Bell pepper is a vegetable with beneficial properties for human nutrition. However, salinity is an abiotic factor affecting bell pepper yield in arid and semi-arid areas of Peru. A very useful study was found recently which aimed to determine the combined effect of humic acid (HA) and Trichoderma harzianum (TH) as a sustainable alternative to improve the yield of sweet pepper under salt stress.
Saline soils are responsible for low yields and economic losses in the bell pepper crop. It should be noted that salinity refers to the accumulation of salts in the soil, which is caused by a high concentration of sodium and chlorine ions up to a level of toxicity, which generates osmotic stress in the plant, hormonal imbalance, low nutrient mobilization and generates reactive oxygen species (ROS) resulting in plant dehydration, wilting and even death. Studies have shown that saline soils cause low yields of peppers, because high ionic concentrations hinder water uptake and consequently the movement of nutrients by mass flow is significantly reduced, causing the fruit to have a smaller size and weight. On the other hand, bell pepper production is concentrated in the coastal regions of Peru, which is characterized by lack of rainfall, high evapotranspiration and to make up for the water deficiency, irrigation is carried out with groundwater, and irrigation with this low quality water causes the accumulation of salts in the soil, which can lead to salinity in agricultural areas. However, global warming and the lack of strategies may increase the salinity of coastal zone soils, jeopardizing the country's food security.
In this situation, several authors have found that humic acid is a sustainable alternative to mitigate the effect of salt stress, because it is an organic compound that is formed through the decomposition of organic matter and improves soil aggregation, resulting in greater water retention, in addition to the chelation of several nutrients, increases the activity of the Rubisco enzyme and reduces the toxicity of some uptake ions. Previous studies have revealed that humic acid increases its salinity tolerance efficiency and increases yield when combined with a beneficial microorganism. Recent studies have shown that the use of Trichoderma harzianum has a highly effective disease control effect and produces organic matter with dissolved ions, thereby improving the effectiveness of fertilizers applied to plants and the absorption rate of nutrients. Therefore, it was hypothesized that the combined application of humic acid and T. harzianum could improve intercellular communication in salt stress signaling, plant homeostasis, and plant performance.
The reason for the slow growth of plants under salt stress is the accumulation of Na + in plant tissues, which triggers electrolyte leakage and leads to slow root and leaf growth. Specifically, the accumulation of salt in the rhizosphere of plants reduces and hinders the entry of water into the root vascular bundle, resulting in a decrease in cell turgor pressure, which leads to physiological imbalance and hinders the absorption of nutrients. Increasing the dose of HA combined with Trichoderma can significantly reduce the negative effects of salinity, promote root development (root dry weight), and improve the absorption of nutrients and water by plants. This effect is due to the dual effects of HA and Trichoderma. For HA, its effect is due to soil aggregation, ion imbalance and moisture retention. Specifically, HA produces soil aggregates by retaining water for a long time, which leads to the flow of nutrients to the roots through the material. It has been reported that soil aggregation is a dynamic process in which plants and microorganisms interact and affect the improvement of soil fertility, accompanied by an increase in nutrient availability.
While the role of Trichoderma is to reproduce in the rhizosphere of plants and synthesize organic compounds that dissolve ions through it, improve the utilization of these ions and use the water retained by soil aggregates, thereby promoting plant root growth. In addition, application from the beginning of transplantation and in the early stages of bell pepper plants tends to promote their proliferation, help seedlings to take root, and stimulate the synthesis of their endogenous hormones (gibberellins and auxins), promote root development, and thus increase nutrient absorption. On the other hand, Trichoderma produces fungal elicitors in the rhizosphere of plants, which can dissolve nutrients and produce mycoparasites in soil pathogenic fungi, playing a dual role as a biofertilizer and a biofungicide.
As for fruit characteristics, soil salinity can cause osmotic stress in plants, which significantly reduces the movement of water from the soil to plant tissues, leading to changes in cell metabolism and affecting the filling of bell pepper fruits. Increasing the application dose of HA improves tolerance to salinity and means that water conduction is continuous, which leads to a continuous flow of carbohydrates and water to the fruit, thereby achieving better fruit size.
In terms of yield, bell pepper yield is significantly reduced under salt stress. Beneficial microorganisms stimulate plant defense mechanisms and biosynthesis of phytohormones, promote root proliferation, and produce organic acids that dissolve nutrients in different mineral forms (sediments and fertilizers), stabilize soil aggregates, and thus improve plant absorption of nutrients and water, so this combination can increase photosynthetic activity and promote the accumulation of carbohydrates and water in fruits.
Moreover, the addition of HA and Trichoderma harzianum to the soil significantly increased the concentrations of K + and Ca 2+ in bell pepper leaves, because HA can promote soil aggregation, causing ions to adhere to sand particles, thereby making K + and Ca 2+ in soil solution more available, and increasing K + absorption through its function as a pseudocytokinin. In addition, when hyaluronic acid is combined with microorganisms, organic acids that dissolve nutrients are produced, thereby increasing K + absorption, and reducing the absorption of Na + ions and Cl - by saline irrigation under saline soil conditions, and increasing the K + /Na + ratio, confirming that HA is more efficient when combined with beneficial microorganisms.
Similarly, salt stress causes plants to synthesize proline as a defense mechanism. However, the study showed that plants without HA and thiourea had lower proline content than those treated with increasing doses of HA and microorganisms, so the osmoregulatory effect of these treatments was stronger. This beneficial finding may be due to the fact that the beneficial microorganisms communicate with the plants and stimulate them to accumulate osmoregulatory substances at the cellular level, which regulate water potential and dissolve potassium ions, thus maintaining the sodium and potassium ratio in the plant. In addition, the microorganisms produce exopolysaccharides, a biofilm that retains water and binds Na + and Cl - ions, thus stabilizing soil aggregates and generating efficient movement of nutrients and water to the roots.
In addition, Trichoderma produces organic acids that dissolve nutrients and humic acids in different mineral forms (precipitates and fertilizers), stabilize soil aggregates, thereby allowing better utilization of fertilizer nutrients, increasing soil microbial communities, photosynthesis, carbohydrate accumulation and water conduction to the fruit. Therefore, with the increase in the amount of humic acid combined with beneficial microorganisms, the plants showed tolerance to soil salinity, resulting in good growth and yield of bell peppers.
In conclusion, saline soil significantly reduces the growth and productivity of bell pepper. However, the combined application of humic acid and Trichoderma harzianum, increases the tolerance to salinity and produces a significant effect on plant height, plant dry weight and increases root development, which improves the uptake of nutrients and water, favoring fruit set, fruit size and fruit weight per plant, resulting in the increase of the total yield of bell pepper in both seasons. It was also found that the plant subjected to salt stress, increases the uptake of sodium and chlorine, causing harmful effects on it. However, the increase of the doses of humic acids combined with Trichoderma harzianum, significantly increases the concentration of K+ and Ca2+ in leaves, also, the proline content increases, generating defense mechanisms to the saline stress.
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