1 Introduction

An important summer vegetables named Okra (Abelmoschus esculentus L.) belongs to the Malvaceae family that is grown all over Bangladesh. According to the USDA [1], okra pods are highly healthy and nutritious since they include P, K, Zn, Cu, Ca, Mg, and Mn, dietary fiber, protein as well as reasonable amounts of vitamins including ascorbic acid, thiamine, riboflavin, and niacin. 100 g of fresh fruit includes 89.6% water, 90 mg calcium, 103 mg potassium, 56 mg phosphorus, 43 mg magnesium, and 18 mg vitamin C [2]. Vegetables are highly produced in winter, but the availability decreases with the start of summer [3]. So, cultivating okra can largely compensate the shortage of vegetables during summer. The total okra production was about 85,000 metric tons [4], which is much lower than any other developed country [5]. Farmers in Bangladesh usually follow traditional practices, and they don’t have standard production techniques. As a result, they don’t get satisfactory yields. Plant growth regulators (PGR) may be help to enhance fruit and vegetable yields while reducing those obstacles [6]. PGRs are crucial appliances for modifying and regulating various development as well as plants physiological processes [7]. These agrochemicals may enhance plant physiological functioning by improving photosynthetic capacity and translocation of photosynthetic products, hence enhancing crop output overall [8]. PGRs are acknowledged as one of the quickest strategies to increase output [9]. GA3 and NAA are two PGRs from the Gibberellin and Auxin families, respectively. GA3 and NAA have been used to promote the growth, development, yield, and reproduction of many plant species. NAA enhances produce quality, yield, and growth [10]. Plant height, number of leaves, internode length, flowering initiation, fruit number and quality, and fruit weights of okra increased by both GA3 and NAA [11, 12]. Mukhtar [13] stated that PGRs helped vegetables grow and produce more efficiently. Gibberellins perform an important function in controlling processes of development in plants [14]. Plant growth is increased by the advanced concentration of GA3 [15]. NAA and GA3 encourage cell elongation, leading to taller and hasten flowering and fruit ripening [16]. Similarly, pod length and diameter increased by GA3 [17]. As mentioned above, these effects are mainly attributed to growth, development and yield attributes. Consequently, the present research sought to identify the optimum GA3 and NAA concentrations, as well as the best GA3 and NAA combination, for okra growth and development, yield, and quality attributes. There are a few studies found that application of GA3, NAA and PGRs improve the vegetative growth of okra. However, no information is known on how GA3 and NAA affect okra quality and yield. As a result, the aim of our research was to identify the effect of various dosages of both GA3 and NAA on improving okra production and quality.

2 Materials and methods

2.1 Experimental location and plant materials

This experiment has been carried out in the agro service center (ASC) research field, BADC, Chehelgazi, Dinajpur (25.66° N latitude, 88.65° E longitude, and 34.5 m altitude), from March to September, 2022 and 2023. During the experimental period, the average temperature ranged from 24 to 32 °C, and the relative humidity (RH) ranged from 70 to 85%. A hybrid dherosh (Green Finger) developed by Lal Teer Seed Limited was used as plant material. The variety is high-yielding and tolerant to yellow mosaic virus (YMV), prolific and high fruit set (50–55 pods), and medium-sized deep green pods with predominant ridges. Seeds were sown on 26 March 2022 and 24 march 2023.

2.2 Treatment and design

The experiment used three replications of the Randomized Complete Block Design (RCBD) and included three levels of GA3 and NAA: 0 ppm, 150 ppm, and 250 ppm, resulting in nine treatment combinations. The treatment combinations were as follows: N0G0: 0 ppm NAA + 0 ppm GA3 (Control), N0G1: 0 ppm NAA + 150 ppm GA3, N0G2: 0 ppm NAA + 250 ppm GA3, N1G0: 150 ppm NAA + 0 ppm GA3, N1G1: 150 ppm NAA + 150 ppm GA3, N1G2: 150 ppm NAA + 250 ppm GA3, N2G0: 250 ppm NAA + 0 ppm GA3, N2G1: 250 ppm NAA + 150 ppm GA3, N2G2: 250 ppm NAA + 250 ppm GA3.

