Luis Tarquino Llerena-Ramos

Heavy metal contamination of agricultural soils is a critical environmental challenge because of its persistence and the risks it poses to ecosystems and human health. Heavy metals cannot be degraded, and changes in soil conditions can remobilize stable fractions into labile, bioavailable forms. Plant growth-promoting rhizobacteria (PGPR) are increasingly used as bioinoculants that assist phytoremediation rather than as a stand-alone technology. Depending on the strain, host plant and soil conditions, PGPR can either mobilize metals and enhance their uptake and translocation (phytoextraction) or immobilize them in the rhizosphere and roots, reducing their transfer to aboveground tissues (phytostabilization). This critical review, supported by a structured and reproducible literature search, synthesizes the evidence on (i) plant growth-promoting traits under metal stress; (ii) direct PGPR–metal interactions, including biosorption by extracellular polymeric substances, metallothioneins, organic acids, redox transformation and microbially induced carbonate precipitation; (iii) the regulation of plant antioxidant defenses; and (iv) root exudate-driven, endophyte-assisted and formulation-based strategies. The evidence, dominated by short-term pot experiments with single metals, is critically compared, and priorities for field validation, mixed contamination and regulatory approval are identified.

In Ecuador, pepper (Capsicum annuum L.) cultivation has gained importance in recent years due to the crop’s nutritional value and the increasing area devoted to its production. Pepper is moderately sensitive to water stress, a constraint that is becoming increasingly relevant in areas with limited or inefficient irrigation. This study evaluated the effect of foliar spraying with SiO₂ nanoparticles (SiO₂ NPs) on 'Nathalie' hybrid pepper plants subjected to temporary water stress caused by interruption of irrigation. The experiment was conducted in a greenhouse at the Faculty of Agricultural and Forestry Sciences, State Technical University of Quevedo, Ecuador. Seedlings were transplanted into containers filled with a homogenized soil and cow manure compost substrate. Treatments were arranged in a completely randomized design and consisted of a non-stressed control without SiO₂ NPs and three SiO₂ NP concentrations (250, 500, and 1000 mg L⁻¹) applied to plants exposed to a 10-day irrigation suspension, after which irrigation was resumed. Foliar application of SiO₂ NPs improved plant height, stem diameter, leaf area index, fruit length, and fruit mass, with the strongest response generally observed at 1000 mg L⁻¹. These results indicate that SiO₂ NPs may help maintain growth and fruit development in pepper plants exposed to short-term water stress. However, because the design did not include a water-stressed treatment without SiO₂ NPs, the stress-mitigation effect should be interpreted with caution and confirmed under field conditions.

The low efficiency of conventional fertilization limits nutrient use in intensive horticultural crops; therefore, microencapsulated bioinputs represent an alternative to improve cucumber productivity under greenhouse conditions. This study aimed to evaluate the dose-dependent response of growth, productive components and yield of Cucumis sativus L. to foliar application of chitosan and micronutrient microcapsules with SiO₂ nanoparticles. The experiment was carried out under greenhouse conditions at the Technical State University of Quevedo, Ecuador, with a mean internal temperature of 27.9 °C. A completely randomized design was used with four treatments and four replications: T1, control with basal fertilization and no foliar spraying; T2, 1,000 mg.L⁻¹; T3, 1,500 mg.L⁻¹; and T4, 2,000 mg.L⁻¹, sprayed at 12, 25 and 35 days after sowing with 100 mL.plant⁻¹ per application. Plant height, stem diameter, days to flowering, number of fruits per plant, fruit length, fruit diameter, fruit weight and yield per plot were assessed. The 2,000 mg.L⁻¹ dose showed the most consistent response, with 456.25 cm plant height at 60 days, 8.00 mm stem diameter, 3.00 fruits per plant, 30.28 cm fruit length, 5.67 cm fruit diameter and 6.72 kg per plot, equivalent to a projected 42.0 t.ha⁻¹ against 24.8 t.ha⁻¹ in the control. The 2,000 mg.L⁻¹ dose showed the best agronomic performance under the evaluated conditions, although validation in additional cycles is required.

