Biochar is gaining significant attention due to its potential for carbon (C) sequestration, improvement of soil health, fertility enhancement, and crop productivity and quality. In this review, we discuss the most common available techniques for biochar production, the main physiochemical properties of biochar, and its effects on soil health, including physical, chemical, and biological parameters of soil quality and fertility, nutrient leaching, salt stress, and crop productivity and quality. In addition, the impacts of biochar addition on salt-affected and heavy metal contaminated soils were also reviewed. An ample body of literature supports the idea that soil amended with biochar has a high potential to increase crop productivity due to the concomitant improvement in soil structure, high nutrient use efficiency (NUE), aeration, porosity, and water-holding capacity (WHC), among other soil amendments. However, the increases in crop productivity in biochar-amended soils are most frequently reported in the coarse-textured and sandy soils compared with the fine-textured and fertile soils. Biochar has a significant effect on soil microbial community composition and abundance. The negative impacts that salt-affected and heavy metal polluted soils have on plant growth and yield and on components of soil quality such as soil aggregation and stability can be ameliorated by the application of biochar. Moreover, most of the positive impacts of biochar application have been observed when biochar was applied with other organic and inorganic amendments and fertilizers. Biochar addition to the soil can decrease the nitrogen (N) leaching and volatilization as well as increase NUE. However, some potential negative effects of biochar on microbial biomass and activity have been reported. There is also evidence that biochar addition can sorb and retain pesticides for long periods of time, which may result in a high weed infestation and control cost.

Contaminated soils can cause a potential risk into the health of the environment and soil as well as the quality and productivity of plants. The objectives of our study were to investigate the integrative advantageous effects of foliar ZnO nanoparticles (NPs) (60 mg Zn NPs L⁻¹), rice straw biochar (RSB; 8.0 t ha⁻¹), cow-manure biochar (CMB, 8.0 t ha⁻¹), and a combination thereof (50% of each) on sunflowers grown in agricultural land irrigated with polluted wastewater for the long term (≈50 years). The availability of heavy metals (HMs) in soil, HMs accumulation in whole biomass aboveground, growth, productivity, and quality characteristics of the sunflower were investigated. The combination treatment significantly minimized the availability of HMs in soil, and, consequently, substantially lessened the uptake of HMs by the sunflower, compared to treatments of ZnO NPs and control (i.e., untreated soil). The application of the combination treatment reduced the availability of Pb, Cr, Cu, and Cd in the soil by 78.6%, 115.3%, 153.3%, and 178.5% in comparison to untreated plots post-harvest, respectively. Compared to untreated plots, it also reduced the Pb, Cr, Cu, and Cd in plant biomass by 1.13, 5.19, 3.88, and 0.26 mg kg⁻¹ DM, respectively. Furthermore, combination treatment followed by biochar as an individual application caused a significant improvement in sunflower productivity and quality in comparison to untreated soil. For instance, seed yield ha⁻¹, 100-seed weight, and number of seeds per head obtained from the combination treatment was greater than the results obtained from the untreated plots by 42.6%, 47.0%, and 50.4%, respectively. In summary, the combined treatment of NPs and both RSB and CMB is recommended as a result of their positive influence on sunflower oil quality and yield as well as on minimizing the negative influences of HMs.

Melatonin, a multifunctional signaling molecule, is ubiquitously distributed in different parts of a plant and responsible for stimulating several physiochemical responses against adverse environmental conditions in various plant systems. Melatonin acts as an indoleamine neurotransmitter and is primarily considered as an antioxidant agent that can control reactive oxygen and nitrogen species in plants. Melatonin, being a signaling agent, induces several specific physiological responses in plants that might serve to enhance photosynthesis, growth, carbon fixation, rooting, seed germination and defense against several biotic and abiotic stressors. It also works as an important modulator of gene expression related to plant hormones such as in the metabolism of indole-3-acetic acid, cytokinin, ethylene, gibberellin and auxin carrier proteins. Additionally, the regulation of stress-specific genes and the activation of pathogenesis-related protein and antioxidant enzyme genes under stress conditions make it a more versatile molecule. Because of the diversity of action of melatonin, its role in plant growth, development, behavior and regulation of gene expression it is a plant’s master regulator. This review outlines the main functions of melatonin in the physiology, growth, development and regulation of higher plants. Its role as anti-stressor agent against various abiotic stressors, such as drought, salinity, temperatures, UV radiation and toxic chemicals, is also analyzed critically. Additionally, we have also identified many new aspects where melatonin may have possible roles in plants, for example, its function in improving the storage life and quality of fruits and vegetables, which can be useful in enhancing the environmentally friendly crop production and ensuring food safety.

