1 Introduction

Nepal is a small landlocked mountainous country bordered by India in south, east and west and China in north. Agriculture is chiefly recognized as the mainstream economic sector in Nepal for the overall development and remained as a major concern to the government, business and to the general people in large (Pokhrel and Rijal, 2017). Agriculture employs 65.7% of the labor force in Nepal. In Nepal, 3,091,000 hectares of land area is under cultivation and 1,030,000 hectares of cultivable land is still not cultivated (MoALD, 2018). The contribution of agriculture sector in natural GDP is 31.23%, where horticulture sub-sector has the most significant role, sharing 21.42% in AGDP of the country (MoALD, 2016).

Cabbage (Brassica oleracea var. capitata) is one of the most consumed and most important leafy vegetables worldwide due to its high nutritional value (Sawant et al., 2010; Shrestha, 2019). It is therefore necessary to improve the seeds of cabbage for more production. This is a plant that grows in certain regions of the temperate zone and is cultivated every two years (Ahmed et al., 2019). However, its cultivation is equally successful in the tropical and sub-tropical regions (Devi et al., 2017). It is used as vegetables in curries, salad, and pickling. It is taken alone or in combination with potatoes for purposes of preparing vegetables (Singh, 2015). As per studies, it was found that 100 g of the green edible portion of the cabbage is present with 92 %water, 18 mg sodium, 170 mg potassium, 1.28 g protein, 5.8 g carbohydrate, 4 % calcium, and 2 % mg iron (Dev et al., 2020). It is also rich in indole-3-carbinol which helps in improving the capability of DNA repair in cells and seems to hinder the formation of cancer cells (Moniruzzaman et al., 2019). The seed production in cabbage can be done by seed to seed and head to seed method. In head to seed method, head intact method is especially practiced where a cross cut is made to facilitate the emergence of the flower stalk (Singh et al., 2018). Despite its importance, cabbage seed production occupies a relatively small portion of the vegetable seed importance sector. It accounts for about 0.5% of the vegetable seed-producing land (Aryal et al., 2022). Seed yield and quality are critical factors influencing cabbage productivity. However, cabbage seed production is often constrained by climatic conditions, particularly in cold and high-altitude environments where low temperatures and short growing seasons can adversely affect flowering, seed set and maturation. The application of plant growth regulators has therefore been studied as a method of enhancing cabbage and reproductive performance.

Gibberellic acid (GA₃) and naphthalene acetic acid (NAA) are widely used plant growth hormones which exhibited beneficial effect in several crops such as cell elongation and floral induction (Chaurasiy et al., 2014). Similarly, application of plant growth regulator is also better to enhance the yield of vegetable without compromising with their quality (Singh and Verma, 2021). Previous studies have reported that GA₃ application can enhance plant height, leaf number, flowering and seed yield in cabbage and other Brassicaceae crops (Moniruzzaman et al., 2019; Dev et al., 2020). Similarly, NAA has been shown to enhance flower retention, pod development and seed maturation by influencing auxin-related physiological mechanisms. However, the majority of these investigations have been carried out in lowland or moderate climatic conditions.

Systemic studies evaluating the combined and comparative effects of GA₃ and NAA on cabbage seed production under high-altitude, cold climate conditions remain limited. Specifically, data regarding ideal hormone levels for improving seed yield in areas like Mustang, Nepal is limited. Therefore, this study was conducted to evaluate the effects of different concentrations of GA₃ and NAA on the growth and seed yield of cabbage in high altitude environments, with the goal of producing location-specific recommendations for cabbage seed production in colder areas.

2 Materials and Methods

2.1 Experimental site and climate

The experiment was conducted at Temperate Horticulture Development Center (THDC) located at Gharapjhong Rural Municipality-2, Marpha, Mustang district, coordinated at 28 °20’ to 29 °05’ N and 83 °30’ to 84 °15’ E with an altitude of 2,650 masl. The area is characterized by a cold, high altitude climate with cool summers and cold winters. The average annual minimum and maximum temperature range reported during the research period were 4.9 °C-20.7 °C. The average annual rainfall of the study site was recorded to be about 36.875 mm. The soil in the experimental area was sandy loam to sandy in texture, well-drained and was prepared by ploughing and leveling before transplanting.

