The bitter gourd is an important vegetable of cucurbitaceae family. The small-fruited bitter gourd Momordica charantia var. muricata with small, tuberculate fruits is considered as progenitor of commercially grown large-fruited bitter gourd (Walters and Decker-Walters, 1988). It is an herbaceous vine with wide variations in leaves, flowers and fruits (Neelavathi et al., 2015). The immature fruits contain charantin, momordicin, polypeptide P, vicine, insulin-like peptides and other steroidal glycosides. Bitter gourd fruits are known to possess antihyperglycemic (Ali et al., 1993; Virdi et al., 2003), antilipidemic (Fernandes et al., 2007), antiviral, antiulcerogenic, antitumorigenic and anti-inflammatory properties. Bitter gourd fruits can regulate uptake of glucose in diabetic rats (Ahmed et al., 2004). The yield of bitter gourd is influenced by genotypes, nutrients, irrigation water, soil pH and soil properties. 

Salt affected soil is a problem in certain pockets of Tamil Nadu. Out of 4.7 lakh hectares salt affected soils in Tamil Nadu, 3.0 lakh hectares in inland and 1.7 lakh hectares in coastal areas. Out of the 3 lakhs inland salt affected soil, 2.00 lakh was due to alkalinity and 1.00 lakh hectares due to salinity. Salinity is a constraint limiting plant growth and productivity of vegetable crops. Salinity inhibits water uptake by plants, causes ionic imbalance leading to ionic toxicity and osmotic stress which affects the growth and fruit yield of horticultural crops. Sodicity is due to the presence of sodium salts in soil. Sodicity tolerance involves a complex of physiological responses and metabolic processes. Understanding the mechanism underlying plant response to salinity and sodicity provides new insights into the improvement of salt tolerance.  Plasma membrane of the cell is a site of salt injury (Mansour and Salama 2004). Electrolyte leakage is a key physiological parameter to evaluate abiotic tolerance (Arvin and Donnelly 2008). Salt tolerant genotypes maintained lower electrolyte leakage under sodic condition compared to salt sensitive genotypes. The plasma membrane integrity is maintained in genotypes having lower electrolyte leakage. 

Plants accumulate a group of metabolites particularly amino acids when exposed to biotic and abiotic stresses. Proline is an amino acid that accumulates in plants under abiotic stresses. Proline accumulation is a common physiological response in many plants in response to a wide range of biotic and abiotic stresses such as drought, salinity, low temperature, heavy metals and high acidity (Verbruggen and Hermans 2008; Hossain et al., 2014). Proline is an excellent osmolyte and compatible solute. It plays a role as a metal chelator, an anti-defence molecule and a signalling molecule. Proline imparts stress tolerance (Farkhondeh et al., 2012; Gharsallah et al., 2016) by maintaining cell turgor or osmotic balance and bringing concentration of reactive oxygen species within normal range. It protects folded protein structures against denaturation, stabilises cell membranes by interacting with phospholipids, functions as a hydroxyl radical scavenger, or serves as an energy and nitrogen source. The accumulation of free proline resulted in osmotic adjustment and salt tolerance in bitter gourd by facilitating water absorption, scavengers and reactive oxygen species molecules. There is a positive correlation between proline accumulation and stress tolerance (Dar et al., 2016; Mansour and Ali 2017). In this context, growth, yield, electrolyte leakage and proline were studied in small-fruited bitter gourd. 

The present investigation was carried out at the farm of Horticultural College and Research Institute for Women, Tamil Nadu Agricultural University, Tiruchirappalli, Tamil Nadu during 2018-20. Fifty genotypes of small-fruited bitter gourd collected from Tamil Nadu were evaluated for growth, yield, electrolyte leakage and proline content. The small-fruited bitter gourd seeds were sown in clay loam soil with three replications and Randomized Block Design (RBD). The experimental soil is sodic in nature with pH of 9.1 and EC of 0.12 dS/m and ESP of 33.62 % (Table 1). The available nitrogen, phosphorus and potash in the soil was 176 kg/ha, 24 kg/ha and 258 kg/ha, respectively. The seeds were sown in the beds at 2 x 1.5 m spacing in December 2018, June 2019 and January 2020. The vines were allowed to creep on the ground.

Table 1: Soil properties of experimental field.

Electrolyte leakage. Ten discs of fresh leaf (0.5 cm diameter) were cut from the fully expanded leaves and the leaves were washed three times with deionized water to remove surface-adhered electrolytes. Leaf discs were placed in test tubes containing 5 ml of deionized water. The initial electrical conductivity of the solution (EC 1) was determined using a conductivity meter. After 30 minutes, electrical conductivity of the solution (EC 2) was determined. The leaf discs were then incubated in a water bath for 10 minutes to release all electrolytes, cooled down to 25°C and their final electrical conductivity (EC 3) was measured. The electrolyte leakage (EL) was calculated as

Electrolyte leakage (%) = (EC 1 – EC2 / EC 3) × 100

Proline. Proline accumulation in leaf tissue was determined via reaction with ninhydrin. Purified proline was used to build a standard curve for proline content quantification. 0.5 gram of fresh leaf samples were homogenized in 10 ml of 3% aqueous sulfosalicylic acid and centrifuged at 3000 rpm for 1 minute. 2 ml of supernatant was reacted with 2 ml of ninhydrin acid and 2 ml of glacial acetic acid for 1 hour at 100°C in a heater. The chromophore was extracted using 2 ml of toluene, and its absorbance at 520 nm was determined by UV Spectrophotometer with toluene used as blank. 