Selenium (Se) and silicon (Si) are considered advantageous elements to induce plants’ tolerance to various environmental stresses. Wheat yield is negatively affected by salinity stress, especially in dry and semi-dry areas. Therefore, the objective of the current study was to investigate the effects of Se, Si and their combinations (0 as control, Se₁₅, Se₃₀, Si₁₅, Si₃₀, Se₁₅ + Si₁₅, and Se₃₀ + Si₃₀ mM) in alleviating the deleterious effects of salinity stress (7.61 dS m⁻¹, real field conditions) on anatomical characteristics as well as the physio-biochemical and productivity parameters of wheat plants. The selenium and silicon treatments and their combinations caused significant amelioration in growth, anatomical and physiological attributes, and grain yields of salinity-stressed wheat in comparison with the untreated plants (control treatment). The integrated application of Se₃₀ + Si₃₀ significantly increased plant growth (i.e., plant height 28.24%, number of tillers m⁻² 76.81%, fresh weight plant⁻¹ 80.66%, and dry weight plant⁻¹ 79.65%), Fv/Fm (44.78%), performance index (PI; 60.45%), membrane stability index (MSI; 36.39%), relative water content (RWC; 29.39%), total soluble sugars (TSS; 53.38%), proline (33.74%), enzymatic antioxidants (i.e., CAT activity by 14.45%, GR activity by 67.5%, SOD activity by 35.37% and APX activity by 39.25%) and non-enzymatic antioxidants (i.e., GSH content by 117.5%, AsA content by 52.32%), yield and its components (i.e., number of spikelets spike⁻¹ 29.55%, 1000-grain weight 48.73% and grain yield ha⁻¹ 26.44%). The anatomical traits of stem and leaves were improved in wheat plants treated with Se₃₀ + Si₃₀. These changes resulting from the exogenous applications of Se, Si or their combinations, in turn, make these elements prospective in helping wheat plants to acclimate successfully to saline soil.

Plant secondary metabolites are known to have a variety of biological activities beneficial to human health. They are becoming more popular as a result of their unique features and account for a major portion of the pharmacological industry. However, obtaining secondary metabolites directly from wild plants has substantial drawbacks, such as taking a long time, posing a risk of species extinction owing to over-exploitation, and producing a limited quantity. Thus, there is a paradigm shift towards the employment of plant tissue culture techniques for the production of key secondary metabolites in vitro. Elicitation appears to be a viable method for increasing phytochemical content and improving the quality of medicinal plants and fruits and vegetables. In vitro culture elicitation activates the plant’s defense response and increases the synthesis of secondary metabolites in larger proportions, which are helpful for therapeutic purposes. In this respect, light has emerged as a unique and efficient elicitor for enhancing the in vitro production of pharmacologically important secondary metabolites. Various types of light (UV, fluorescent, and LEDs) have been found as elicitors of secondary metabolites, which are described in this review.

Salinity is one of the major issues that limits field crop productivity in an arid and semiarid environment. Therefore, two field trials were carried out over two seasons of 2018 and 2019 to investigate the enhancement of different methods of potassium application (i.e., recommended soil amendment (control; K₂O), seed soaking (SS) and foliar spray (FS) in the form of potassium sulfate (K₂SO₄, 6 mM)) on antioxidant protection, physio-biochemical, yield and quality traits of soybean (cv. Giza 22) grown in normal (electrical conductivity; EC = 2.68 dS m⁻¹) and saline soil (EC = 7.46 dS m⁻¹). Physio-biochemical attributes (total chlorophyll, carotenoids, K⁺ and K⁺/Na⁺ ratios, performance index and catalase (CAT) activity), growth traits (i.e., shoot length, number and area of leaves plant⁻¹ and shoot dry weight), yield and its components and seed quality (number of pods plant⁻¹, 100-seed weight, seed yield ha⁻¹ and seed protein and oil contents) were significantly decreased when soybean plants were grown in saline soil compared with those grown in normal soil. In contrast, activity of enzymatic antioxidants (i.e., superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione peroxidase (GPX)), contents of non-enzymatic antioxidants and osmoprotectants (i.e., total soluble sugars, free proline, ascorbic acid and α-tocopherol), Na⁺, Cl⁻, H₂O₂ and malondialdehyde (MDA) were increased in soybean plants grown in saline soil compared with normal soil. However, under salt-stressed conditions, potassium applied through SS or FS significantly enhanced all soybean growth, photosynthetic efficiency, K⁺ content, ratio of K⁺/Na⁺ and activity of CAT, SOD, APX and GPX as well as improved yield and quality traits, while potassium application did not affect the contents of non-enzymatic antioxidants and osmoprotectants. For instance, foliar potassium application (FS) increased seed yield ha⁻¹ by 92.31% and protein content by 63.19% compared with the control under the salt stress condition. In addition, both applications of potassium significantly reduced Na⁺, Cl⁻, H₂O₂ and MDA contents in soybean plants compared with those obtained from control treatments. Exogenous application of K₂SO₄ was more effective than SS at improving soybean physio-biochemical attributes, yield and seed quality traits under soil-salinity stress.