2.2 Plant material and experimental design

The experiment was conducted using cabbage (Brassica oleraceae var. capitata) plants of Copenhagen market variety. The head of the matured cabbage was transplanted at the first week of Bhadra. Transplanting was done at a spacing of 60 × 75 cm and plot size was maintained 2.40 × 3.00 m. The experiment was laid out in a single-factor Randomized Complete Block Design (RCBD) with 7 treatments and 3 replications. The treatments namely T₁, T₂, T₃, T₄, T₅, T₆ and T₇ consisted of different concentrations of GA₃ and NAA including a control which is shown in (Table 1).

2.3 Cultural practices

Standard agronomic practices were followed throughout the experimental period. Land preparation involved deep ploughing followed by harrowing to make soil free from large soil clods and weeds prior to transplanting. Seedlings of uniform size and age were transplanted carefully. Fertilizers were applied according to recommended doses, with basal and topdressing applications. Irrigation was provided immediately after transplanting to maintain adequate soil moisture, then it was given at 7-8 days interval. Weeding and plant protection measures were carried out uniformly across all plots to minimize biotic stress.

2.4 Data collection and measurements

Data were recorded from central 2 plants per plot resulting in a total of 42 sampled plants. Observations were taken at 15 days after treatment (DAT), 30 DAT and at harvest. Growth parameters included peduncle height, number of leaves, number of branches and canopy cover. Yield attributes such as number of inflorescences, pods per inflorescence, seeds per pod, Thousand Grain Weight (TGW) and seed yield per plant were recorded following standard measurement procedures.

2.5 Statistical analysis

Data entry was done with the help of MS Excel following standard format then data were subjected to analysis of variance (ANOVA) appropriate to one-way randomized complete block design technique using RStudio. All the analyzed data were subjected to Duncan’s Multiple Range Test (DMRT) for mean comparison at 5% level of significance and findings were discussed related with available literature (Gomez and Gomez, 1984).

3 Results and Analysis

3.1 Growth parameters

3.1.1 Peduncle height

The result showed that peduncle height was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 2). At 15 DAT, the maximum peduncle height was recorded from GA₃ at 50 ppm (106.33 cm) followed by GA₃ at 100 ppm (96.33 cm) which was statistically similar with NAA at 100 ppm (87 cm). Monoruzzaman et al. (2019) also reported the maximum peduncle height from GA₃ at 50 ppm. The control treatment gave the lowest peduncle height (58.67 cm) which was statistically at par with NAA at 50 ppm (61.67 cm). At 30 DAT, the maximum peduncle height was recorded from GA₃ at 100 ppm (136.33 ppm) which was statistically similar with GA₃ at 50 ppm (118.83 cm) whereas the minimum peduncle height was recorded from control treatment (95.33 cm).

At harvest, the mean peduncle height was found to be 121.61 cm. The maximum peduncle height was recorded from GA₃ at 100 ppm (136.33 cm) which was statistically similar with NAA at 100 ppm (130.33 cm). The minimum peduncle height was recorded from control treatment (110.33 cm) which was statistically at par with NAA at 50 ppm (113 cm) and NAA at 75 ppm (117.33 cm). However, the minimum peduncle height was recorded all in control treatment at 15 DAT, 30 DAT and a harvest.

3.1.2 Number of leaves

The analyzed data revealed that the number of leaves was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 2).

At 15 DAT, the mean leaf number was found to be 131.52. The maximum leaf number was observed from GA₃ at 50 ppm (147.33) followed by GA₃ at 100 pm (136) and NAA at 100 ppm (41.33). Kumar et al. (2023) also reported the maximum and minimum leaf number from GA₃ at 50 ppm and from control treatment. The minimum leaf number was observed from control treatment (120.33) and NAA at 50 ppm (120.33).

At 30 DAT, the mean leaf number was found to be 137.85. The maximum leaf number was recorded from GA₃ at 50 ppm (167.67) and NAA at 100 ppm (162) whereas the minimum leaf number was observed from control treatment (105).

However, the maximum and minimum leaf number was recorded from GA₃ at 50 ppm and control treatment at both 15 DAT and 30 DAT.

