Effect of seed priming on seed germination and seedling growth of papaya (Carica papaya L.)

Introduction

Papaya (Carica papaya L.), a member of the family Caricaceae (2n = 18), is an important tropical and subtropical fruit crop grown throughout India. Native to Central America and southern Mexico, it is valued for its delectable flavour, quick returns and rich nutritional and medicinal properties. In India it is cultivated on a large scale in Andhra Pradesh, Gujarat, Maharashtra, West Bengal, Karnataka, Madhya Pradesh and Chhattisgarh. Papaya is nutritionally rich, supplying carbohydrates, vitamin A, vitamin C and minerals in the edible portion (Ram, 2005) [20] . Papaya is commercially propagated through seeds as no economically viable vegetative method is available (Lay et al., 2013) [15] . However germination in papaya is slow, incomplete and erratic largely because the seeds are enclosed in a gelatinous sarcotesta that acts as a physical and biochemical barrier to germination (Hossain et al., 2023) [12] . Germination is further influenced by temperature, moisture, oxygen and the growing substrate by naturally occurring germination inhibitors (Olivas, 2021) [16] . Seed priming is a simple low-cost pre-sowing technique that improves the rate and uniformity of germination and enhances seedling vigour. Plant growth regulators such as gibberellic acid (GA₃) promote uniform germination by activating hydrolytic enzymes that mobilize food reserves, while auxins such as naphthalene acetic acid (NAA) and salts such as potassium nitrate (KNO₃) are also used to break dormancy. Biofertilizers such as Azotobacter and Phosphate-Solubilising Bacteria (PSB) produce growth-promoting hormones that stimulate root and shoot development (Sukhade et al., 1995) [25] . In view of the high cost of chemical growth regulators, farmers are also turning to affordable organic alternatives such as cow urine and cow dung, which contain nutrients and bio-active substances that support seedling growth (Subramaniam, 2005) [24] . The growing medium comprising soil, sand and farmyard manure (FYM) also plays a key role in providing anchorage, aeration and nutrients for germinating seeds (Abad et al., 2002; Srivastava et al., 1998) [1,23] . The high cost of hybrid cv. Red Lady seed coupled with irregular germination makes efficient nursery raising critical for papaya cultivation. Keeping these facts in view the present investigation was undertaken to evaluate the comparative efficacy of different seed priming agents plant growth regulators, salts, organic inputs and biofertilizers on the germination and seedling growth of papaya cv. Red Lady.

Materials and Methods

The experiment was carried out during 2023-24 at the Horticulture Farm, Department of Fruit Science, College of Agriculture, Indira Gandhi Krishi Vishwavidyalaya, Raipur (Chhattisgarh), which is situated at 21.25° N latitude and 81.62° E longitude at an altitude of 298.15 m above mean sea level (Chhattisgarh Plains) in the seventh agro-climatic zone of India. The crop was raised under open-field conditions from November 2023 to February 2024. The investigation was laid out in a Completely Randomized Design (CRD) with eleven treatments replicated three times. Seeds of papaya cv. Red Lady were soaked separately for 12 hours in the respective priming solutions before sowing: T 0 (control, water soaking), T 1 (cow urine 100%), T 2 (cow dung 100%), T 3 (GA₃ @ 150 ppm), T 4 (GA₃ @ 200 ppm), T 5 (NAA @ 150 ppm), T 6 (NAA @ 200 ppm), T 7 (Azotobacter @ 10%, 0.5 g per 10 g seed), T 8 (PSB @ 10%, 0.5 g per 10 g seed), T 9 (KNO₃ @ 1000 ppm) and T 10 (KNO₃ @ 2000 ppm). Thirty seeds were used per treatment (330 seeds in total). GA₃ and NAA (150 mg each) were dissolved in 5 ml ethyl alcohol and 2-3 ml of 1N NaOH respectively and made up to one litre with distilled water the required quantities of cow urine, cow dung, Azotobacter and PSB were prepared as fresh solutions. The growing medium for all treatments consisted of soil, sand and FYM in a 1:1:1 ratio filled into 6″ × 4″ (200 gauge) black polybags. Treated seeds were sown one per bag at a depth of 2-2.5 cm followed by light irrigation. The bags were irrigated regularly kept weed-free and protected with imidacloprid (0.5 ml L⁻¹) at 15-day intervals and with mancozeb and copper oxychloride against fungal diseases. Observations on germination parameters (days taken to germination, rate of emergence and germination percentage) were recorded during the germination period, while growth parameters (seedling height, number of leaves, stem girth and shoot length) were recorded at 30, 45 and 60 days after sowing (DAS). Fresh and dry weights of sapling and root, Root: Shoot ratio, Seedling Vigour Index and root parameters were recorded at 60 DAS on five randomly selected seedlings per replication. The parameters were computed as follows: Rate of emergence (%) = (No. of seedlings emerged 5 DAS / No. of seedlings emerged 15 DAS) × 100 Germination percentage (%) = (Total no. of seeds germinated / Total no. of seeds sown) × 100 Root: Shoot ratio = Dry weight of root / Dry weight of shoot Seedling Vigour Index-I = Germination (%) × Total seedling length at 60 DAS Seedling Vigour Index-II = Germination (%) × Dry weight of seedling at 60 DAS The data were subjected to analysis of variance (ANOVA) for a Completely Randomized Design following Fisher and Yates (1963) [11] . The significance of treatment effects was tested by the F-test, and treatment means were compared using the critical difference (CD) at the 5% level of significance.