2.3 Application of GA3 and NAA

The stock solution was firstly made by diluting a small amount of absolute alcohol in each weighed substance before adding appropriate distilled water to make an acceptable solution. GA3 and NAA were applied as a combined foliar spray. The solutions were mixed together before application to ensure a homogenous mixture of the two hormones. At the three-leaf stage, the combined GA3 and NAA solutions were applied using a hand sprayer, ensuring uniform coverage of the plant foliage. On the other hand, in the control treatment, only water was applied.

2.4 Nutrient management

Following the recommendation of the Bangladesh Agricultural Research Institute, urea, TSP, and MoP were applied at the rates of 150, 100, and 150 kg per hectare, respectively [18]. During the final land preparation, total TSP and one-fourth of the MOP fertilizers except urea were administered as the basal dose. At 20, 40, and 60 DAS, three equal splits of total urea and the remaining fertilizers were applied.

2.5 Measurement of growth and yield characters

Data on plant height, leaves number, and branch per plant at several DAS were collected and averaged among five plants. A portable leaf area meter named LI-LAM8 was used to determine the leaf area. Harvesting of the pod was done at 2-day intervals. The length and diameter of the internode and pod were measured by digital vernier calipers. Pod yield plant−1 and plot−1 were counted by portable digital balance. The dry matter content of the pod was estimated through the following equation:

$${\text{Dry matter content of pod}}\left( \% \right) ={\text{(Dry weight of pod/Fresh weight of pod}})\times {1}00$$

2.6 Measurement of quality attributes

2.6.1 Measurement of TSS

A Refractometer (Atago ATC-1, 32-10 Honcho, Itabashi-ku, Tokyo 173-001, Japan) was used to determine Total soluble solids (TSS) at 25 °C. A few drops of okra pod fluid were placed on the prism surface of the refractometer, and the findings were expressed as % Brix.

2.6.2 Measurement of Vitamin C

The method of Ranganna [19] was used to determine Vitamin C.

2.6.3 Measurement of β-carotene

The equations of Barros et al. [20] and Igbokwe et al. [21] were used to determine the β-carotene concentration.

2.6.4 Measurement of minerals

A complexometric titration method with Na2-EDTA as a complexing agent use to determine Ca, Mg, and Zn contents of okra [22], a flame emission spectrophotometer was used to measure potassium levels Golterman [23] and Ghosh et al. [24] respectively.

2.7 Statistical analysis

Statistix v.10. (Analytical Software, Tallahassee, FL, USA) was use to analysis the acquired data on different parameters. The mean was calculated for all treatments, and the F (variance ratio) test was conducted to examine the variance for every character. The difference between treatment means was calculated using LSD at a 5% level of significance, as stated by Gomez and Gomez [25].

3 Results

3.1 Effect of GA 3 and NAA growth attributes

Application at several levels GA3 and NAA had a significant influence on okra plant height, leaf surface area, number of leaves, internode number, length, number of branches, and number of buds (Table 1). The findings demonstrate that in 1st year and 2nd year the tallest plant (114.40 cm, 137.90 cm), the most significant number of leaves (57.03, 58.10), and the largest number of branches plant-1 (5.43, 5.46), were identified in 150 ppm NAA + 150 ppm GA3 respectively. In contrast, the lowest values were obtained in the control in 1st year 95.70 cm, 42.20, 3.23 and in 2nd year 114.87 cm, 41.70, 3.10 respectively). Similarly, during 1st year leaf area, internode number, internode length and number of buds were also found to be superior in those plants that were exposed to 150 ppm NAA + 150 ppm GA3 and were 56.54%, 32.15%, 35.34% and 31.44% as well as in the next year the results were 62.4%, 35.76%, 42.68% and 31.17% respectively higher compared to control.

