Silicon improves salinity tolerance and affects ammonia assimilation in maize roots

Silicon Mitigates Ammonium Toxicity in Cabbage (Brassica campestris L. ssp. pekinensis) ‘Ssamchu’

Ammonium (NH4+) toxicity hinders the cabbage yield because most subspecies or varieties exhibit extreme sensitivity to NH4+. Current knowledge indicates that silicon (Si) can alleviate or reverse the ammonium toxicity severity. However, few investigations have been conducted on NH4+-stressed cabbage to elucidate the mechanism underlying the Si alleviation. The study described herein analyzes induced physio-chemical changes to explore how Si helps mitigate NH4+ toxicity. We applied one of three NH4+:NO3- ratios (0:100, 50:50, and 100:0) at a constant N (13 meq·L−1) to the cabbage plants, corresponding with two Si treatment levels (0 and 1.0 meq·L−1). Chlorosis and foliage necrosis along with stunted roots occurred following 100% NH4+ application were ameliorated in the presence of Si. The NH4+ toxicity ratio was reduced accordingly. Similarly, inhibition on the uptake of K and Ca, restricted photosynthesis (chlorophyll, stomatal conductance, and Fv/Fm), and accumulation of reactive oxygen species (ROS, O2·-, and H2O2), as well as lipid peroxidation (MDA, malondialdehyde) in NH4+-stressed cabbages were mitigated with added Si. The lower observed oxidative stresses in solely NH4+-treated plants were conferred by the boosted antioxidant enzymes (SOD, superoxide dismutase; CAT, catalase). Concomitantly, Si-treated plants showed higher activities of key NH4+ assimilation enzymes (GS, glutamine synthetase; GOGAT, glutamate synthase; NADH-GDH, glutamate dehydrogenase) and NH4+ content in leaves. However, excessive NH4+ assimilations cause the acidic stress, which has been demonstrated to be the primary cause of NH4+ toxicity. Therefore, further investigation regarding the Si effects on H+ regulation and distribution should be warranted.

Minimizing the Adversely Impacts of Water Deficit and Soil Salinity on Maize Growth and Productivity in Response to the Application of Plant Growth-Promoting Rhizobacteria and Silica Nanoparticles

The development of new approaches for sustaining soil quality, leaf health, and maize productivity are imperative in light of water deficit and soil salinity. Plant growth-promoting rhizobacteria (PGPR) and silica nanoparticles (SiNP) are expected to improve soil chemistry leading to improved plant performance and productivity. In this field experiment, water deficit is imposed by three irrigation intervals—12 (I1), 15 (I2), and 18 (I3) days. Plants are also treated with foliar and soil applications (control, PGPR, SiNP, and PGPR + SiNP) to assess soil enzymatic activity, soil physicochemical properties, plant physiological traits, biochemical analysis, nutrient uptake, and productivity of maize (Zea mays L.) plants grown under salt-affected soil during the 2019 and 2020 seasons. With longer irrigation intervals, soil application of PGPR relieves the deleterious impacts of water shortage and improves yield-related traits and maize productivity. This is attributed to the improvement in soil enzymatic activity (dehydrogenase and alkaline phosphatase) and soil physicochemical characteristics, which enhances the plants’ health and growth under longer irrigation intervals (i.e., I2 and I3). Foliar spraying of SiNP shows an improvement in the physiological traits in maize plants grown under water shortage. This is mainly owing to the decline in oxidative stress by improving the enzymatic activity (CAT, SOD, and POD) and ion balance (K+/Na+), resulting in higher photosynthetic rate, relative water content, photosynthetic pigments, and stomatal conductance, alongside reduced proline content, electrolyte leakage, lipid peroxidase, and sodium content under salt-affected soil. The co-treatment of SiNP with PGPR confirms greater improvement in yield-related traits, maize productivity, as well as nutrient uptake (N, P, and K). Accordingly, their combination is a good strategy for relieving the detrimental impacts of water shortage and soil salinity on maize production.

