INTRODUCTION

Salinity has emerged as a major issue for agriculture all over the world, including Bangladesh. Approximately 40% of the earth’s land area may be at risk due to salinity issues globally (Naher et al., 2011; Imran et al., 2024). Bangladesh occupies an area of 147,570 km². Approximately 20% of the nation and more than 30% of the net cultivable land are coastal. It extends inside up to 150 km from the coast. Approximately 0.83 million hectares, or more than 30% of Bangladesh’s total cultivable land area, are arable lands, out of 2.85 million hectares of coastal and offshore areas (Haque, 2006; Howlader et al., 2018). The cultivable areas in coastal districts are subject to different degrees of soil salinity. Bangladesh’s coastal and offshore areas include tidal, estuaries, and river floodplains in the southern Bay of Bengal. Agricultural land usage in these places is quite poor, accounting for around 50% of the national average (Petersen & Shireen, 2001; Haque et al., 2024).

Salinity generates unfavorable environmental and hydrological conditions, limiting normal crop production throughout the year. Salt stress influences plant growth in three distinct ways: (i) Increased osmotic potential of the soil solution, (ii) Ionic imbalance, and (iii) Ionic toxicity (Munns et al., 2012). The quantity of salts in the soluble solution directly correlates with a decrease in crop yield (Tavakkoli et al., 2011). Plant morphological and physiological activities such as photosynthesis, transpiration, stomatal conductance, plant water status, and growth, can all be impacted by salt stress (Zamani et al., 2017). In Bangladesh’s coastal regions, newly deposited alluviums from upstream become salinized upon contact with seawater and continue to be flooded at high tides and seawater intrusion through creeks. Tidal flooding during the wet season (June- October), direct inundation by saline or brackish water, and upward or lateral migration of saline ground water during the dry season (November-May) are the variables that significantly contribute to the development of salty soils (Howlader et al., 2018). As a result, increased salinity and salt effect severely limit normal agricultural yield. Even saline incursion has an impact on the supply of water for drinking, irrigation, and industrial use (Miah et al., 2020). Saline soils are rich in Na⁺ and Cl⁻ ions, which make water unavailable and hinder crop growth. High salt concentrations negatively impact plant development through multiple pathways, including water stress, nutritional imbalances, ion toxicity, oxidative stress, disruption of metabolic processes, cell membrane damage, and reduced cell expansion and division (Shu et al., 2017). Salinity leads to Na⁺ toxicity and ionic imbalance, disrupting vital metabolic processes like protein synthesis, enzymatic activity, and photosynthesis. High Na⁺ concentrations compete with essential nutrients, affecting leaf water potential, sap movement, and stomatal conductance. Additionally, osmotic stress from salinity impairs photosynthesis by reducing CO₂ uptake and stomatal function (Angon et al., 2022). The salinity problem arises when the ion concentration of various salts in soils exceeds the threshold level required for normal germination, growth, and physiological activities of plants in root zones. Standard agricultural soils have a salt level of ~4 dS m⁻¹, comparable to 40 mM NaCl, as assessed by the electrical conductivity of the saturated extract (Shrivastava & Kumar, 2015).

Okra (Abelmoschus esculentus L.) is an annual herbaceous plant growing in tropical and subtropical regions that provides carbs, lipids, vitamins, and minerals (Sharmin et al., 2023). It is grown for its fibrous pods. It is more heat and drought tolerant than most vegetables, and it can produce more in heavy soils and soils with low moisture content, but it is less resistant to chills and frosts (Ayub et al., 2018). Okra includes essential components for human health, including vitamins, potassium, calcium, carbs, and unsaturated fatty acids like linolenic and oleic acids (Asare et al., 2016). Fruit of the okra plant contains 88% water, 2.1% protein, 0.2% fat, 8% carbohydrate, 1.7% fiber, 0.2% ash, 88 IU of vitamin A, 9.8 mg of vitamin C, and 116 mg of calcium per 100 g. Its fruits can be boiled, cut, or fried and served as vegetables. Its soft texture, which is very mucilaginous and helpful in thickening soup, is the major reason it is farmed by so many farmers (Akanbi et al., 2010). According to Maas & Hoffman (1977), it is categorized as a vegetable crop with a moderate salt tolerance. Despite the fact that salt stress has been shown to lower okra yield (Nitawaki et al., 2021). It is a significant summer crop, producing 56,145 tons from 11,458 hectares in 2017-2018 (BBS, 2018). The development of this particular vegetable crop is significant in Bangladesh because it acts as a critical way of meeting the country’s vegetable demand, especially during the summer season when vegetables are scarce in the market. So, okra is crucial to the nation’s ability to meet its vegetable needs (Sharmin et al., 2023). As Bangladesh’s land diminishes and its population and food demand grow, utilizing coastal areas for crop cultivation becomes crucial. This study was conducted to evaluate the salinity tolerance of okra by assessing its growth and development under varying levels of salt stress, analyzing its response to different salt concentrations in the soil.

