Introduction

The global population is projected to increase by 8.6 billion in 2030, 9.8 billion in 2050, and 11.2 billion in 2100 [1], and food security is under-challenged due to an ever-increasing human population, limited cropland, and the risk of climate change [2]. Abiotic stresses caused by climate change progressively affect the cropping systems, which poses serious threats for global food production. Salinity is an abiotic environmental stress that seriously limits seed germination and hampers the seedlings and plant growth of different crops [3-7]. Salt stress is increasing day by day due to human action and climate change, which has a dramatic response to the morpho-physiology of plants. The coastal areas like the Ganges delta in Bangladesh are especially prone to salinity, which necessitates developing salt tolerant crop cultivars. The population in South Asian countries like Bangladesh is increasing rapidly from 75 million (1971) to 170 million (2022), whereas the production of pulse crops is decreasing rapidly because of salinity [8].

Mungbean (Vigna radiata L. Wilczek) belongs to the family of Fabaceae and is a major pulse crop for people in Asian countries like Bangladesh [9]. It is extensively cultivated in Bangladesh owing to its rapid stand establishment and early maturity. It is a vital constituent of diet owing to its superior nutritional value having 62.5% carbohydrates, 26% protein, 4.2% fibers, and 1.4% fat along with numerous minerals and vitamins [10]. It is the cheapest source of dietary protein for humans and livestock, extensively used in various culinary preparations, and recommended for diabetes [6]. The population of Bangladesh is increasing rapidly while cultivable land is decreasing day by day [11], and hence, it is indispensable to produce more food from our restricted land. Farmers use their fresh and fertile soils to grow more cereal crops like rice, wheat, and maize to meet the basic demand for food. For this reason, farmers do not use their fresh-fertile land in pulse cultivation [11]. With an increasing demand for grain food, farmers truly have no scope to use good soils for pulses, and this situation is projected to incur a serious shortfall of pulses soon [12]. Therefore, the crop should be shifted to the problem soils like saline soils. To overcome the protein deficiency, as well as malnutrition, problem pulses like the mungbean crop should be cultivated in the newly reclaimed lands, especially under saline conditions of such soil.

The productivity of mungbean is very low due to soil salinity that hampers plant growth and slices yield through specific ion toxicity [13], altered water relations [14], and the nutritional imbalance that causes huge aggregation of sodium ion and decreased potassium, calcium, and micronutrients [15-17]. The introduction of salt-tolerant mungbean genotypes after thorough screening in existing cropping patterns might enhance the utilization efficiency of salt-affected areas [18]. Therefore, it is now very important to screen out existing salt-tolerant cultivars and develop new varieties having novel ameliorative mechanisms to cope with the deleterious effects of a saline environment.

The sort out of salt-tolerant genotypes or lines has been done by many researchers on different species but no evidence exists regarding the response of existing mungbean genotypes to varying levels of salt stress. Thus, our research hypothesis was that mungbean genotypes might differ in their potential for salt tolerance and these could respond differently to varying levels of salinity. Therefore, the present research was conducted to identify the potential salt tolerance ability of existing mungbean cultivars under subtropical environments.

Experimental

Placement of Seeds for Germination

Before placing, the seeds were sterilized using mercuric chloride (0.5% w/v) solution for two minutes and washed properly with distilled water after sterilization. Required amounts of NaCl were dissolved in tap water to prepare 75 and 150 mM NaCl salt solutions. Fifty seeds were kept in each plastic pot (25 cm length, 20 cm width, and 15 cm deep) containing sand as a growing medium, and a total of 400 (50 x 8) seeds were used for the study. Tap water was irrigated in control treatment pots and prepared solutions of salt were irrigated as per treatment. Pots were retained at room temperature.

Fig. 1.

Daily maximum, minimum, and average temperature and relative humidity during the experimental period.

Table 1.

Characteristic features of the mungbean genotypes.

