Mitigation of salt stress in Phaseolus vulgaris L. by dopamine: Effects on growth, physiological parameters and antioxidant activity

The natural environment for plants is composed of abiotic and biotic stresses (Cramer et al., 2011). Abiotic stress conditions cause extensive losses in agricultural production worldwide. Stress conditions such as drought, salinity or heat have been the subject of intense research (Dos Santos et al., 2022). Environmental stresses can inhibit seed germination, delay growth, promote senescence and even lead to plant death (Li et al., 2019).

High salinity is one of the most widespread abiotic stress factors that severely restricts crop productivity (Yadav et al., 2020; Iftikhar et al., 2024). It is reported that about 7% of the Earth’s land and 20% of the total arable area are affected by high salt contents (Rasool et al., 2013). High soil salinity is a severe and growing problem, and it is preventing the achievement of sustainable agriculture (Vijayan et al., 2008; Roy et al., 2014). Soil salinity can hinder plant growth in two primary ways. First, the salt in the soil water makes it harder for plants to absorb the necessary water, slowing their growth. This is known as the osmotic or water-deficit effect of salinity. Second, excessive salt absorbed by the plant through its transpiration stream can damage cells in the leaves, further impeding growth. This is referred to as the salt-specific or ion-excess effect (Carillo et al., 2011; Haghshenas et al., 2024). Salinity can cause a decrease in leaf water potential, ionic imbalance and cell damage in plants, as well as causing increased accumulation of reactive oxygen species (ROS), negatively affecting protein synthesis, enzymatic activities and photosynthesis (Ait-El-Mokhtar et al., 2019; Hayat et al., 2024).

Phytohormones play an important role in alleviating abiotic stresses in plants by facilitating growth, development, nutrient distribution and source–sink relationships. Some important phytohormones that alleviate salinity stress in plants include abscisic acid (ABA), gibberellins (GA3), brassinosteroids (BRs), ethylene (ET), jasmonic acid (JA) and nitric oxide (NO). All these phytohormones show diversity in their modes of action under salt tolerance. Due to the prominent role of growth regulators in plants, there is a demand for novel PGR to facilitate the increasing demand for biochemical developmental processes (Verma et al., 2022). Dopamine (DA) is an important catecholamine neurotransmitter in the nervous system (Wang et al., 2018). It is common in animals and plants where it plays a variety of important roles, including in stress responses (Gao et al., 2020). Although widely found in animals, it was first recognized in plants as an antioxidant with greater anti-oxidative capacity than catechin, glutathione and flavonoids (Kulma and Szopa, 2007). It has a strong reducing capacity, allowing it to scavenge free radicals and maintain a dynamic balance of ROS. Therefore, it plays a vital role in the regulation of intercellular ion osmosis and chloroplast photophosphorylation (Kanazawa and Sakakibara, 2000). There is a relationship between DA and plant stress resistance, and studies have shown that DA can improve plant adaptation to adverse conditions (Liu et al., 2020; Jiao et al., 2021; Yildirim et al., 2024).

Legumes are rich in proteins and are very important components of human and animal nutrition because of their protein content and converting the free nitrogen of the air into a form that plants can benefit from (Jukanti et al., 2012). Plants in this group are the most sensitive to salt among cultivated plants. The increase in salt concentration in the soil causes an increase in osmotic pressure, making it difficult for the plant to take water from the soil. As a result, the structure of the soil deteriorates, and plant growth slows down or may even stop. Bean (Phaseolus vulgaris L.) is an important legume providing a major protein source for the human worldwide. The mature dry seed and fresh pods are often consumed as vegetables, and the rest of the plants are used as animal fodder (Berber and Yaşar, 2011). Bean has been reported to be a very sensitive crop to salinity conditions (Garcia et al., 2019).

The time-consuming and very expensive methods of reclaiming the areas with salinity problems reveal the necessity of alternative applications. It is possible to eliminate soil salinity by some methods, which causes significant losses in agricultural production. Irrigation with water of high salt content, washing the soil, establishing a good drainage system and applying some chemicals to the soil are among the measures that can be taken. The limited measures and the difficulty of implementation, the long-term effects and the expensiveness limit their use. Along with these measures, the emergence of salt stress-tolerant varieties and their breeding offer more permanent and effective solutions in crops (Singh et al., 2021). However, it takes a long time to develop resistance genotypes against salinity stress. To mitigate the negative impacts of salt stress, some hormones and plant growth regulators, including DA, have been used (Oral et al., 2020; Dadasoglu et al., 2021, 2022; Yildirim et al., 2022; Ahammed and Li, 2023).

Several studies have been conducted to determine the effect of DA against stress in plants (Liu et al., 2020). However, to our knowledge, no study has been found on the effects of DA in bean plants exposed to salt stress. Therefore, this research was conducted to reveal the effects of DA against salt stress on plant growth, physiological and biochemical characteristics of bean plants.

