Studies on foliar application of bio-stimulants on growth and flowering of Petunia

Introduction

Flowers are an inseparable part of human life and hold great importance from birth to death. Nowadays, with the increasing standard of living, social and religious ceremonies are considered incomplete without the use of flowers in various forms. Their beauty and fragrance have the power to make people feel happy, relaxed and cheerful. Apart from their aesthetic value, flowers also play an important economic role. Petunia hybrida, commonly known as garden petunia, belongs to the family Solanaceae and is one of the most important ornamental bedding plants cultivated worldwide. It originated in South America, and the name Petunia is derived from 'Petun', the Brazilian name for tobacco (Sink, 1984) [22] . Petunia is considered one of the earliest cultivated bedding plants and has remained commercially important due to its attractive flowers, wide range of colours and prolonged flowering period (Griesbach, 2007) [9] . Modern petunias are mainly derived from species such as Petunia axillaris and P. violacea. The crop is widely used for landscape beautification as bedding plants, borders, potted plants, window boxes and hanging baskets. Petunia is an annual herbaceous plant with attractive, colourful flowers available in shades of pink, red, white, purple and yellow, including several bi-coloured forms. It is commercially propagated through seeds and is widely cultivated as a seasonal ornamental crop. However, its growth and flowering are affected by drought, high temperature, frost and excessive rainfall during the flowering period (Jundan et al., 2004; Ando et al., 2001) [2,11] . Hence, suitable crop management practices are essential for obtaining vigorous growth, profuse flowering and better seed production. Excessive dependence on chemical fertilizers may adversely affect soil properties and increase production costs. Therefore, sustainable nutrient management through the use of bio-stimulants has gained importance in modern floriculture. Bio-stimulants are substances or microorganisms that stimulate natural plant processes, improve nutrient use efficiency, enhance tolerance to abiotic stresses and contribute to better crop quality (Traon et al., 2014; Drobek et al., 2019) [8,25] . Humic substances, seaweed extracts and other bio-stimulants have been reported to improve plant growth, nutrient uptake, flowering and quality in flower crops (Suguna, 2005; Bulgari et al., 2015) [5,24] . Thus, the use of bio-stimulants offers considerable potential for improving growth, flowering, seed yield and seed quality of Petunia while promoting sustainable floricultural production.

Material and Methods

The present investigation was conducted during 2025-26 at the Main Agricultural and Horticultural Research Station (MAHRS), College of Agricultural Sciences, Iruvakki, under Keladi Shivappa Nayaka University of Agricultural and Horticultural Sciences, Shivamogga. The experimental site is situated in the Western Ghats and represents the Hill Zone (Zone-9) of Karnataka, at 14°25′ N latitude and 75°11′59.23″ E longitude, with an altitude of 650 m above mean sea level. The experiment was conducted under open field conditions during the Rabi season using Petunia (Petunia hybrida Vilm.) cv. Supercascade Pink. The experiment was laid out in a Randomized Complete Block Design (RCBD) comprising nine treatments with three replications. The treatments consisted of T₁ (Humic acid @ 0.4%), T₂ (Humic acid @ 0.8%), T₃ (GA₃ @ 150 ppm), T₄ (GA₃ @ 300 ppm), T₅ (Biovita @ 0.4%), T₆ (Biovita @ 0.8%), T₇ (Spic Cytozyme @ 0.4%), T₈ (Spic Cytozyme @ 0.8%) and T₉ (untreated control with water spray). The experiment was conducted in plots of 2.1 × 1.2 m with a spacing of 30 × 30 cm, accommodating 84 plants per treatment. A basal dose of vermicompost and cocopeat at the rate of 2:1 Kg per m 2 was mixed in the soil and levelling of bed was done using straight stick, after bed preparation, irrigation was given before transplanting. Healthy seedlings were transplanted on flat beds, and the respective bio-stimulants were applied as foliar sprays at 30, 45 and 60 days after transplanting (DAT). The required concentrations of humic acid, Biovita and Spic Cytozyme were prepared at 0.4 and 0.8 per cent, while GA₃ solutions were prepared at 150 and 300 ppm by dissolving the required quantity of the respective bio-stimulant and making the final volume with water. The untreated control received water spray at the same intervals. Irrigation, weeding and other necessary intercultural and plant protection measures were carried out uniformly for all the treatments throughout the crop period. The observations were recorded from randomly selected and tagged plants at appropriate growth and flowering stages for different parameters such as plant height, plant spread, internodal length, stem girth, number of leaves, leaf area, leaf area index and days taken for flower bud initiation, flower opening, 50 per cent flowering and days taken for peak flowering were statistically analysed to determine the significance of differences among the treatments.

