Enhancement of Salinity Tolerance in Green Gram (CO 8) through Plant Growth Regulators: Physiological and Biochemical Responses

The experiment was carried out to determine the effect of plant growth regulators that mitigate the effects of salinity in green gram (CO 8). The research was conducted as a laboratory study at the Crop Physiology laboratory, Department of Crop Management, Thanthai Roever Institute of Agriculture and Rural Development, Perambalur, Tamil Nadu, India, to study the “Effect of plant growth regulators on green gram (CO 8) under salinity stress”. The experiment was laid out under a completely randomized block design with eight treatments and three replications. The seeds of green gram (CO 8) were placed in petriplates. The petridishes for the experiment were sterilized using 0.01 per cent HgCl₂ and 70 per cent ethanol, and finally repeated washing with distilled water. The salinity was imposed using NaCl at concentrations of 75 mM, 100 mM, 150 mM, and 200 mM. Among these NaCl concentrations, no seed germination occurred at 200 mM. At 75 mM and 100 mM, almost all the seeds germinated, along with the control. However, only 50 per cent of the seeds germinated at 150 mM NaCl compared to the control (Fig. 1). Hence, 150 mM NaCl was standardized for the experiment to evaluate the effect of plant growth regulators in mitigating salinity stress in green gram. Seeds were soaked in T₃: NAA 100 ppm, T₄: NAA 200 ppm, T₅: kinetin 50 ppm, T₆: kinetin 100 ppm, T₇: GA₃ 100 ppm and T₈: GA₃ 200 ppm. After that, the seeds were dried in the shade for 4 hours. These treated seeds were later placed on a germination sheet in each petri plate separately, with untreated seeds in the control and absolute control. The germination paper was moistened at regular intervals with a 150 mM NaCl solution for salinity and with water for the absolute control. The petri plates were kept in the laboratory at room temperature. The seeds were allowed to germinate by pouring approximately 10 ml of 150 mM NaCl solution once every 3 days. Distilled water was used to maintain absolute control. Germination percentage was recorded every 24-hour interval up to 7 days. Seeds were considered germinated when the radicle length reached at least 2 mm. Finally, germination was recorded on the 7^th day, and the number of seeds germinated was expressed as a percentage. Root and shoot lengths were measured 7 days after sowing and expressed in cm. For total dry matter production, 10 seedlings from each treatment were randomly selected and placed in a hot-air oven at 80 °C for 48 hours, then weighed to determine total dry weight. The seed vigour index was calculated using the following formula proposed by AbdulBaki and Anderson (1973). Stress tolerance index of the seeds was calculated using the following formula proposed by Dhopte and Livera-Munoz (1989). The contents of chlorophyll ‘a’, ‘b’, and total chlorophyll were estimated by adopting the procedure of Arnon (1949), and the content was expressed as mg g^-1 of fresh weight. The relative water content (RWC) was estimated according to Barrs and Weatherley (1962) and calculated using the following formula, expressed as per cent. The data on various parameters were analyzed statistically using AGRES software. Wherever the treatment differences were found significant, critical differences were calculated at the 5% probability level, and the values were furnished. The treatment differences that were not significant were denoted as “NS”.

Germination percentage (%)

When the seeds are exposed to salinity stress, the germination percentage was reduced compared to the absolute control. A significant difference was observed among all treatments with respect to seed germination (Fig. 2). Absolute control (T₁) showed the highest germination percentage (92.26%), and the control (T₂) showed the lowest germination percentage (48.21%). Seed treatment with PGRs resulted in a significant increase in germination percentage. Among the PGRs, the GA₃ 200 ppm (T₈) treatment showed the highest germination percentage (79.68%), followed by GA₃ 100 ppm (T₇) (76.27%) and Kinetin 100 ppm (T₆) (72.21%). However, the lowest germination percentage was observed in the NAA 200 ppm (T₄) treatment (59.78%) (Table 1). It may be due to GA₃ at 200 ppm (T₈) inducing seed germination by inhibiting abscisic acid in the seed. GA₃ is known to play an essential role in seed germination (Fahad et al., 2015). A similar result was reported by Khan and Panda (2008), who stated that GA₃ increased germination percentage in caper plants under salinity.

Shoot length (cm)

Significant differences were observed across all treatments in shoot length. The absolute control (T₁) recorded the highest shoot length (23.87 cm), and the control (T₂) recorded the lowest shoot length (4.49 cm). The result confirmed that salinity reduces shoot length. The PGR treatments showed a positive response against salinity. GA₃ 200 ppm (T₈) had the most prominent result (17.3 cm), followed by GA₃ 100 ppm (T₇) (17.3 cm), Kinetin 50 ppm (T₅) (9.3 cm). The lowest shoot length was recorded by NAA 200 ppm (T₄) (5.4 cm) (Table 1).