Proline content =

((μg proline/mL × mL toluene)/115.5 μg/μmole) × (g sample/5) = μmoles proline gram FW−1

Statistical analysis. The data were statistically analysed (Panse and Sukhatme 1985). Level of significance is 5 per cent.

Electrolyte leakage. Electrolyte leakage has been used as an indicator of cell membrane permeability under abiotic stresses. The electrolyte leakage from plasma membranes is reported as one of the most important selection criterion for identification of salt-tolerant plants (Ashraf and Ali 2008). The electrolyte leakage was significantly influenced by bitter gourd accessions. The electrolyte leakage value varies with bitter gourd accessions. The lowest electrolyte leakage indicates decrease in membrane permeability and increased cell tolerance to salt stress. The lowest electrolyte leakage was recorded in MCM 10 (29.85%) followed by MCM 5 (31.43%) and MCM 2 (32.18%). The genotypes with higher electrolyte leakage are not desirable not only due to salt stress but also higher content of potassium (Mansour and Salama 2004). Demidchik et al. (2014) also stated that electrolyte leakage is mainly related to the efflux of K+ in plant cells.

Proline content. Accumulation of proline in plants is an indication of disturbed physiological condition, triggered by biotic or abiotic stress condition. Proline is a measure of stress in vegetable crops (Claussen, 2005). Determination of free proline levels is a useful assay to monitor physiological status and to assess salt tolerance of plants. There was a significant difference in accumulation of proline in the leaves of bitter gourd (Table 2). The higher accumulation of proline was recorded in MCM 41 (0.610 mg/g) followed by MCM 14 (0.547 mg/g), MCM 1 (0.545 mg/g) and MCM 16 (0.543 mg/g). Free proline content can increase upon exposure of plants to salinity and drought (Ábrahám et al., 2010). In some of the bitter gourd genotypes, the accumulation of higher quantity of proline was positively correlated with yield per plant. Protective role of proline against salt stress was reported by Huang et al., 2009. The proline content was positively or negatively correlated with yield that could be due to genetic character of the bitter gourd genotypes. The difference in the yield is due to the role of proline in flower transition (Saxena et al., 2008).

Table 2: Electrolyte leakage and proline content in leaves of small fruited bitter gourd.

Growth and yield characteristics. The pooled data on growth and yield parameters of small-fruited bitter gourd grown during 2018-2020 was calculated. The presence of salts was greatly influenced the germination of bitter gourd seeds. The germination percentage was ranged from 51.33 to 100. The number of days taken for germination ranged from 5.79 to 6.58. The number of days for germination was significantly lower (5.79) in MCM 3, followed by MCM 50 (5.95).  

The yield parameters of small fruited bitter gourd under saline soil is presented in Table 3. The number of days taken for first male flower opening ranged from 34.28 to 49.85. The number of days for first male flower opening was significantly lower (34.28) in MCM 25. The number of days taken for first female flower opening ranged from 41.52 to 59.27. The number of days for first female flower opening was significantly lower (41.52) in MCM 25, followed by MCM 7 (22.78) and MCM 24 (44.94). The fruit length ranged from 2.35 cm to 5.97 cm. The significantly higher value (5.97 cm) for fruit length was recorded in MCM 45, followed by MCM 41 (5.23). The individual fruit weight was significantly higher (9.94 g) in MCM 45, followed by MCM 39 (8.88) and MCM 41 (8.82). The number of fruits per plant ranged from 29.43 to 48.38. The significantly higher number (48.38) of fruits/plant was recorded in MCM 12, followed by MCM 1 (48.33). The higher yield per plant was recorded significantly higher in MCM 45 (399.4 g) followed by MCM 41 (324.3g) and MCM 1(309.3 g). Number of seeds/fruit ranged from 3.66 to 12.33. A significantly higher number (12.33) of seeds/fruit was recorded in MCM 47, followed by MCM 39 (9.23). The level of salt tolerance varied with cultivars which are corrected with growth (Arvin and Donnelly 2008). The growth of New Zealand spinach varied with different soil texture and salinity (Kim et al., 2011) and water spinach (Yousif et al., 2010).

Table 3: Yield characteristics of small-fruited bitter gourd.

Among 50 small-fruited bitter gourd genotypes evaluated, MCM 12 (44.38 fruits/plant) and MCM 1 (48.33 fruits/plant) recorded better yield performance. The lowest electrolyte leakage was recorded in MCM 10 (29.85 %) followed by MCM 5 (31.43 %) and MCM 2 (32.18 %). The higher accumulation of proline was recorded in MCM 41 (0.610 mg/g) followed by MCM 14 (0.547 mg/g), MCM 1 (0.545 mg/g) and MCM 16 (0.543 mg/g). The presence of wide variation in yield, electrolyte leakage and proline content in small-fruited bitter gourd offered a great scope for selecting the suitable genotypes in the breeding programmes for development of varieties/hybrids for sodic soil.

Study on proline and electrolyte in response to salt stress will provide the better understanding of developing technologies and varieties for salt affected soils. Proline induces the expression of salt‐stress‐responsive proteins and may improve the adaptation of plants to salt‐stress. Addressing the problems of salinity is necessary to sustain the vegetable production in future. The improvement in understanding of the responses of vegetable crops to salinity is one of the necessary prerequisites for increased cultivation and their use for phytoremediation could contribute to solving the problem of salinization.