The agricultural use of silicon dioxide nanoparticles (SiO2 NPs) has the potential to improve the yield, sensory quality, and nutraceutical properties of various plant species. The objective of this study was to evaluate different concentrations of SiO2 NPs (0, 5, 7, and 9 mg L−1) on the growth, yield, and nutraceutical quality of peppers (Capsicum annuum L.) cv. Quetzal grown under greenhouse conditions. A completely randomized experimental design was established with four treatments and three replicates. SiO2 NP solutions were applied via drenching at specific phenological stages. Agronomic variables (plant height, stem diameter, and dry biomass), productive variables (number and weight of fruits and yield per hectare), and biochemical variables (total phenolic compound content, flavonoids, and antioxidant capacity) were measured. The results indicate that concentrations of 7 and 9 mg L−1 of SiO2 NPs significantly promoted vegetative growth, with increases in plant height (+12.40%) and dry biomass (+60.02%), respectively. Furthermore, increases in fruit number and weight, as well as total yield, were observed. In terms of nutraceutical quality, higher levels of phenolic compounds (+37.14%), flavonoids (+45.44%), and antioxidant activity (+3.71%) were recorded in fruits treated with these concentrations. These findings suggest that the application of SiO2 NPs at optimal doses can improve both the yield and nutraceutical quality of peppers, positioning it as a promising agronomic input for this crop.

Broccoli (Brassica oleracea) is a horticultural crop with high nutritional and economic value that is sensitive to the availability of essential micronutrients. In order to determine the biostimulant effect of selenium on crop growth and yield, a trial was conducted under controlled conditions in the greenhouse of the Faculty of Agricultural and Forestry Sciences of the Quevedo State Technical University, “La María” campus (65 m above sea level). A completely randomized design was applied with four treatments (0, 3, 5, and 7 mg L⁻¹ of selenium) and five replicates. The results showed that the 7 mg L⁻¹ treatment promoted the highest vegetative development and yield, registering averages of 54.30 cm in height, 5.55 cm in stem diameter, 20 leaves per plant, 297.96 g head weight, 9.26 cm equatorial diameter, and 19,847.62 kg ha⁻¹ yield, with a profitability of 159%. In contrast, the control had 8,415.4 kg ha⁻¹ and the lowest profitability (17.88%). It is concluded that the application of selenium acts as a physiological biostimulant, significantly improving the growth, productivity, and economic efficiency of broccoli, making it a sustainable agronomic strategy with the potential to optimize intensive horticultural systems.

Organic fertilization has been associated with improvements in soil fertility and vegetable performance. The effect of organic fertilizers on the agronomic and productive performance of cucumber (Cucumis sativus) in greenhouses was evaluated. A completely randomized design with four treatments was used: T0 (control), T1 (compost), T2 (bocashi), and T3 (worm humus), with four replicates (one plant per replicate). The variables were plant height (cm), stem diameter (mm), days to flowering, fruit length and diameter (cm), fruit weight (g), yield (kg ha⁻¹), and benefit/cost ratio. Significant differences were detected between treatments. Worm humus (T3) showed greater vigor: at 30 days, 74.9 cm and 5.75 mm; at 45 days, 161.5 cm and 6.73 mm; and at 60 days, 341.5 cm and 6.73 mm. It also had a shorter time to flowering (44.5 days). In terms of fruit characteristics, T3 showed greater length (25.51 cm), diameter (6.09 cm), and average weight (480.75 g), along with the highest profitability (profit/cost 2.03). In conclusion, worm humus consistently improved the growth, precocity, and production indicators of greenhouse cucumbers compared to the control.