MATERIALS AND METHODS

Experimental Location

The experiment was conducted at Sher-e-Bangla Agricultural University’s Central Farm, Dhaka, from September to December 2021. The experimental site is located at 90.20°N latitude and 23.50°E longitude, with an altitude of 8.2 meters above sea level.

Experimental Setup

A Completely Randomized Design (CRD) was used with four treatments and three replications. The study was carried out on a rooftop using pots, each with a capacity of 8 kg of soil, prepared with a mixture of cow dung and fertilizers. Pots were randomly assigned their respective treatments and replications to minimize bias. The experiment aimed to evaluate the performance of the Super Shomy hybrid variety of okra obtained from Bayer CropScience (https://www.bayer.com/en/).

Growth Conditions and Treatment Applications

Salinity treatments were applied to the pots 25 days after sowing. The salt solution was administered in three installments at five-day intervals following the designated treatment schedule. To meet the treatment requirements, a total of 8.1L of saline solution containing 4738.50 g of NaCl was prepared and diluted accordingly using the standard dilution formula (S₁V₁ = S₂V₂), where S₁ represents the initial NaCl concentration, V₁ is the required stock volume, S₂ is the target concentration, and V₂ is the final volume. The prepared solution was used for different treatments, including T₀ (Control - no saline application), T₁ (100 mM NaCl), T₂ (70 mM NaCl), and T₃ (40 mM NaCl). The saline solution was not directly applied to the pots; instead, each treatment was divided into three portions and incrementally applied to ensure uniform distribution and controlled exposure to salinity stress.

Measurement of Morphological Traits

To evaluate the impact of salinity treatments on plant growth, ten plants were randomly selected from each treatment and replication, and key morphological traits were measured following standard procedures as described by Shahid et al. (2011). Plant height (cm) was recorded from the base of the stem to the apex using a measuring scale. Leaf length (cm) and leaf width (cm) were measured from fully expanded leaves at the midpoint of the plant. Leaf area (cm²) was determined using the standard leaf area formula as described by Shahid et al. (2011). The number of leaves per plant and the number of branches per plant were manually counted to evaluate plant vigor and canopy development. These parameters were recorded at different growth stages to analyze the overall response of okra to increasing salinity levels and to determine the effectiveness of treatments in mitigating salt stress effects.

Statistical Analysis

All recorded data were subjected to analysis of variance (ANOVA), and mean separation was conducted using Least Significant Difference (LSD) at a 5% probability level, following the method of Gomez & Gomez (1984). Data were analyzed using SAS 9.3 software to determine the statistical significance of treatment effects on plant growth parameters.

RESULT AND DISCUSSION

Effects of salinity stress on plant height

Salinity stress poses a significant threat to plant growth, with even moderate salt levels causing noticeable reductions in plant height. There were no significant differences in plant height among the treatments at 20 days after sowing (DAS) because no saline application was applied. However, following the saline application, significant variations in plant height was observed at 40 and 60 days after sowing (DAS) (Table 1). The tallest plants were observed at 40 DAS in the control (T₀) with 64.90 cm, while the shortest plants were found in T₁ with 61.60 cm and T₂ with 62.13 cm, both of which were statistically similar. Similarly, the tallest plants were observed at the age of 60 DAS in the control (T₀) with 90.83 cm and lowest plants height were observed from T₁, T₂ and T₃ respectively. These findings suggested that more saline concentration had more negative impact on the height of okra plants which reduced the plant height. Similarly, Shahid et al. (2011) reported that maximum plant height (45.67 cm) was observed in plants grown under control, while the lowest height (26.33 cm) was found at 75 mM NaCl salinity levels, where salinity was at its maximum. So, it is clear that the highest salinity level exerted the most drastic effects on plant height as compared to the other salinity levels.

Effects of salinity stress on leaf length

Salinity stress significantly limits leaf development, reducing leaf length even at moderate salt concentrations. The highest leaf lengths were recorded at 30 DAS in the control (T₀) with 6.37 cm and in T₃ with 6.33 cm which was statistically similar, while the shortest leaf length was found in T₁ with 5.97 cm, where the salt concentration was high (Table 2). Similar trends were observed at 40 DAS and 60 DAS, with consistently higher leaf lengths in T₀ and shortest in T₁. At 20 DAS, no saline applications were made, and there were no differences observed in leaf length. These findings highlight the significant negative impact of saline concentration on okra leaf length at different stages of growth. According to Salik et al. (2021), the findings of this study reveal that saline levels in irrigation water caused a significant reduction in leaf length. The highest reduction in leaf length occurred at the 20 mM treatment level (6.8 cm), followed by 10 mM (7 cm), 5 mM (8.13 cm), and control (9.17 cm) which is consistent with the current study.