Variety	Source	Release year	TGW (g)	Life span (days)	Major diseases and pests
BARI Mung-7	Pulse Research Centre, BARI, Gazipur	2015	49-51	60-62	Powdery mildew, Cercospora leaf spot and yellow mosaic virus tolerant
BARI Mung-8	Pulse Research Centre, BARI, Gazipur	2015	30-32	60-62	Powdery mildew, Cercospora leaf spot and yellow mosaic virus tolerant
BU Mug-2	BSMRAU, Gazipur	2001	50-55	60-62	Have the ability to resist leaf spot and yellow mosaic virus disease
BU Mug-4	BSMRAU, Gazipur	2006	50-55	40-50	Have the ability to resist leaf spot and yellow mosaic virus disease

Results

Discussion

Seed germination is a crucial indicator that determines plant population and ultimately economic yield of pulses. In this study, the GP of various mungbean varieties was significantly affected by salinity levels. The GP was lessened gradually with rising salinity from 0-150 mM NaCl in all mungbean cultivars (Table 2). The cultivar BARI Mung-8 showed the minimum reduction of GP (23.20 to 77.41%), while the highest reduction (59.33 to 91.45%) was in BU Mug-2 at 75 and 150 mM NaCl concentration, respectively. These findings are in good agreement with the observations of Pattnaik et al. [26], who observed that increasing concentration of salinity progressively inhibited the germination of mungbean, and such findings have been reported previously by other researchers as well [27]. Similar results were also attained in several crops like pea [28], wheat [29] mountain rye (Secale montanum) [30], where significant reduction was obtained in germination and seedling growth characters. Several studies also recorded a significant variation in GP with rising salt stress [31-34]. Under salinity stress, seed germination is restricted due to low water potential [35]. The reduction of GP occurred owing to less absorption of water which altered the metabolic activity of plant cells [36] along with perturbing the hormonal balance of plants [37]. The reduction of GP owing to some metabolic disorders as well as specific ion toxic level concentrations is reported under salinity [38, 39], which lowered the water supply [40]. The decline of germination percentage under various salinity stresses can occur due to increasing dormancy in the seed [40]. The RG of mungbean varieties was also decreased imposing the NaCl-induced toxicity, and the magnitude of reduction varied among the varieties. The RG followed similar trends as the GP due to imposing salinity stress. Nevertheless, the minimum reduction of RG (16.00 and 3.00%) was opined in BARI Mung-8 at 75 and 150 mM NaCl stress, respectively, and reduction of RG can be ranked as BARI Mung-8 < BARI Mung- 7 (27.78 and 42.22%) < BU Mug-4 (37.86 and 54.26%) < BU Mug-2 (40.00 and 60.00%), respectively (Table 2). The rate of germination decreased in saline conditions due to the high osmotic pressure of salt water at seed depth than lower soil profile [41]. The same result was found in black gram by Hasan et al. [6] who stated that the RG decreased due to the creation of external osmotic potential of the germination media. These observations are coined by Kandil et al. [40], Shahzad et al. [42], and Babbar et al. [43] in mungbean.

Fig. 2.

Salt tolerance index (STI) based on dry weight of shoot of mungbean under saline environment.

CVG was decreased significantly in mungbean genotypes owing to NaCl osmotic potential (Table 3). However, the reduction of CVG shadowed the similar tendency of GP and RG among the varieties used in our study. This observation was coined with the findings of Tsague et al. [44], who stated that NaCl osmotic potential negatively changed the CVG of cowpeas. Similar results were also attained in black gram [6], cowpea [45], and Tephrosia purpurea [46] seeds. The MGT was significantly enhanced by rising salinity levels (Table 3). Salinity delays the germination by damaging the seed. A similar result was also obtained in common bean by Cokkizgin [47], who found that salinity affects the germinating seeds negatively and at salt stress during germination, salinity damaged seed-germination features and finally increased MGT. A similar finding was also attained by Naveed et al. [48], who found a significant decrease in emerging seeds percentage as well as a delayed emergence process due to increasing salt stress. NaCl delays the seeds’ emergence owing to the negative action of ion toxicity [49], and osmotic stress due to NaCl inhibits the imbibition of water by seeds, resulting in the delay of radicle emergence [50], which might have increased the MGT.

Similar to germination, salt stress adversely affects the growth and development process of mungbean cultivars [6, 51-53]. In our study, significant reductions in lengths of shoot and root of mungbean genotypes were attained due to increasing salt toxicity. This might occur due to fewer physiological activities resulting from water and essential ion stress [54]. Less transfer of metabolites to the expanding tissues may also reduce lengths because metabolite synthesis is greatly hampered at high salinity conditions [39]. Our experimental findings are coined with the observations of the study conducted by Hassan et al. [5], Hajier et al. [34], Niamat et al. [55], Ungar [59], El-Kafafi et al. [56]. NaCl-induced toxicity enhanced abnormal seedling production as well as decreased the lengths of seedlings [48]. Al-Mutawa [57] observed reduced radicle lengths due to imposing salinity. Mohammed [58] found that salt stress decreased the growth traits of the mungbean cultivar by reducing root lengths and lateral root numbers. Rising saline conditions reduced the height of the root as stated by Qu et al. [33]. Similar results were investigated in root length reduction with increasing salinity levels by Hassan et al. [5], and Hasan et al. [6]. Mohammed [58] also reported a reduction in growth attributes like lengths of shoot and root of mungbean cultivars.