Table 1 Effect of GA3 and NAA on growth parameters of okra
Full size table

3.2 Effect of GA 3 and NAA on yield and yield attributes

The okra plants treated to varying concentrations of GA3 and NAA had a substantial influence on yield and yield-related characteristics (Table 2). In 1st year plants exposed to 150 ppm GA3 + 150 ppm NAA increased yield-related okra attributes such as number of pods (24.63), pod length (15.93 cm), pod diameter (2.02 cm), dry matter content of pod (21.33%), and thousand seeds weight (75.80 g) by 60.35%, 27.84%, 32.89%, 35.34%, and 27.76%, respectively, similarly in case of 2nd year the number of pods, pod length, pod diameter, dry matter content of pod and thousand seed weight are 35.79%, 27.84%, 22.72%, 29.54% and 20.89% respectively, compared to the control group. In contrast, in the 1st year highest yield (419.93 g) was found in 150 ppm GA3 + 150 ppm NAA while minimum (310.87 g) from control and in 2nd year highest yield (404.43 g) was found while minimum (404.43 g) from control. The results show that using 150 ppm GA3 and 150 ppm NAA together enhanced yield in both years by 35.08% and 27.01% respectively, compared to the control group.

Table 2 Effect of GA3 and NAA on yield and yield attributes of okra
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3.3 Effect of GA 3 and NAA on quality attributes

The application of GA3 and NAA had a substantial impact on okra's qualitative qualities (Table 3). Results revealed that in 1st and 2nd year maximum vitamin C (8.28 mg and 8.23 mg respectively) and was found in 250 ppm GA3 + 0 ppm NAA while minimum (6.45 mg) in 150 ppm GA3 + 250 ppm NAA and (6.58 mg) in 0 ppm GA3 + 250 ppm NAA treatment combination. When okra plants were exposed to GA3 and NAA @ 150 ppm, TSS, β–carotene, magnesium and zinc increased in 1st year by 81.2%, 31.74%, 22.73% and 21.43%, respectively, and in 2nd year TSS, Potassium, Calcium, magnesium and zinc increased by 50.57%, 5.26%, 9.41%, 18.07% and 33.33% respectively, compared to control. In contrast, potassium and calcium were gradually decreased in 1st year and in 2nd year β–carotene was decreased with increasing the levels of GA3 and NAA compared to control.

Table 3 Effect of GA3 and NAA on quality attributes of 100 g fresh pod of okra
Full size table

4 Discussion

Increasing vegetable production to reduce the unavailability of vegetables in summer is crucial. Application of different concentration of GA3 and NAA to enhance the growth, yields and qualitative attributes of okra was studied in this present experiment (Tables 1, 2, 3).

The findings revealed that the combination of GA3 and NAA considerably enhancing the plant height, leaf number and branches, area of leaf, internode length, and diameter (Table 1). These improvements might be attributed to the stimulatory action of GA3 and NAA on rapid cell division, expansion, and elongation of the meristematic zone of the vegetative portions. Previous research has shown that GA3 and NAA increase cell division, cell and shoot elongation, which causes plants to grow taller, with more leaves and branches per plant [16]. The foliar application of GA3 enhanced plant height, leaf area, number of leaves, internode length and diameter of okra [26] GA3 along with NAA accelerate cell proliferation and elongation resulting in enhancement of vegetative and reproductive growth and development of okra [27]. Moreover, it was seen that the combined application of both GA3 with NAA @150 ppm demonstrated the most superior performance in terms of growth characteristics over other the treatments. These findings are consistent with those of [26]. Kumari et al. [28], Singh et al. [10], Gadade et al. [29] and Khan et al. [30] revealed that application of GA3 and NAA promoted plant height, leaves number, branching etc. in okra var. ‘Parbhani ok’. In the study plant height showed a visible variation between the two years whereas other morphological parameters did not show significant differences between the 2 years (Table 1). The variation in plant height between the 2 years can be attributed to differences in weather conditions [31, 32]. However, the consistent performance of other morphological parameters across both years highlights the efficacy of GA3 and NAA in stabilizing growth under varying conditions. The ability of these plant growth regulators to promote cell division and elongation likely offset the minor environmental fluctuations, ensuring uniform development. These observations are consistent with previous research, which has demonstrated the robust effects of GA3 and NAA in promoting uniform plant growth and development under varying environmental conditions [16, 26, 27].