Effects of salinity stress on leaf width

Salinity stress significantly reduces leaf width in okra, highlighting its detrimental impact on leaf expansion and overall growth. Before the application of saline concentration at 20 days after sowing (DAS), there were no significant differences observed in okra leaf width among the treatments (Table 3). The highest leaf widths were recorded in the control (T₀) with 14.10 cm and in T₃ with 13.63 cm, which was statistically similar, while the shortest leaf widths were found in T₁ with 12.30 cm and T₂ with 12.83 cm, where the salt concentration was high. These findings highlight the significant impact of saline concentration on okra leaf width at different stages of growth. The decrease in leaf length and width due to elevated salt levels primarily resulted from reduced cell division and elongation. Consequently, this led to slower leaf growth and shorter leaves, as noted in Ifediora et al. (2014). Our study’s findings align with those of Ifediora et al. (2014), which observed smaller leaf size and delayed leaf emergence under high salt concentrations in irrigation water. This similarity underscores the osmotic impact of saline water, where leaf cells dehydrate due to a rapid rise in soil salinity.

Effects of salinity stress on leaf area (cm²)

Salinity stress significantly reduces okra leaf area, underscoring its adverse effects on leaf expansion and overall plant productivity. The maximum leaf area was recorded at 30 DAS in the control (T₀) with 86.47 cm² and in T₃ with 85.13 cm² which was statistically similar, while the shortest leaf area was found in T₁ at 73.23 cm² and T₂ at 75.70 cm², where the salt concentration was high (Table 4). Similar trends were observed at 40 DAS and 60 DAS, with consistently higher leaf area in T₀ and shortest in T₁. These findings highlight the significant impact of saline concentration on okra leaf area at different stages of growth. Abbas et al. (2013) concluded that the percent reduction in leaf area was high under 80 mM NaCl stress which was maximum salt stress and there was no reduction in control. They also indicated that reduction of leaf area in okra depends on the level of salt stress.

Effects of salinity stress on number of leaves

Salinity stress leads to a noticeable decline in the number of okra leaves, reflecting its inhibitory effect on leaf production and overall plant vigor. Prior to the application of saline concentration at 20 days after sowing (DAS), there were no notable differences observed in the number of okra leaves among the treatments. The control group (T₀) showed the highest leaf count at 10.77, whereas the lowest number of leaves was recorded in T₁ at 9.13, where salt concentration was higher at the age of 40 DAS (Table 5). This trend remained consistent across all data collection periods, indicating a clear influence of saline concentration on the number of okra leaves at various growth stages. According to Qados (2011) the harmful influence of salinity on leaf number increases with the increase in sodium chloride concentration. This result validates the findings of our study.

Effects of salinity stress on number of branches

Salinity stress acts as a silent yet powerful inhibitor of plant growth, drastically reducing the number of branches as salt concentration increases. The highest branches at 60 DAS were recorded in the control (T₀) with 2.73, while the lowest branches were found in T₁ with 2.13, where the salt concentration was high (Table 6). This trend remained consistent across all data collection periods, indicating a clear influence of saline concentration on the number of branches of okra at various growth stages. According to Kareem et al. (2021), there was a significant decrease in the number of branches as salinity stress increased in okra. Control plants exhibited the highest number of branches, while plants subjected to 100 mM NaCl stress showed the lowest number. Decrease in number of branches with increase in salinity level was also recorded by Al-Zubaidi (2018) when he subjected two varieties of eggplant to salinity stress.

CONCLUSIONS

The results demonstrated significant variations in the morphological characteristics of okra across different salt concentrations. Higher levels of salt had a significant negative impact on these characteristics, leading to reduced values across various morphological parameters. Specifically, the control group exhibited the highest values, while the highest salt concentration treatment (T₁-100 mM) showed the lowest values. Interestingly, the lowest salt concentration treatment (T₃-40 mM) approached values similar to those of the control (T₀). These findings highlight that increasing salt concentration corresponded with decreased morphological parameters compared to the control treatments. The moderate tolerance observed at 40 mM NaCl suggests that this concentration could be a potential threshold for recommending okra cultivation in mildly saline soils, offering a viable strategy for improving crop productivity in saline-prone areas. Future studies should explore physiological and molecular mechanisms underlying salt tolerance to enhance breeding efforts for resilient crop varieties.