Results and Discussion

Growth parameters

The data presented in Table 1 shows that growth parameters were significantly affected by the application of different bio-stimulants. The treatment GA₃ @ 300 ppm (T₄) recorded the maximum plant height (29.37 cm), number of leaves (143.20), leaf area index (4.75), plant spread (43.57 cm and 41.77 cm in E-W and N-S directions, respectively), number of branches (21.33 plant⁻¹), leaf length (8.57 cm) and leaf width (5.27 cm), followed by GA₃ @ 150 ppm (T₃), which recorded plant height (21.83 cm), number of leaves (125.13 plant⁻¹), leaf area (3.20 cm²), plant spread of 35.06 cm (E-W) and 36.00 cm (N-S), number of branches (16.37 plant⁻¹), leaf length (7.70 cm) and leaf width (4.53 cm). In contrast, the untreated control (T₉) recorded the minimum plant height (15.60 cm), number of leaves (90.83), leaf area index (1.36), plant spread of 24.70 cm (E-W) and 25.43 cm (N-S), number of branches (10.23 plant⁻¹), leaf length (5.40 cm) and leaf width (3.85 cm). The pronounced increase in plant height observed in response to (GA₃ @ 300 ppm) may be attributed to the growth-promoting action of gibberellic acid, which stimulates cell division and cell elongation. In addition, GA₃ enhances cell wall extensibility, promotes active meristematic growth and improves photosynthetic efficiency, resulting in increased assimilate production and efficient translocation of photosynthates to the growing tissues. These physiological and biochemical processes collectively contribute to vigorous vegetative growth and greater plant height. Similar findings were reported by and Sravya et al. (2024) [23] in Petunia and Vinutha et al. (2017) [26] in China aster. The maximum number of leaves recorded under GA₃ @ 300 ppm may be attributed to the stimulatory effect of gibberellic acid on shoot apical meristem activity, which accelerated the initiation and development of leaf primordia. In addition, GA₃ promoted the expansion of newly formed leaves and delayed leaf senescence. These responses resulted in continuous leaf production and a significantly higher number of leaves. These results are supported by Manasa et al. (2017) [14] in Gladiolus, Vyshnavi et al. (2023) [27] in Chrysanthemum and Sharma and Collis (2017) [19] in Petunia. The superior branch production recorded under GA₃ @ 300 ppm may be attributed to the enhanced responsiveness of axillary buds to gibberellin signalling, promoted the growth of already established axillary buds into lateral shoots, thereby increasing the number of branches. Similar findings were reported by Ragini et al. (2019) [17] in Asiatic lily and Patra et al. (2015) [16] . The higher leaf area, leaf length and leaf width under GA₃ @ 300 ppm may be attributed to the prolonged expansion phase of developing leaves through delayed leaf maturation, allowing the lamina to attain a larger final size. In addition, gibberellin stimulates cell wall remodelling through the activation of expansion and xyloglucan-modifying proteins, facilitating greater lateral expansion of the leaf blade and resulting in increased leaf area. Similar findings were reported by Singh and Singh (2024) [21] in Petunia, Jayashree et al. (2020) [10] in Asiatic lily and Vyshnavi et al. (2023) [27] in Annual chrysanthemum. The enhancement in leaf area index with (GA₃ @ 300 ppm) may be explained by the role of gibberellins in maintaining favorable hormonal balance during vegetative growth. This promoted continuous canopy establishment and reduced premature leaf abscission, resulting in greater cumulative leaf area per unit ground surface. These results are supported by Kumar et al. (2015) [13] in China aster and Ragini et al. (2019) [17] in Asiatic lily.

Flowering parameters

The data presented in Table 2 shows that flowering parameters were significantly affected by the application of different biostimulants. The treatment receiving GA₃ @ 300 ppm (T₄) recorded the minimum days taken for flower bud initiation (35.55), days taken for flower opening (7.01), days taken for 50 per cent flowering (49.63) and days taken for peak flowering (54.33), followed by GA₃ @ 150 ppm (T₃), which recorded (37.18) days for flower bud initiation, (7.76) days for flower opening, (52.76) days for 50 per cent flowering and (57.75) days for peak flowering. In contrast, the untreated control (T₉) recorded the maximum days taken for flower bud initiation (46.38), flower opening (9.18), 50 per cent flowering (59.85) and peak flowering (66.58).

The earliness in flowering under (GA₃ @ 300 ppm) may be associated with increased activity of enzymes involved in carbohydrate metabolism. GA₃ enhances photosynthetic efficiency and respiratory activity and metabolic energy required for rapid floral bud differentiation. It also improves the availability and utilization of assimilates at the shoot apex, favouring early floral induction and bud development, resulting in an extended flowering duration and increased flower production compared to the untreated plants. Similar observations were reported by Sharma and Collins (2017) [19] in Petunia, Munikrishnappa and Chandrashekar (2014) [15] in China aster, Delvadia et al. (2009) [7] in Gaillardia, Salem et al. (2016) [18] in Gerbera, Shreshtha et al. in Calendula, Chaithra et al. (2020) [6] in Spider lily, Acharya et al. (2021) [1] in African marigold, Bordolui and Ray (2020) [4] in Marigold and Kaur et al. (2021) [12] and Bhargavi et al. (2018) [3] in Chrysanthemum.

Conclusion

From the present study, it can be concluded that foliar application of GA₃ @ 300 ppm (T₄) significantly enhanced the growth and flowering performance of Petunia. The application of GA₃ @ 300 ppm promoted better vegetative development with improved plant height, number of leaves, leaf area, plant spread, number of branches, leaf length and leaf width. In addition, it induced early flower bud initiation, flower opening, 50 per cent flowering and peak flowering compared to the untreated control. Thus, foliar application of GA₃ @ 300 ppm can be recommended as an effective growth regulator treatment for obtaining vigorous plants with early and uniform flowering, thereby improving the ornamental performance of Petunia.

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