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Assessment of Salinity Tolerance in Greengram [Vigna radiata (L.) Wilczek]

P. Shanthi1,*, M. Parameshwaran2, M. Umadevi1, S. Aadhilakshmi2, S. Keerthivasan2
1Department of Genetics and Plant Breeding, Tamil Nadu Agricultural University, Coimbatore-641 003, Tamil Nadu, India.
2Agricultural College and Research Institute, Tamil Nadu Agricultural University, Kudumiyanmalai, Pudukkottai-62 2104, Tamil Nadu, India.
Background: Greengram is one of the important and salt sensitive food legume crops with high nutritional quality. The area under salinity is increasing gradually due to various reasons like global warming, continues irrigation of bore well water and use of more organic fertilizers etc. Identification of salinity tolerant genotype in greengram is highly essential to improve the production and productivity. Salinity stress condition increases the absorption of more Na+ whereas the reduction of K+ absorption was noticed, which thus decreases the K+ /Na+ ratio. The first visible symptom due to salinity stress was retarded growth because of more uptake of Na+ ions leads to reduction of physiological activities. The next important character affected by salinity stress was radical growth.

Methods: A total of 20 greengram genotypes were evaluated for salinity tolerance under four EC levels. Screening was done by roll towel method with two replications. The roll towels were placed in different buckets filled with different EC level of saline water and observations were recorded on the 8th day of germination.  

Result: The salinity level of more than 4.0 EC dsm-1 causes gradual reduction in radicle length and at 12.0 EC level almost in all the genotypes the radicle length was reduced to half as compare to control. The correlation analysis indicated that the plumule length and radicle length were the important criteria for selection to salinity tolerance. While observing the other growth parameters like plumule and radical length reduction, dry matter weight and salinity tolerance and susceptible index the genotypes IPMD 14-10 and VGG 17-038 were identified as more tolerant genotypes and DGGV 80, PUSA-BM5 and VBN 4 were more sensitive genotypes. 
Greengram [Vigna radiata (L.) Wilczek] is one of the important food grain legume with high nutritional quality, high digestibility (Siemonsma and Na Lampang, 1992) with short duration (55-80 days) and helps to improve soil fertility level by its nitrogen fixation ability leads to sustainable agriculture production. It is grown in different ecologies and seasons across India invariably in most of the states. In Tamil Nadu this greengram crop is grown as irrigated crop during kharif and summer seasons and as rainfed crop during rabi season. Food and nutritional security are in question mark day by day in the increasing trend of global population and decreasing arable lands due to abiotic stresses. Among the abiotic stresses drought and salinity are the most important stresses. Presence of high concentration of soluble salts in the agriculture soils especially in the root zones refers to soil salinity. Worldwide the cultivable area affected due to salinity is 23 per cent and sodicity is 37 per cent (Khan and Duke, 2001). Soil salinity arises  due to geo-historical process or man-made one. Sea water intrusion is the main reason for coastal salinity and also increasing the salt concentration in ground water, the interior areas become saline due to continuous irrigation of borewell water with more salt concentration.

Salinity reduces more than 50 per cent of the yield in pulses including greengram. Presence of more salts in soil increases the osmotic pressure in the seed, which restricts the water absorption of the seeds (Tester and Davenport, 2003) and also the enzyme called α-amylase is inhibited. This ultimately affects the germination of seeds followed by the growth of the plants. Salinity delays the nucleic acids and RNAase synthesis leads to reduce the chlorophyll content of the plant. Increase in salt concentration affects germination percentage, shoot and root length, photosynthesis and yield attributes (Ghosh et al., 2016) in greengram. On this note the present experiment was carried out to identify the salinity tolerance genotypes in greengram during germination and young seedling stage.
A total of 20 greengram genotypes were evaluated for salinity tolerance under four EC levels. This experiment was carried out at Agriculture College and Research Institute, Kudumiyanmalai, Pudukkottai District of Tamil Nadu during 2019-21. The evaluation was done on the basis of morphological characterization of young seedlings traits since germination is one of the important stages to maintain plant population and initial vigor. A total of 20 different greengram genotypes  were  used for this experiment.   Four levels of salinity (4.0 EC, 8.0 EC, 12.0 EC and 16.0 EC dsm-1) were compared with control (0.0 EC). Since salinity is not due to the single salt stress the experiment solution with different EC levels were prepared using combination of salts viz., NaCl, Na2SO4, KCl, K2SO4, MgCl2, MgSO4 and CaCl2 in the ratio of 2:1:1:1:1:1:1, respectively dissolved in deionized water (Singh et al., 2009; Shanthi et al., 2021).  Screening was done by roll towel method with two replications and 20 seeds per replication were placed for germination in each treatment. The roll towels were placed in different buckets filled with different concentration of saline water till observations were recorded. To screen the genotype for salt tolerance, germination and seedling characters viz., Germination percentage, Plumule length, radicle length, Plumule-Radicle Ratio and Dry weight were taken into consideration. The observations were recorded on the 8th day of germination. The observations recorded from the genotypes in both the replications were subjected to two factor analysis of variation (Gomez and Gomez, 1984). ANOVA was carried out using STAR package and correlation was calculated. Salt tolerance index (STI) was calculated as (Goudarzi and Pakniyat, 2008) by the following formula.
The plants grown under salt stress condition were unable to absorb adequate water for its metabolic processes or maintain turgidity due to low osmotic potential (Shrivastava et al., 2015). Salinity stress condition increases cations uptake such as Mg, Na, Ca and causes different kinds of nutritional imbalances finally leads to different ranges of toxicity. Among the cations, the most important one is  NaCl toxicity, plants absorb more amount of Na+, which thus decreases the K uptake by plants (Ahmad and Prasad, 2012). The greengram genotypes grown under different salinity conditions affect its biochemical mechanism and leads to reduction in growth (Shanthi et al., 2011).