Root length (cm)

The root length (Table 1) was affected more pronouncedly by salinity than shoot length. Significant differences were observed across all treatments in root length. The maximum root length was observed in the absolute control (T₁) (8.54 cm), and the minimum root length was measured in Kinetin 50 ppm (T₅) (1.9 cm). The results support the idea that salinity causes a negative relationship with root length. However, the PGR treatments showed positive effects against salinity. Among the treatments, GA₃ 200 ppm (T₈) (6.4 cm), GA₃ 100 ppm (T₇) (4.5 cm), and NAA 100 ppm (T₃) (2.52 cm) showed notable effects. Kinetin 50 ppm (T₅) (1.9 cm), NAA 200 ppm (T₄) (1.15 cm), and Kinetin 100 ppm (T₆) (2.7 cm) were on par with control (T₂) (2.52 cm) (Table 1).

Vigour index

The vigour index results exhibited significant variation among the treatments. The highest vigour index was recorded in absolute control (T₁) (2989), and the lowest vigour index was recorded by control (T₂) (320). Among the treatments, GA₃ 200 ppm (T₈) recorded the highest vigour index (1745), followed by GA₃ 100 ppm (T₇) (1670) and Kinetin 100 ppm (T₆) (809). Significantly lowest vigour index was observed in NAA 200 ppm (T₄) (510) (Table 1). This might be due to increased germination percentage; shoot and root length were recorded under the GA₃ treatment.

Total dry matter production (g)

The data (Table 2) on total dry matter production showed a significant difference between the treatments. The highest total dry matter production was observed in the absolute control (T₁) (0.35 g), and the control (T₂) recorded the lowest value (0.09 g), indicating that total dry matter production decreased under salinity stress. Among the ameliorants used, GA₃ at 200 ppm (T₈) showed the highest value (0.25 g). The treatment NAA 200 ppm (T₄) showed the minimum value (0.12 g).

Stress tolerance index (%)

The results (Table 2) showed significant differences in the tolerant potential among the treatments. The highest stress tolerance index was noticed in GA₃ 200 ppm (T₈) treatment (83.09%), followed by GA₃ 100 ppm (T₇) (79.54%) and Kinetin 100 ppm (T₆) (38.54%); the lowest stress tolerance index was recorded by NAA 200 ppm (T₄) (24.31%). This increment may be due to GA₃ at 200 ppm (T₈) inducing germination, vigour index, and shoot and root length under a saline environment.

Chlorophyll content

The salinity treatments reduced the chlorophyll content compared to control. A significant difference was observed in all treatments with respect to chlorophyll content. Among the PGRs, the highest chlorophyll content was observed in seed treatment with Kinetin at 100 ppm (T₆) (1.22 mg g^-1), followed by GA₃ at 100 ppm (T₇) (1.09 mg g^-1). However, the lowest chlorophyll content was observed in the NAA 100 ppm (T₃) treatment (0.43 mg g^-1), followed by the NAA 200 ppm (T₄) treatment (0.69 mg g^-1). Chlorophyll content of the absolute control (T₁) is 0.90 mg g^-1 (Table 2). Kinetin improves chlorophyll content; Pakar et al. (2016) reported that foliar application of kinetin improved chlorophyll levels in salt-stressed plants.

Relative water content

Significant differences were observed across all treatments in relative water content. The GA₃ 200 ppm (T₈) treatment showed the highest RWC (88.44%). The control (T₂) had the lowest RWC (61.85%). Among the PGRs, RWC increased significantly. The highest RWC was observed in GA₃ 200 ppm (T₈), followed by Kinetin 100 ppm (T₆) (82.33%) and GA₃ 100 ppm (T₇) (80.70%). The lowest RWC among treatments was NAA 100 ppm (T₃) (68.43%), followed by NAA 200 ppm (T₄) (72.12%). The absolute control (T₁) RWC is 82.10% (Table 2).

Tables

Table 1. Effect of PGRs on Germination percentage (%), Shoot length (cm), Root length (cm), Vigour index, and Total dry matter production (g) of green gram under Salinity.

Table 2. Effect of PGRs on Stress tolerance index (%), Relative water content (%), Chlorophyll a (mg/g), Chlorophyll b (mg/g) & Total Chlorophyll content (mg/g) of green gram under Salinity.

Figures and Graphs

Fig. 1. Effect of Standardized NaCl on seedling growth of green gram CO 8

Fig. 2. Effect of plant growth regulators on seedling growth of green gram CO 8 under salinity

Abbreviations

Salinity stress significantly reduced germination percentage, shoot and root length, vigor index, total dry matter production (TDMP), Stress tolerance index (%), Relative water content (%), & Total Chlorophyll content (mg g-1) in green gram. The study standardized 150 mM NaCl as the optimal salinity level for evaluating plant growth regulator (PGR) treatments. Among the treatments, GA₃ 200 ppm (T8) showed the highest effectiveness, increasing germination by 31.56 %, shoot length by 46 %, root length by 46 %, vigor index by 70 %, TDMP by 87 %, and stress tolerance index (70 %) compared to the saline control. GA₃ 100 ppm (T7) was the second-best treatment, showing considerable improvements in growth parameters. Kinetin 100 ppm (T6) enhanced chlorophyll content under stress. These findings suggest that GA₃ 200 ppm is the most effective treatment for mitigating salinity-induced damage, with GA₃ 100 ppm as a suitable alternative.

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