Influence of leaf scrap amendments and humic acid seed priming on germination of acid lime (Citrus aurantifolia Swingle)

Introduction

Acid lime (Citrus aurantifolia Swingle) is an economically important citrus crop widely cultivated in India for its high vitamin C content and year-round market demand. Successful orchard establishment depends largely on the availability of uniform and vigorous nursery seedlings; however, seed propagation of acid lime is often constrained by slow and irregular germination (Chadha, 2001; Hartmann et al., 2011) [5,9] . Suboptimal physical and biological properties of nursery growing media further aggravate this problem. Incorporating organic amendments, such as leaf litter, can improve soil structure, moisture retention, and microbial activity, thereby creating a more favourable environment for seed germination. Leaf scraps from perennial fruit trees gradually decompose and contribute organic matter and nutrients to the growing medium (Swift et al., 1979) [16] . In parallel, humic acid, a biologically active component of soil organic matter, has been reported to enhance seed germination by improving membrane permeability, water uptake, and early metabolic activity (Nardi et al., 2002; Canellas et al., 2002) [13,4] . The present study was undertaken to evaluate the combined influence of different leaf scrap amendments and humic acid seed priming on the germination behaviour of acid lime, to identify an efficient and sustainable nursery practice.

Materials and Methods

Experimental Site and Duration

The experiment was conducted during the 2024-2025 season at the Fruit Research Station, Imaliya, Department of Horticulture, Jawaharlal Nehru Krishi Vishwa Vidyalaya (JNKVV), Jabalpur, Madhya Pradesh. The region falls under a subtropical climatic zone.

Experimental Design and Treatments

The experiment was laid out in a Randomized Block Design (RBD) with thirteen treatments and three replications. Seeds of acid lime were subjected to different combinations of potting soil amendments and humic acid seed priming. Treatments consisted of:  Control: soil alone without humic acid  Soil amended with mango, sapota, or karonda leaf scrap at 100 g or 200 g per kg of soil  Seed soaking in humic acid solutions at 1500 ppm or 2000 ppm for 24 hours prior to sowing

Preparation of Media and Seed Treatment

Leaf scraps were air-dried, crushed, and incorporated thoroughly into the soil as per treatment specifications. Seeds were surface sterilised and soaked in the respective humic acid solutions for 24 hours, while control seeds were soaked in distilled water.

Observations Recorded

The following germination parameters were recorded:  Days to initial germination: Number of days taken for the emergence of the first seedling  Days to 50% germination: Number of days required for 50% of seeds to germinate  Germination percentage: Calculated at 30 days after sowing (DAS)

Statistical Analysis

The recorded data were subjected to analysis of variance (ANOVA) as per the procedure described by Gomez and Gomez (1984) [6] . Treatment means were compared at the 5% level of significance.

Results and Discussion

Days to first germination

Leaf scrap and humic acid treatments significantly influenced the time taken for initial seedling emergence (Table 1; Fig. 1). The earliest germination (11.00 DAS) was recorded in Soil + mango leaf scrap 100 g + humic acid 2000 ppm (T 3 ), which was statistically comparable with Soil + sapota leaf scrap 100 g + humic acid 1500 ppm (T 6: 12.33 DAS), Soil + sapota leaf scrap 200 g + humic acid 2000 ppm (T 9: 12.33 DAS), and Soil + sapota leaf scrap 200 g + humic acid 1500 ppm (T 8: 13.33 DAS). The maximum delay in germination was observed in the control (T 1: 19.00 DAS). The enhanced early emergence under T 3 may be attributed to the biostimulant effect of humic acid on enzymatic hydrolysis of seed reserves and improved cell membrane permeability, which accelerates water uptake and metabolic processes [Nardi et al., 2002] [13] . Similar reductions in days to germination due to humic substances have been reported in tomato and maize, supporting the present findings [Ampong et al., 2022] [1] .