In addition, the yield related characteristics like pod number, pod length, diameter and dry matter content and yield were superior over control (Table 2). The primary causes of these out comes might be due to the stimulatory effect of GA3 on plant development which leads to biosynthesis, accumulation and translocation of photosynthetic products in the growing point and causes higher pod development, increase pod size ultimately production increased. The corresponding results were also noticed by Khatun et al. [33]. Mohammadi et al. [34] and Meena et al. [27] noticed that GA3 and NAA increased pod size, pod number, pod yield of okra. It was also noticed that combined application of GA3 and NAA at lower concentration produced maximum yields but with increasing the concentration yield is decreased (Table 2). This could be due to the complex interaction between these plant growth regulators, where NAA might be affecting the synthesis and action of gibberellins, leading to an imbalance at higher concentrations. The treated plants still showed significant performance compared to the control, suggesting that the higher doses might affect the hormonal balance and gibberellin synthesis, as noted in other studies [33, 34]. Singh et al. [35] and Bhosle et al. [36] observed that greater concentrations of GA3 and NAA had an antagonistic influence on the number of the flowers in tomato but to our best knowledge, there is no research have been conducted on the function of GA3 and NAA in together at different concentration to use in okra plants for qualitative parameters. Our research reported that applying GA3 and NAA @150 ppm enhances the quality of okra. Besides these, qualitative attributes of okra like vitamins and minerals Content were considerably manipulated by both GA3 and NAA (Table 3). The increase in ascorbic acid with the increasing level of GA3 might be due to application of gibberellins which promote the activity of acid invertase, which results in an increase in hexose levels in plant tissue, or it might be due to the result of protecting synthesized ascorbic acid from oxidation through the enzyme ascorbic acid oxidizes [10].

Verma et al. [37] and Mistry et al. [38] found comparable findings in tomatoes. The level of beta-carotene in pod was negatively affected by GA3 while positively by NAA and vice versa. Nevertheless, the combined treatment of GA3 at 150 ppm and NAA at 250 ppm revealed the highest amount of beta-carotene in okra. Khandakar et al. [39] reported that lower concentration of GA3 combined with higher level of NAA increased carotenoids in Wax apple. Similar effects were observed in tomato by Ishiwu et al. [40]. Similarly, the vitamin C was increased by GA3 but decreased by NAA. TSS was remarkably enhanced due to application of both GA3 and NAA. Our results revealed that plants treated with both GA3 and NAA increased TSS level. It might be due to the impact of growth regulators, which may accelerate the metabolic processes and result in greater TSS [9]. Also Khan et al. [30] demonstrated that GA3 treatment may enhance TSS in sweet oranges when compared to the control. It was also noticed that potassium and calcium gradually decreased with increasing the concentration of both GA3 and NAA than control. Sharif Hos et al. [2] observed that the application of both GA3 and NAA at highest concentration lower level of calcium in okra. Along with magnesium and zinc was positively enhanced due to use of growth regulators. It may also be related to the impact that GA3 and NAA on biological processes such as carbohydrate synthesis, which results in an increase in magnesium and zinc concentrations [25].

5 Conclusion

According to the study's results, GA3 and NAA notably improved okra growth, yield, and quality. The application of GA3 with NAA @ 150 ppm potentially increased yield as well as quality attribute of okra. These findings provide valuable insights for optimizing okra production practices in Bangladesh, potentially mitigating the seasonal shortage of vegetables during summer months. This is the first experiment on single variety so further research with different variety will be needed to strengthen out the present results.