFT5 ameliorated these adverse effects. Decreases in plant biomass due to salinity stress were significant in both uninoculated control plants and in plants inoculated with OFT2 compared with plants without NaCl stress. However, no reductions in total biomass were observed in plants that were inoculated with the OFT5 strain. Strain OFT5 influenced growth, physiological status, and ionic balance of tomato plants more efficiently than strain OFT2 under NaCl stress. In particular, inoculated OFT5 reduced salt-induced ethylene production by tomato seedlings, and although it did not reduce shoot uptake of Na, it promoted shoot uptake of other macronutrients (P, K, and Mg) and micronutrients (Mn, Fe, Cu, and Zn). These nutrients may activate processes that alleviate the effects of salt, suggesting that OFT5 can be used to improve nutrient uptake and plant growth under moderate salt-affected conditions by reducing stress-related ethylene levels.

Genotypic variation in rice for grain yield and quality as affected by salt-affected field conditions

ABSTRACTA field experiment was conducted under two natural field conditions at the Research Farm (normal soil) and Proka Farm (salt-affected soil) of The Institute of Soil and Environmental Sciences (ISES), University of Agriculture, Faisalabad, Pakistan, to evaluate the performance of 11 rice genotypes in normal and salt-affected conditions. The experiment was laid out in randomized complete block design (RCBD) with three replications. The gas exchange attributes were measured at vegetative stage whereas the grain and straw yields and the yield components were recorded at maturity. After harvesting, the ionic parameters including sodium (Na+) and potassium (K+) were determined. Afterward, grain quality in terms of length, width, milling recovery, broken fraction, and chalkiness was also determined for the selected genotypes. Salt-affected conditions adversely affected the physiology, yield, and quality of the tested genotypes. The genotypes KS-282 followed by Shaheen Basmati showed significantly higher photosynthetic rate, transpiration rate, and stomatal conductance under both normal and salt-affected conditions, whereas the genotypes 99404 followed by 99417 showed minimum values of gas exchange attributes. The grain and straw yields were the highest in the case of KS-282 at both sites, whereas the lowest grain and straw yields were observed in the case of 99440 followed by 99417 under both normal and salt-affected conditions. Regarding the quality attributes, Super Basmati produced longer grains but with higher broken fraction and lower milling recovery, whereas the reverse was observed in the case of KS-282.

Assessment of Ammonia Volatilization Losses and Nitrogen Utilization during the Rice Growing Season in Alkaline Salt-Affected Soils

The objectives of this study were to evaluate the effects of different fertilizer types and application rates on ammonia volatilization loss and to explore nitrogen distribution and nitrogen use efficiency using the 15N isotope tracing technique in different alkaline salt-affected conditions in the Songnen Plain, Northeast China. The results showed a decreasing trend in ammonia volatilization loss from ammonium nitrate and ammonium sulfate, but not that from urea, as the electrical conductivity gradient increased, whereas the reverse trend was found as the pH gradient increased. Ammonia volatilization loss increased in moderately salt-affected soil compared with that in slightly salt-affected soil, particularly during the tillering stage, regardless of the N fertilizer rate. The percentage of N absorbed by rice plants increased from urea but decreased from the soil as the amount of nitrogen was increased. Interestingly, the N retention rate in soil decreased and rice grain yield and nitrogen agronomic efficiency increased as the amount of nitrogen increased in both salt-affected soil conditions. The nitrogen application amount of highest N physiological efficiency was 225 kg·N/ha. Considering high rice production and a minimal environmental threat, we should fully consider controlling ammonia volatilization losses by adjusting the fertilizer type and the crop stage when the fertilizer is applied.