3.1.3 Number of branches

It is clear from the data that the application of GA₃ and NAA significantly affected on number of branches of cabbage at 30 DAT whereas the treatment was found non-significant at 15 DAT (Table 2). It might be due to various environmental factors like light, temperature and nutrient availability which can modulate the plant’s response to hormones. The mean branch number at 15 DAT was found to be 31.14. The maximum branch number was observed from GA₃ at 100 ppm (49.83) which was significantly similar with NAA at 100 ppm (43.17). The minimum number of branches was observed from control treatment (27.33) along with GA₃ at 50 ppm (29.17) and GA3 at 75 ppm (29.17) which was statistically at par with NAA at 50 ppm (32.83) and NAA at 75 ppm.

3.1.4 Canopy cover

The analyzed data revealed that the canopy cover was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 2). The canopy cover was also found non-significant at 15 DAT. The mean canopy cover recorded was 31.14 cm.

At 30 DAT, canopy cover was significantly affected by the application of different concentrations of GA₃ and NAA. The mean canopy cover was 111.80 cm. The maximum canopy cover was recorded from GA₃ at 100 ppm (124.67 cm) which was statistically at par with GA₃ at 50 ppm (118.83 cm). GA₃ at higher concentration leads to leaf expansion which helps to spread plants. This similar work was presented by Dev et al. (2020) who reported that maximum canopy cover would be obtained with the treatment of highest dose of GA₃. GA₃ at 75 ppm (107.33 cm), NAA at 75 ppm (112.33 cm) and NAA at 100 ppm (107 cm) showed the similar effect on canopy cover whereas the minimum canopy cover was observed from control treatment (95.33 cm).

3.2 Yield parameters

3.2.1 Number of inflorescences

The analyzed data revealed that the number of inflorescences was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 3). The mean number of inflorescences was found to be 127.04. The higher number of inflorescences was found from GA₃ at 100 ppm (140) which was statistically similar to GA₃ at 75 ppm (134.67) and NAA at 100 ppm (136). Similarly, GA₃ at 75 ppm and NAA at 100 ppm showed the similar effects which was statistically similar with GA₃ at 50 ppm (131). The lower number of inflorescences was observed in control treatment (104.67) as usual like in other parameters.

3.2.2 Number of pods per inflorescence

The analyzed data revealed that the number of pods per inflorescence was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 3). The mean number of pods per inflorescence was found to be 15.99. When the data for the number of pods per inflorescence parameter were analyzed, the best results were given by the application of GA₃ at 50 ppm (22.19) followed by GA₃ at 100 ppm (17.10) and NAA at 100 ppm (16.67). Even though the GA₃ at 100 ppm and NAA at 100 ppm treatment statistically mirrored each other, the effect of GA₃ at 100 ppm gave the maximum number of pods per inflorescence. GA₃ at 75 ppm (14.37) showed the intermediate effect which was statistically similar with NAA at 50 ppm (15.55) and NAA at 75 ppm (14.05). Number of pods per inflorescence was found lower in control treatment (12).

3.2.3 Number of seeds per pod

The analyzed data revealed that the number of seeds per pod was significantly influenced by the application of different concentrations of GA3 and NAA (Table 3). The mean number of seeds per pod was found to be 23. When the data for the number of seeds per pod parameter were analyzed, the best results were given by the application of GA₃ at 50 ppm (24.67) which was statistically at par with GA₃ at 75 ppm (23.33) and GA₃ at 100 ppm (24). All NAA treatments with concentrations 50 ppm (22.67), 75 ppm (22.33) and 100 ppm (22.33) gave the similar results. Although they gave the similar results, NAA at 50 ppm gave the higher seeds per pod. GA₃ at 75 ppm gave the intermediate effects of all i.e. 23.33. The number of seeds per pod was found lower in control treatment (21.67) as usual.

3.2.4 Thousand grain weight (TGW)

The analyzed data revealed that the Thousand Grain Weight (TGW) was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 3). When the data for the Thousand Grain Weight (TGW) parameter were analyzed, the best results were given by the application of GA₃ at 50 ppm (1.17 g) and GA₃ at 100 ppm (1.28 g). Even though the GA₃ at 50 ppm and GA₃ at 100 ppm treatment statistically mirrored each other, the effect of GA₃ at 100 ppm was greater than the effect of GA₃ at 50 ppm. The lowest Thousand Grain Weight (TGW) was recorded from the control treatment (1.17 g) and from NAA at 50 ppm (1.17 g).