Results and Discussion

Germination parameters

The pre-sowing priming treatments significantly influenced all germination parameters (Table 1). The earliest germination (14.31 days) was recorded under T 4 (GA₃ @ 200 ppm), which was statistically at par with T 3 (14.44 days), whereas the control (T 0 ) took the longest (19.84 days). GA₃ accelerates germination by activating α-amylase and other hydrolytic enzymes that convert stored starch into simple sugars, supplying energy for early embryo growth. Similar results were reported by Barche et al. (2010) [5] and Babu et al. (2010) [4] in papaya and Yadav et al. (2018) [30] in custard apple. The maximum rate of emergence (47.48%) was also recorded under T 4 followed by T 3 (42.97%), while the minimum (28.63%) was under the control. The germination percentage ranged from 50.00% (T 0 ) to 86.67% (T 4 ); T 4 was followed by T 3 (83.33%), T 6 (80.00%) and T 5 (78.33%), which were statistically at par. The improvement in germination under GA₃ may be attributed to its role in overcoming physiological dormancy imposed by the seed coat, thereby enhancing water uptake and metabolic activity. These findings corroborate those of Chacko and Singh (1966) [6] , Deb et al. (2010) [8] and Kumawat et al. (2014) [13] in papaya.

Growth parameters

Seedling height, number of leaves, stem girth and shoot length were significantly influenced by the priming treatments at all growth stages (Table 2). At 60 DAS, the tallest seedlings (25.01 cm) were recorded under T 4 (GA₃ @ 200 ppm) followed by T 3 (24.59 cm) while the control (T 0 ) produced the shortest seedlings (17.36 cm). The increase in seedling height under GA₃ may be due to enhanced nutrient uptake and cell elongation resulting in greater internodal length. Comparable results were reported by Wagh et al.

(1998) [29] in aonla, Pampanna and Sulikeri (2001) [17] in sapota and Rajput et al. (2020) [19] in custard apple.

The maximum number of leaves per seedling (6.76, 8.79 and 12.01 at 30, 45 and 60 DAS) was recorded under T 4 , whereas the control recorded the minimum (4.40, 6.62 and 9.39). Stem girth was also highest under T 4 (1.67, 3.28 and 3.70 mm) the minimum under T 0 (0.43, 1.72 and 2.07 mm); the increase in girth is associated with GA 3 induced cambial activity and cell division (Dhankar and Singh, 1996) [10] . Similarly, shoot length was maximum under T 4 (5.89, 8.87 and 13.65 cm) and minimum under T 0 (2.80, 5.05 and 7.83 cm). The enhanced shoot growth under GA 3 reflects rapid mobilization of carbohydrates by α-amylase, providing energy for cell expansion, in agreement with Vincent and Thandapani (2003) [28] , Desai et al. (2017) [9] and Zainudin and Adini (2019) [31] in papaya.

Fresh and dry weight, Root: Shoot ratio and seedling vigour

The fresh and dry weights of sapling, Root: Shoot ratio and Seedling Vigour Indices differed significantly among treatments (Table 3). The maximum fresh weight (16.67 g) and dry weight (1.63 g) of sapling were recorded under T 4 (GA₃ @ 200 ppm), and the minimum (9.02 g and 0.82 g) under the control. Greater biomass under GA 3 is attributed to more efficient mobilization of water and nutrients, promoting the production and translocation of photosynthates as reported by Lay et al. (2013) [15] and Anjanawe et al. (2013) [3] in papaya.

The highest Root: Shoot ratio (0.31) was recorded under T 1 (cow urine 100%) followed by T 3 (0.30) and T 4 (0.28) while T 0 , T 2 and T 9 recorded the lowest (0.22). The higher ratio under cow urine may be due to sustained cell turgidity that favours cell division and elongation (Sutheesh et al. (2016) [26] . Seedling Vigour Index-I and Index-II were maximum under T 4 (1183.00 and 140.98) and minimum under T 0 (391.50 and 41.17) reflecting the combined improvement in germination, seedling length and dry weight under GA 3 priming in agreement with Pandit et al. (2001) [18] , Deb et al. (2010) [8] and Shukla et al. (2012) [22] . T 0 -T 10 as defined in Table 1 Root parameters All root parameters were significantly influenced by the priming treatments (Table 4). At 60 DAS, T 4 (GA₃ @ 200 ppm) recorded the maximum root length (25.83 cm), number of roots per seedling (85.59), number of primary roots (9.08), number of secondary roots (72.86), fresh weight of root (2.49 g) and dry weight of root (0.46 g), whereas the minimum values were recorded under the control (T 0 ). The enhanced root development under GA₃ is attributed to increased synthesis of photosynthates and their translocation through the phloem to the root zone together with cell elongation in the sub-apical region of the roots. These results are in agreement with Anburani and Shakila (2010) [2] , Deb et al. (2010) [8] , Lanjhiyana et al. (2020) [14] and Choudhary et al. (2022) [7] in papaya, and Verma et al.

Conclusion

The present investigation clearly demonstrated that seed priming markedly improves the germination and seedling growth of papaya cv. Red Lady. Among all the treatments evaluated, seed priming with GA₃ @ 200 ppm (T 4 ) proved the most effective, giving the earliest germination, the highest germination percentage and rate of emergence and superior seedling growth, biomass and root development.

Whereas the untreated control consistently recorded the poorest performance. GA₃ @ 200 ppm can therefore be recommended as an effective and practical seed priming treatment for raising vigorous papaya seedlings in the nursery. As these findings are based on a single season, further multi season and multi-location trials are suggested for confirmation.

Acknowledgement

AcknowledgementThe authors are grateful to the Department of Fruit Science, College of Agriculture, Indira Gandhi Krishi Vishwavidyalaya, Raipur (Chhattisgarh), for providing the necessary facilities to carry out this investigation.

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