Secondary consolidation of soft soils in salt-affected conditions related to sea lever rise, case study: hanoi soft soils

Secondary consolidation of soft soils in salt-affected conditions related to sea lever rise, case study: hanoi soft soils

Halophyte agriculture: Success stories

The world's food production will need to increase by up to 70% by 2050 to match the predicted population growth. Achieving this goal will be challenging due to the decreased availability of arable land, resulting from urbanization and land degradation. Soil salinity is a major factor contributing to the latter process. While some improvement in crop yields in saline soils may be achieved as a consequence of single gene transfers, the real progress may be achieved only via a painfully slow “pyramiding” of essential physiological traits. Given the time constraints, a safer solution to meet the 2050 challenge may be to find alternative crop and forage species for farming in salt-affected conditions and to restore salt-affected areas. This review focuses on the suitability of halophytes to become important components of 21st century farming systems. We provide a comprehensive summary of the current use of halophytes for human food consumption, for forage and animal feeds, as oilseed and energy crops, and for desalination and phytoremediation purposes. We argue that the use of halophytes may be a viable commercial alternative to ease pressure on the requirement of good quality land and water for conventional cropping systems and the utilization of land degraded by salinity.

Efficacy of Rhizobium and Pseudomonas strains to improve physiology, ionic balance and quality of mung bean under salt-affected conditions on farmer's fields

High ethylene concentration under different environmental stresses such as salinity is one of the contributing factors for premature senescence of different plant parts. Plants under salinity stress produce increased levels of ethylene which inhibit the plant growth and physiology thus deteriorating the quality of the produce. Some plant growth promoting rhizobacteria (PGPR) have the ability to improve quality of crops through reduction in detrimental effects of salinity on plant growth and physiology by lowering endogenous level of ethylene along with other mechanisms. Two field trials were conducted to evaluate the Rhizobium and Pseudomonas containing ACC-deaminase for their efficacy to reduce the effect of salinity on physiology, ionic and nutrient balance of mung bean. Results showed that salinity stress adversely affected the physiological parameters of mung bean. It decreased the CO(2) assimilation, stomatal conductance of water, relative water content, photosynthetic rate, transpiration rate and chlorophyll contents in mung bean but inoculation of either Rhizobium or Pseudomonas alone reduced adverse effect of salinity significantly. However, co-inoculation with Rhizobium and Pseudomonas was the most effective treatment and it diluted the adverse effects of salinity on relative water contents and CO(2) assimilation rate thus improving the photosynthetic rate, water use efficiency and chlorophyll content over the un-inoculated control. Co-inoculation improved the ionic balance and also increased the phosphorus and protein concentration in grain of mung bean. The results suggested that these strains could be effectively used to improve the growth, physiology and quality of mung bean under salt-affected conditions.

The combined application of rhizobial strains and plant growth promoting rhizobacteria improves growth and productivity of mung bean (Vigna radiata L.) under salt-stressed conditions

Salinity stress induces higher levels of ethylene in plants in arid and semi-arid regions. This increased concentration of ethylene can be inhibited by using plant growth promoting rhizobacteria (PGPR) containing ACC-deaminase. A pot trial was conducted under salt-stressed conditions to evaluate the potential of combined application of Rhizobium phaseoli (M6 and M9), and PGPR (Pseudomonas syringae, Mk1; Pseudomonas fluorescens, Mk20 and Pseudomonas fluorescens Biotype G, Mk25) to improve the productivity of mung bean. The results showed that salinity stress decreased significantly mung bean growth, yield and physiological parameters but inoculation with either rhizobia or PGPR alone enhanced these parameters significantly. However, the combined application of rhizobia and PGPR was more effective for reducing the depressing effect of salinity on mung bean. Co-inoculation increased the shoot fresh weight (145%), root fresh weight (173%), number of pods plant−1(150%), pod fresh weight (182%), total dry matter (269%), relative water content (19%), water use efficiency (51%), potassium concentration in leaves (33%), sodium concentration in leaves (56%) and nitrogen concentration in grains of mung bean (99%), compared with the uninoculated control. The results imply that combined application of Rhizobium and Pseudomonas strains can improve the productivity of mung bean. Thus, these strains could be evaluated in intensive field trials for developing biofertilizers to improve the productivity of mung bean under salt-affected conditions.