Physiological role of silicon in radish seedlings under ammonium toxicity

BACKGROUND

High concentrations of ammonium as the sole nitrogen source may result in physiological and nutritional disorders that can lead to reduced plant growth and toxicity. In this study, we hypothesized that ammonium toxicity in radish seedlings (Raphanus sativus L.) might be mitigated by the incorporation of silicon (Si) into applied nutrient solution. To examine this possibility, we conducted a hydroponic experiment to evaluate the effects of five concentrations of ammonium (1, 7.5, 15, 22.5, and 30 mmol L^-1 ) on the photosynthesis, green color index, stomatal conductance, transpiration, instantaneous water-use efficiency, and biomass production of radish in the absence and presence (2 mmol L^-1 ) of Si. The experimental design was a randomized block design based on a 2 × 5 factorial scheme with four replicates.

RESULTS

The highest concentration of applied ammonium (30 mmol L^-1 ) was found to reduce the photosynthesis, transpiration and total dry biomass of radish seedlings, independent of the presence of Si in the nutrient solution. However, at lower ammonium concentrations, the application of Si counteracted these detrimental effects, and facilitated the production of seedlings with increased photosynthesis, greater instantaneous water-use efficiency, and higher total dry biomass compared with the untreated plants (without Si). Transpiration and stomatal conductance were affected to lesser extents by the presence of Si.

CONCLUSION

These findings indicate that the addition of Si to nutrient solutions could provide an effective means of alleviating the unfavorable effects induced by ammonium toxicity at concentrations of less than 30 mmol L^-1 . © 2020 Society of Chemical Industry.

Plant Growth Promoting Rhizobacteria and Silicon Synergistically Enhance Salinity Tolerance of Mung Bean

The present study explored the eco-friendly approach of utilizing plant-growth-promoting rhizobacteria (PGPR) inoculation and foliar application of silicon (Si) to improve the physiology, growth, and yield of mung bean under saline conditions. We isolated 18 promising PGPR from natural saline soil in Saudi Arabia, and screened them for plant-growth-promoting activities. Two effective strains were selected from the screening trial, and were identified as Enterobacter cloacae and Bacillus drentensis using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry and 16S rRNA gene sequencing techniques, respectively. Subsequently, in a 2-year mung bean field trial, using a randomized complete block design with a split-split plot arrangement, we evaluated the two PGPR strains and two Si levels (1 and 2 kg ha(-1)), in comparison with control treatments, under three different saline irrigation conditions (3.12, 5.46, and 7.81 dS m(-1)). The results indicated that salt stress substantially reduced stomatal conductance, transpiration rate, relative water content (RWC), total chlorophyll content, chlorophyll a, chlorophyll b, carotenoid content, plant height, leaf area, dry biomass, seed yield, and salt tolerance index. The PGPR strains and Si levels independently improved all the aforementioned parameters. Furthermore, the combined application of the B. drentensis strain with 2 kg Si ha(-1) resulted in the greatest enhancement of mung bean physiology, growth, and yield. Overall, the results of this study provide important information for the benefit of the agricultural industry.

Mechanisms of silicon-mediated alleviation of drought and salt stress in plants: a review

Drought and salinity are the main abiotic stresses limiting crop yield and quality worldwide. Improving food production in drought- and salt-prone areas is the key to meet the increasing food demands in near future. It has been widely reported that silicon (Si), a second most abundant element in soil, could reduce drought and salt stress in plants. Here, we reviewed the emerging role of Si in enhancing drought and salt tolerance in plants and highlighted the mechanisms through which Si could alleviate both drought and salt stress in plants. Silicon application increased plant growth, biomass, photosynthetic pigments, straw and grain yield, and quality under either drought or salt stress. Under both salt and drought stress, the key mechanisms evoked are nutrient elements homeostasis, modification of gas exchange attributes, osmotic adjustment, regulating the synthesis of compatible solutes, stimulation of antioxidant enzymes, and gene expression in plants. In addition, Si application decreased Na(+) uptake and translocation while increased K(+) uptake and translocation under salt stress. However, these mechanisms vary with plant species, genotype, growth conditions, duration of stress imposed, and so on. This review article highlights the potential for improving plant resistance to drought and salt stress by Si application and provides a theoretical basis for application of Si in saline soils and arid and semiarid regions worldwide. This review article also highlights the future research needs about the role of Si under drought stress and in saline soils.