The two way ANOVA (Table 1) results clearly revealed that the main effects of salinity, genotypes and its interaction were highly significant in all parameters studied.

Table 1: Analysis of variance of different characters and various salt concentrations in greengram genotypes.

Hence the greengram genotypes taken for this study were varied in their response to different levels of salinity in germination and other characters in its seedling stage. The coefficient of variation varied among different characters and the maximum was recorded by the plumule radicle length ratio (15.39) and the minimum was recorded by dry matter weight (2.06). Similar result was also reported in blackgram (Shanthi et al., 2021a). This was also confirmed by the critical difference (CD) for all characters taken for this experiment were compared for different factorial plots and subplots with their interactions.

The reduction in germination percentage was noticed for increasing salinity levels invariably in all the genotypes (Table 2).

Table 2: Mean germination percentage of greengram genotypes at different levels of salt concentration.

The genotypic difference was observed within and among the salinity levels. In overall performance, while compared with control (0.0 EC level), the reduction in germination percentage was very less at 4.0 EC level and very high at 16.0 EC level. Similar results were observed in greengram (Prakash, 2017) and Shanthi et al., (2021a) in blackgram. Variability in salinity tolerance among rice varieties was reported by (Shanthi et al., 2011) and variation in germination has also been reported by Hakim et al., (2010).

Plumule length is the most important seedling character which affected due to salinity. Kumar et al., (2021) reported that the salinity is a major abiotic stress that significantly affects the plant growth by causing osmotic stress and inducing ionic and nutrient imbalance. The first visible symptom due to salinity is retarded growth (Shanthi et al., 2021b) prolonged salinity the plumule length was reduced. The maximum plumule length after 8 days of germination was recorded by the genotype IPMD 14-10 (18.9 cm) (Table 3) and it was reduced to half in most of the genotypes studied under 8.0 EC level and the length was reduced high at 16.0 EC level.

Table 3: Mean plumule length of greengram genotypes at different levels of salt concentration.

This character can be considered as one of the important criteria for identification of salinity tolerant genotype(s). These results were in accordance with the findings of Prakash (2017) in greengram.

The radicle length is another important parameter for salinity screening in seedling stage. Beyond the 4.0 EC level the gradual reduction in radicle length was observed and under 12.0 EC level almost in all the genotypes the radicle length was reduced to half as compare to control (0.0 EC) (Table 4).

Table 4: Mean radicle length of greengram genotypes at different levels of salt concentration.

Under 16.0 EC salinity condition all the genotypes recorded below 2.75 cm of radical length. These results indicated that the plumule length was more under control condition and reduced under stress conditions; whereas the radicle length was slightly increased up to 8.0 EC level and at 12.0 and 16.0 EC level both plumule and radicle lengths were reduced. The plumule length was severely affected by salinity and the radicle length was increased by slight increase in salt concentration to increase the water absorption rate and later at high salinity level (12.0 and 16.0 EC level) the radicle growth was also affected. The results were in accordance with Hanumantha Rao et al., (2016).

The average dry matter weight of single seedling under control is less than the seedling under salinity stress condition. The salinity level the dry matter weight also increased gradually and under higher saline concentration of 16.0 EC (Table 5).

Table 5: Mean dry weight of greengram genotypes at different levels of salt concentration.

Due to high level of salinity the sodium salt absorption by the plant may be increased that leads to accumulation in plants. Similar findings were also confirmed in blackgram by Shanthi et al., (2021b) and Priyadharshini et al., (2019).

Salt tolerance index and salt injury index were used to determine the degree of tolerance for different genotypes towards salinity. Salt tolerant index for plumule length, radicle length and dry matter weight were calculated. The salt tolerance index was reduced by increasing salt concentration for both plumule and radicle length and salt injury index was increased vice a versa. This results clearly showed that the increasing salt concentration affect the plant height. While comparing root length the damage due to salinity was comparatively low in root compare to shoot. Similar result was recorded by Shanthi et al., (2021a). Salinity stress significantly reduces the net photosynthetic rates, due to salt exclusion mechanism the energy losses was increased, nutrient mobilization largely decreased, cell division and cell elongation also reduced all this activity finally reduced the plant growth (Seeman and Sharkey, 1986).
Simple linear correlation
The linear correlation analysis was carried out for germination percentage, plumule length, radicle length, plumule radicle length percentage and dry matter weight under different EC level. The significant and positive correlation was recorded for germination percentage with plumule length at 4.0 EC level and 8.0 EC level and radicle length of control and 4.0 EC level. These results clarify that the lower level of salinity (up to 4.0 EC level) the germination percentage, plumule length and radicle length  were not affected more. W-hereas beyond 4.0 EC level the germination percentage, radicle and plumule length were drastically affected (Table 6).

Table 6: Simple correlation analysis for the growth characters at different salinity level for the greengram genotypes.

Similar results were reported by Shanthi et al., (2021a). Hence the radicle and plumule length can be taken as criteria for selection to identify the salt tolerant genotypes. Whereas the dry matter weight is concerned it has recorded negative and significant correlation with plumule length it clearly showed that the increasing salinity concentration reduced the plumule length and reduces the dry matter weight.
Identification of salinity tolerant greengram genotype is highly essential due to the gradual increase in salinity affected area day by day due to global warming and application of more inorganic fertilizers for cultivation. Among the 20 genotypes evaluated, based on the parameters IPMD 14-10 and VGG 17-038 are identified as most tolerant genotypes and DGGV 80, PUSA-BM5 and VBN 4 are more sensitive genotypes.
The authors are thankful to National Pulses Research Centre for providing seeds for this experiment.

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