Inducing salt tolerance in mung bean through coinoculation with rhizobia and plant-growth-promoting rhizobacteria containing 1-aminocyclopropane-1-carboxylate deaminase

Twenty-five strains of plant-growth-promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase and 10 strains of rhizobia were isolated from rhizosphere soil samples and nodules of mung bean. They were screened in separate trials under salt-stressed axenic conditions. The three most effective strains of PGPR (Mk1, Pseudomonas syringae ; Mk20, Pseudomonas fluorescens ; and Mk25, Pseudomonas fluorescens biotype G) and Rhizobium phaseoli strains M1, M6, and M9 were evaluated in coinoculation for their growth-promoting activity at three salinity levels (original, 4dS·m(-1), and 6 dS·m(-1)) under axenic conditions. The results showed that salinity stress significantly reduced plant growth but inoculation with PGPR containing ACC deaminase and rhizobia enhanced plant growth, thus reducing the inhibitory effect of salinity. However, their combined application was more effective under saline conditions, and the combination Mk20× M6 was the most efficient for improving seedling growth and nodulation. The effect of high ethylene concentrations on plant growth and the performance of these strains for reducing the negative impact of saline stress was also evaluated by conducting a classical triple-response bioassay. The intensity of the classical triple response decreased owing to inoculation with these strains, with the root and shoot lengths of inoculated mung bean seedlings increasing and stem diameter decreasing, which is a typical response to the dilution in a classical triple response bioassay. Thus, coinoculation with PGPR containing ACC deaminase and Rhizobium spp. could be a useful approach for inducing salt tolerance and thus improving growth and nodulation in mung bean under salt-affected conditions.

Inhibition kinetics of salt-affected wetland for municipal wastewater treatment

A mathematical model was developed in order to describe the system behavior and performance of a constructed wetland (CW) treatment under salt-affected conditions. The rate of biodegradation of organic wastes was modeled using the first-order kinetics while the effect of salt concentrations was accounted by growth inhibition. Experimental data were used to determine model constants of the mathematical model. The experimental units were planted with cattail ( Typha angustifolia) and fed with spiked municipal wastewater. The hydraulic retention time varied from 12 to 120 h and wastewater conductivity was in the range of 4–32 mS/cm. At specified conditions the model was found to well describe the trend of the experimental data in terms of BOD removal with the Pearson correlation of 0.872. The model also permits construction of a nomograph which can be used to aid the design and prediction of CW treatment under salt-affected conditions.

Growth of Three Forage Species in Saline Conditions

Extensive areas of salt deposits are rendering agricultural soils in arid regions unsuitable for arable cropping. Identifying native plant species to grow in salt-affected soils or to grow with saline water may sustain productivity of salt affected lands. The objective of this greenhouse study was to compare biomass production and ion accumulation (i.e., Ca 2+ , Mg 2+ , Na + , and Cl m ) in potential salt tolerant forage species grown under simulated saline conditions found near La Junta region in southeast Colorado, USA. Seed-derived plants of tall fescue (Festucaarundinacea), narrowleafed birdsfoot trefoil (Lotus glaber), and broadleafed birdsfoot trefoil (Lotus corniculatus) were grown in pots filled with soil collected from nonsaline and saline areas near La Junta. Pots were irrigated with one of three qualities of water differing in EC: 0.13, 0.27, and 0.64 S m m 1 . For all treatments, F.arundinacea produced almost twice as much dry matter (DM) as L. glaber and L.corniculatus. Soil/water quality treatments appeared to have minimal effect on the DM yield of all species in 319 d, although DM yield appeared to be stimulated in L. glaber by either saline water or when grown in saline soil. Herbage Ca 2+ concentrations in all plants were higher when grown in nonsaline soil and decreased as water quality decreased. Herbage Mg 2+ concentrations were not affected by soil or water quality treatments. In contrast, herbage Na + and Cl m concentrations increased in all three species grown in either saline soils or irrigated with saline water. Soil EC increased at harvest for all plant species irrigated with saline water in both saline and nonsaline soils. Based on the growth responses observed for the three forage species under saline conditions, they all appear to be capable of growing in the salt-affected conditions of the La Junta region. Salt management practices will eventually be necessary to sustain productivity.