Salinity, which is one of the factors limiting crop production, is increasing its pressure on agricultural lands day by day (Carillo et al., 2011). In this study, the morphological, physiological and biochemical effects of exogenous DA applications in bean plants under moderate and high salinity stress were investigated. Salinity adversely affects the growth and yield of several crop plants. However, the interaction of DA with several crops in saline conditions reduces the extent of poor growth and thus helps plants to survive in adverse conditions. Our results suggest that DA promote better growth of plants under salt treatment. The growth traits were improved by DA application. In the interaction between salinity and DA, it is important to note that, under high salinity stress of 100 mM, the rate of 100 μM and 150 μM DA recorded the highest increase in plant growth parameters.

The results obtained from the study showed that salt stress negatively affected plant growth parameters and significantly reduced plant height, stem diameter, leaf area, shoot FW, shoot DW and root fresh and DWs to the control (non-salinity) (Table 3). Bean has been reported to be a very sensitive crop to salinity conditions (Garcia et al., 2019). One of the first observed effects of salt stress is a decrease in the growth rate of plants. Salt stress inhibits plant growth by preventing the uptake of some plant nutrients and the continuity of photosynthesis and causing metabolic toxicity (Ran et al., 2021). Salt-stressed plants had lower CRV than unstressed plants (Table 4). The amount of photosynthetic pigment generally decreases in plants under salt stress. Earlier researches have indicated that salt stress reduces the chlorophyll content in plants (Shams and Yildirim, 2021; Wang et al., 2022; Assouguem et al., 2024). Taibi et al. (2016) suggested that the decrease in the amount of photosynthetic pigment in plants under salt stress is an indicator of oxidative stress and that chlorophyllase, a proteolytic enzyme, is activated by salt stress and breaks down the pigments. According to the results obtained from the study, salt stress decreased LRWC values (Table 4). Plants with more water in their tissues in saline conditions are considered more tolerant to salinity. It was determined that as the salt concentration applied to the plants increased, the water potential and osmotic potential had more negative values (Cocozza et al., 2013). This study pointed out that salinity stress elevated EL, H₂O₂ and MDA content of bean plants (Table 2). Similarly, previous studies showed salinity caused an increase in EL, H₂O₂ and MDA content in various crops (Chen et al., 2020; Zahedi et al., 2021). It is thought that EL increases due to the deterioration of the cell membrane under salt stress. MDA, the end product of lipid peroxidation, is one of the important biomarkers of oxidative damage. Abiotic stresses cause the formation of superoxide (O₂^·−) and H₂O₂ as a result of redox reactions (Martinez et al., 2016; Dadasoglu et al., 2022).

At different salinity levels, DA had a positive effect on the growth and physiological parameters. In terms of the interaction between salinity and DA, the exogenous treatments of DA alleviated the negative effects of salinity stress on the plant growth, physiological and biochemical characteristics of bean plants. DA-treated bean seedlings had higher plant height, leaf area, plant FW, plant DW, root FW and root DW than non-treated plants in both non-saline and saline conditions (Table 3). Furthermore, DA improved CRV (SPAD) and LRWC values of bean plants under salinity stress (Table 4). This molecule helped to regulate chlorophyll concentrations and stomatal behaviour, while also altering the uptake, transport, partitioning and resorption of nutrients within the whole plant (Liang et al., 2018). We propose that this LRWC efficiency, regulatory role of DA has a positive influence on salt tolerance and offers new opportunities for its use in agriculture, especially in regions that are challenged by such stress conditions in the field. DA has been reported to enhance the photosynthetic activity in plants under salt stress. In addition, exogenous DA treatments enhance water-use efficiency (Liu et al., 2020). DA applications decreased the increased MDA and H₂O₂ levels caused by salt stress (Table 4). DA-treated bean plants had higher antioxidant enzyme activity in both saline and unsalted conditions than non-DA-treated bean plants (Figure 1). DA, a water-soluble antioxidant, has been reported to be involved in plants’ tolerance to abiotic stresses, modulating various metabolic processes in plant cells, such as oxygen scavenging processes, plant sugar metabolism and photophosphorylation of chloroplasts. It has been reported that DA applications to plants exposed to abiotic stress factors such as salinity promote the functioning of the antioxidant defence mechanism, strengthen the plant body, have positive effects such as photosynthesis and water use efficiency, and increase the resistance of plants to stress factors by reducing lipid peroxidation and membrane permeability in plants under stress (Ahammed and Li, 2023).