Tomato crops are treated with high concentrations of synthetic fertilizers and insecticides to increase yields, but the careless use of these chemicals harms the environment and human health and affects plant pathogen resistance. The effect of foliar spray of three concentrations of chitosan (500, 1000, and 2000 mg L⁻¹) on plant growth, yield, fruit quality, and physiological performance in two tomato varieties (Floradade and Candela F1) was studied. Physiological traits such as photosynthesis, chlorophyll content, and leaf area index of the plants were positively affected by chitosan, an effective compound that biostimulates growth, with increases in biomass of organs with respect to the control treatment. Chitosan also improved tomato quality, such as increases in polyphenols, antioxidant capacity, flavonoids, carotenoids, vitamin C, and total soluble solids in both tomato varieties. Finally, yield increased by 76.4% and 65.4% in Floradade and Candela F1, respectively. The responses of tomato plants to chitosan application were different depending on the variety evaluated, indicating a differential response to the biostimulant. The use of chitosan in agriculture is a tool that has no negative effects on plants and the environment and can increase the productive capacity of tomato plants.

Although favorable effects of Selenium nanoparticles (SeNPs or nSe) in tomato have been reported, research has concentrated on stress alleviation and disease management. From the above it is noticeable that the effect of NPs varies greatly depending on the model plant, nanoparticle (concentration, size, shape), and application (foliar or drenching). For this reason, the objective of this study was to investigate the impact of biostimulating tomato plants under no stressor conditions (Solanum lycopersicum cv. ‘Pomodoro’ L.) with SeNPs on morpho-physiological and fruit quality parameters. Three doses of Selenium nanoparticles (5, 15, and 30 mg L⁻¹), and a control were applied via a foliar application after transplanting. The results indicate that a 5 mg L⁻¹ SeNP treatment improved the growth and yield of the tomato, with the exception of the root length and leaf weight. Moreover, all doses modified the evaluated physiology, bioactive compounds, and fruit quality parameters. This research helped in understanding the SeNPs’ effect on tomato plants in greenhouses under a no stressor condition.

The tomato (Solanum lycopersicum L.), considered one of the most desirable horticultural crops in the world for its nutritional and nutraceutical qualities, is also used as a model for many studies. The objective of this research was to compare different concentrations of chitosan nanoparticles (NPs) on the growth and development of tomato plants, as well as on fruit quality. Three concentrations of chitosan and a control were applied to tomato leaves in a randomized block design with three replicates and four treatments. Different growth variables (plant height, stem diameter, root length and fresh and dry biomass of different plant organs) as well as fruit yield and polar and equatorial diameter were evaluated. Fruit quality was evaluated by determining the contents of total soluble solids (TSS, %), vitamin C (mg in 100 g), polyphenols (mg gallic acid per g), antioxidant capacity (µmol Trolox per g), flavonoids (mg catechin per g) and carotenoids (µg β-carotene per g). The positive effect of foliar applications of chitosan nanoparticles was proven. The higher concentrations favoured yield and fruit quality, increasing yield and contents in the different quality variables, while the medium concentration stimulated growth in general.

The application of nanotechnology in horticultural crops optimizes physiological performance, growth, nutraceutical quality, and resistance to adverse environmental factors. The objective of this study was to evaluate the effect of silica dioxide nanoparticles (SiO₂ NPs) on tomato crops in greenhouses. Three doses were used (25 mg L^-1, 35 mg L^-1, and 45 mg L^-1) along with a control. A randomized block experimental design with three replications was used. The variables analyzed included physiological, morphological, yield, and nutraceutical quality aspects. It was observed that plant height was higher with 45 mg L^-1 of SiO₂ NPs, while 25 mg L^-1 of SiO₂ NPs showed the highest values for stem diameter, root length and yield. The dose of 25 mg L^-1 of SiO₂ NPs caused higher chlorophyll content, net photosynthetic rate, water use efficiency, and leaf water status. Regarding nutraceutical quality (soluble solids, vitamin C, total phenols, flavonoids, carotenoids, and antioxidant capacity), the best results were obtained with 35 mg L^-1 of SiO₂ NPs. These findings suggest that SiO₂ NPs at concentrations of 25 and 35 mg L^-1 improve the tomatoes’ physiological traits, growth, yield, and nutraceutical quality, which is relevant for agricultural production and the promotion of healthy diets.