NaCl-induced toxicity significantly reduced the growth attributes (fresh and dry weights, and lengths of shoot and root) of mungbean cultivars [59]. However, shoot growth was affected severely compared to root growth. Salinity hampers plants’ growth and development by decreasing emergence [60], and seedling growth [61]. In the present study, NaCl significantly affected the various growth attributes like the fresh weight of the shoot and root of mungbean varieties studied. From 0-150 mM NaCl, the shoot fresh weight was adversely damaged by salt toxicity, and the highest (87.69%) and the lowest (49.76%) reduction was examined in BARI Mung-8 and BU Mug-2, respectively (Table 5). The highest and lowest reduction of root fresh weight was attained at 150 mM salt stress in BARI Mung-8 and BU Mug-2, respectively (Table 5). Seedling fresh weight might be decreased due to low uptake of water by seedlings under induced saline toxic conditions [59, 62]. Anantharaju and Muthiah [63] also found similar results in their study. In the control condition, seedlings accumulate a huge amount of dry matter in the root and shoot compared to the seedlings in the salt stress condition [64]. An adverse effect of salinity on decreasing seedling length (shoot and root length) was also observed by Hasan et al. [6], Ullah et al. [27], Hosseini and Rezvani Moghaddam [65]. El-Kafafi et al. [56] observed salt stress drastically reduced the root fresh weight. Naher and Alam [66] also stated that the increasing salinity levels significantly reduced the growth of roots.

Salt stress significantly reduced the shoot and root dry weight. The reduction was severe in susceptible genotypes under NaCl-induced toxicity compared to the control treatment. Dry matter synthesis under salt stress might be due to the assembly of organic solutes and inorganic ions in an osmotic environment, while lower dry matter content at higher salinity levels might be due to the obstacle of reserved food hydrolysis and their distribution to the growing shoots [67]. The results are in agreement with the findings of Hasan et al. [6], and Saha et al. [68]. The reduction of shoot dry weight may happen due to the shoot osmotic adjustment ability under salinity [39]. This result also agreed with the observations of El-Kafafi et al. [56]. Fresh and dry weights of mungbean cultivars were decreased under salt stress [6, 56, 69, 70].

The seed vigor index was substantially lessened with increasing levels of salinity (Table 7). The SVI decreased with increasing levels of salinity, which indicates that higher salt stress caused a detrimental effect on the seed. Similar findings were attained by Khajeh-Hosseini et al. [71] in soybean, Segatoleslami [72] in the seeds of medicinal plants (Satureja hortensis L.), (Cynara scolymus L.) and Cokkizgin [47] in common bean seeds. On the other hand, the SVI expressed a substantial correlation with the GP, shoots, and root lengths of the fenugreek variety [73]; and with GP, CVG (Coefficient of velocity of germination) of common bean [46]. Reduction in seed vigor was also observed by Hokmalipour [74]. The findings of the study were also coined with the study of Foeniculum vulgare under salinity in which they observed that seed vigor was reduced under a saline environment [75].

To assess salt tolerance, one tolerance index was applied (STI) according to quantitative criteria based on their shoot dry weight in saline conditions. Higher STI means that the genotypes with high yielding capacity and high stress tolerance ability [6]. An increment of salt stress successively lessens the STI values [76].

Conclusions

Our observations are aligned with the hypothesis related to genetic divergences of mungbean cultivars to varying salinity levels regarding germination indices and seedling growth characters. From the findings, it could be concluded that NaCl-induced toxicity significantly varied the germination and growth of seedlings of all mungbean genotypes, while BARI Mung-8 was observed as a moderately salt-tolerant genotype. Based on the emergence and seedling growth characteristics, BU Mug-2 could be tagged as a susceptible genotype. Thus, the general adoption of BARI Mung 8 might be recommended following results validations under varying agroecological conditions.