3.2.5 Seed yield

The analyzed data revealed that the seed yield was significantly influenced by the application of different concentrations of GA₃ and NAA (Table 3). When the data for the seed yield were analyzed, the best results were given by the application of GA₃ at 50 ppm (2.91 t/ha) followed by NAA at 100 ppm (2.53 t/ha) which was statistically similar with GA₃ at 100 ppm (2.68 t/ha). NAA at 75 ppm (1.82 t/ha) was statistically at par with NAA at 50 ppm. The control treatment gave the lowest seed yield (1.48 t/ha) like other parameters which was expected.

4 Discussion

The growth and yield parameters of cabbage were significantly affected by the application of different concentrations of GA₃ and NAA. Notably, GA₃ at 50 ppm produced the highest seed yield (2.91 t/ha) and also resulted in superior performance across several traits, including peduncle height, number of leaves, pods per inflorescence and seeds per pod. Similar findings were reported by Roy and Nasiruddin (2011) who observed maximum cabbage yield at 50 ppm, suggesting that moderate concentrations of gibberellin are more effective than higher doses for optimizing plant productivity. Likewise, Kumar et al. (2023) reported the highest leaf number in plants treated with GA₃ at 50 ppm and the lowest in the control treatment which is in agreement with the findings of the present study. The enhanced leaf production observed under GA₃ application may be attributed to its stimulatory effect on the apical meristem, promoting cell division, cell elongation and nucleo-protein synthesis thereby increasing leaf initiation and development.

Furthermore, GA₃ at 50 ppm significantly increased the number of inflorescences, pods per inflorescence and seeds per pod, aligning with the findings of Dev et al. (2020). This suggests that a lower concentration of GA₃ enhances growth and seed yield more effectively than higher doses, which is consistent with the observation by Moniruzzaman et al. (2019) and Prodhan et al. (2022) that excessive GA₃ can induce disproportionate vegetative growth or physiological stress, ultimately reducing reproductive output.

NAA treatments also improved growth and yield parameters compared with the control, particularly at 100 ppm, which produced seed yield statistically comparable to GA₃ at 100 ppm. Auxins such as NAA are known to enhance cell enlargement, nutrient translocation and sink strength thereby supporting reproductive growth and seed formation. However, the overall response of cabbage to NAA was lower than that observed for GA₃ indicating that gibberellin played a more dominant role in regulating flowering and seed yield under the environmental conditions of Mustang. Similar positive effects of NAA on cabbage growth and yield have been reported by Neelam et al. (2023).

From a practical perspective, the findings indicate that foliar application of GA₃ at 50 ppm during head initiation and flowering can be recommended as an effective strategy for improving cabbage seed production in temperate regions of Nepal. Nevertheless, the study was conducted at a single location during one growing season. Therefore, further multi-location and multi-year studies are necessary to validate these findings under varying environmental conditions. Further investigations should also examine physiological and biochemical responses associated with plant growth regulator application to better understand the mechanisms responsible for enhanced seed yield.

5 Conclusion

On the basis of the above findings, it can be concluded that different concentrations of GA₃ and NAA has significant effect on growth parameters like peduncle height, number of leaves, number of branches, canopy cover and yield parameters like number of inflorescences, number of pods per inflorescence, number of seeds per pod, Thousand Grain Weight (TGW) and seed yield per plant.

GA₃ at different ppm has shown more effect than NAA at different concentrations whereas the control treatment has the least effect in all measured parameters above. GA₃ at 50 ppm has given the best result in maximum parameters like plant height, leaf number, number of pods per inflorescence, TGW and seed yield per plant.

Authors’ contributions

ST handled the data collection, data analysis and manuscript writing. SK, AC and NP helped in sample collection. PNA and AS served as the primary supervisor and helped in choosing the research topic. BBK involved in the manuscript revision and providing the final structure to the manuscript. All authors read and approved the final manuscript.

Acknowledgement

We would like to acknowledge Agriculture and Forestry University, Rampur, Chitwan, Nepal and Temperate Horticulture Development Center, Marpha, Mustang for the constant support and guidance during the research period.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be constructed as a potential conflict of interest.

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