It has been reported that DA applications to plants exposed to abiotic stress factors such as salinity promote the functioning of the antioxidant defence mechanism, strengthen the plant body, have positive effects such as photosynthesis and water use efficiency, and increase the resistance of plants to stress factors by reducing lipid peroxidation and membrane permeability in plants under stress (Ahammed and Li, 2023). Similarly, Abdulmajeed et al. (2022) signified that DA had remarkable anti-stress effects, which could help regulate plant self-defence systems, reflecting satisfactory plant growth and productivity in bean under cadmium stress. The findings of this study revealed that salt stressed bean plants had more SOD, POD and CAT activity (Figure 2). When the plant is exposed to any abiotic stress such as salinity, its sensitivity to stress conditions increases and the H₂O₂ level changes. Reducing the ROS level is achieved by antioxidant enzymes such as SOD, POD and CAT (Dadasoglu et al., 2021). Exogenous DA treatments enhance antioxidant activity (SOD, POD, CAT) and reduce the production of H₂O₂ of crops under salinity conditions. Akcay et al. (2024) determined exogenous DA treatments lowered EL under drought stress, MDA levels under salinity stress in tomato seedlings. When plants face stressful conditions, DA can have some surprising benefits. Studies show it can increase their rate of photosynthesis, allowing them to capture more sunlight for energy. Additionally, DA helps plants to use water more efficiently and strengthens their antioxidant defences (Akcay et al., 2024).

DA also has been shown to modulate the photosynthetic oxygen reduction process. The ameliorative impact of DA on plants under salinity conditions can be attributed to its function as an oxygen reduction factor, allowing oxygen reduction to participate in energy conversion during photosynthesis. Thus, DA may have an ameliorative impact in plants under salinity conditions by preventing oxidative stress-induced tissue damage. This may also be due to DA oxidation leading to the production of melanin, an effective free radical scavenger. This study is in concordant with Li et al. (2015) who determined DA treatments could enhance SOD, CAT and POD activities and suppress H₂O₂ in apple plants under salinity conditions. Just like vitamin C and catechin, DA and its by-product, melanin, act as powerful antioxidants within plants. They directly eliminate harmful molecules called ROS that can damage cells. When these ROS appear, DA transforms into a red pigment called dopachrome, signalling the stressed area. Dopachrome then gets further oxidized into the familiar black pigment, melanin (Roshchina, 2022). In this study, it was determined salt stressed plants had much more proline content than non-stressed plants (Figure 2). Proline accumulation is a common phenomenon in many monocot and dicot plant species under salt stress (Elrys et al., 2020). In this study, DA treatments had positive impact on proline content in bean under salinity stress (Figure 2). Positive impact of DA on plant growth and physiological parameters in stressful conditions can be directly or indirectly attributed to the role of DA in nutrient uptake and root system architecture, photosynthetic capacity and pigments, stomatal regulation, N fixation and photophosphorylation (Farouk et al., 2023).

Salinity conditions had significant impacts on the hormone content of seedlings. Our findings demonstrated that salt stress resulted in decreased IAA and SA content in bean seedlings (Figure 2). However, salt-stressed seedlings had more ABA content than the non-stressed ones. ABA, known as a stress hormone, accumulates in significant amounts in plants under stress conditions and helps the plant survive (Waśkiewicz et al., 2013; Yu et al., 2020). Samancioglu et al. (2016) reported that drought stress elevated ABA content but reduced IAA and GA in cabbage plants. Our study findings are in concordance with those of Kazan (2013), who found that that the IAA concentration decreases and the ABA amount increases in potatoes, rice and tomatoes under salt stress. Plant hormones are active members of signalling compounds involved in the induction of stress responses in plants (Kaya et al., 2009). Moreover, DA treatments caused elevated IAA and SA content while decreasing ABA content of bean plants. DA is coordinated with phytohormone activity to regulate growth and enable plants to fine-tune their stress responses (Kulma and Szopa, 2007).

Salinity stress decreased the K/Na⁺ ratio compared with the control treatment (Figure 2). With the increase of Na⁺ concentration in the rhizosphere region, while Na⁺ entry into the plant stem cell increases, the uptake of K⁺ into the cell decreases, and accordingly, the K/Na⁺ balance of the plant stem cell is disturbed. This is because Na⁺ competes with K⁺ for areas where K⁺ will bind (Ahmad et al., 2015). Along with the fact that excess salt decreases the water potential in the cell in plants, it also disrupts the ion balance in the cell, negatively affecting plant growth (Hao et al., 2021). K/Na values increased with DA treatments in salinity conditions (Figure 2). Na and K content can be used as a good indicator of tolerance to salinity stress in plants. DA treatments could reduce Na⁺ uptake by plants while keeping of K⁺ content. In fact, earlier researches determined that DA treatments could enhance the water-use efficiency in salt stressed plants. High water use efficiency can decrease salt uptake by plants and prevent water shortage. It has been suggested that plant cells under salt stress may lower the concentration of Na⁺ in the cells by expelling them or compartmentalizing them into vacuoles, thus decreasing the negative effect of the salt stress (Liu et al., 2020).

Salinity has an important role in plant production. High concentrations of salinity can cause toxic effects for plants. To our knowledge, this is the first study to reveal the effects of DA treatments on plant growth of bean during seedling under NaCl stress. This study shows that DA may be used to decrease the effect of salt stress by inducting the systemic tolerance of plants. In conclusion, DA mitigates NaCl stress by regulating antioxidant enzymes activity, plant nutrient and phytohormones content in bean. Further studies should be conducted under field conditions at large scale. Moreover, economic analysis should be made to determine availability.