Days to 50% germination

Significant differences among treatments were also observed for days taken to attain 50% germination (Table1). Treatment T 3 recorded the minimum number of days (16.00 DAS), followed by T 6 (17.36 DAS) and T 9 (19.51 DAS), whereas the control (T 1 ) required the maximum time (28.66 DAS) to reach 50% germination. Faster achievement of 50% germination under T 3 indicates better synchronisation and uniformity of seedling emergence, which is desirable for nursery management and subsequent cultural operations. Improved germination performance in enriched organic substrates has also been reported for acid lime, suggesting that organic-humic amendments create a more favourable microenvironment for seed germination [Nainar and Muthulakshmi, 2021] [12] .

Germination percentage

Germination percentage at 30 DAS varied significantly among treatments (Table 1). The highest germination percentage (77.33%) was recorded in T 3 , which was significantly superior to all other treatments. Sapota-based treatments also showed comparatively higher germination, particularly T 6 (65.33%), while the lowest value (56.00%) was observed in T 4 . The superior performance of T 3 can be attributed to enhanced soil microbial activity, improved nutrient release from decomposing mango leaf scrap, and the synergistic hormonal effects of humic acid on early seedling development. Previous work has highlighted the positive influence of decomposed leaf litter on soil fertility and germination in plantation crops, supporting the role of organic residues in improving germination [Guruprasad, 1992] [7] [Harishma, 2017] [8] .

Seedling vigour indices

Seedling vigour index I and II exhibited marked enhancement under treatments involving the combined application of leaf scrap and humic acid (Table 2). The maximum values for SVI I (2931.85) and SVI II (19096.23) were obtained in T 3 , which were significantly higher than all other treatments; in contrast, the lowest vigour indices were recorded in the control (SVI I: 615.57; SVI II: 2399.40). Improved seedling vigour under T 3 is likely due to enhanced root growth, better nutrient and water uptake, and increased antioxidant activity induced by humic substances, which collectively promote more robust seedling growth [Ampong et al., 2022] [1] . Similar positive effects of humic acid on seedling vigour have been documented in cereals and horticultural crops, emphasising its broad agronomic utility [Bag et al., 2020] [2] .

Survival percentage

Survival percentage at 150 DAS also differed significantly among treatments (Table 2). The highest survival (81.00%) was observed in T 3 , followed by sapota-based treatments T 6 and T 9 (73.00%), while the control (T 1 ) recorded the lowest survival (58.33%). The higher survival under T 3 reflects the cumulative positive effects of improved vigour indices, better root-shoot balance, and enhanced stress tolerance, which enable seedlings to withstand environmental fluctuations. The results are in agreement with observations that organic-humic amendments improve seedling resilience and survival in nursery crops by improving physical, chemical, and biological properties of the growth medium [Prisa and Caro, 2023] [4] .

Conclusion

The study clearly indicated that the combined application of leaf scrap and humic acid significantly influences seed germination and seedling establishment of acid lime under nursery conditions. Among the tested treatments, Soil + mango leaf scrap 100 g + humic acid 2000 ppm (T 3 ) consistently outperformed other combinations by recording the earliest germination, shortest time to 50% germination, highest germination percentage, superior seedling vigour indices, and maximum survival percentage. These findings confirm the synergistic benefits of mango leaf scrap as an organic substrate and humic acid as a bio-stimulant in promoting early and vigorous growth of acid lime seedlings, and suggest that T 3 can be recommended as a practical and cost-effective nursery technology for improving the efficiency of acid lime propagation.

References

1. Ampong I, Asare PA, Annor P, Amponsah B. Humic substances in plant growth promotion: mechanisms and agronomic significance. Int J Plant Soil Sci. 2022;34(2):1-12.
2. Bag K, Bandyopadhyay KK, Sehgal VK, Sarangi A, Krishnan P. Effect of tillage, residue and nitrogen management on radiation interception and efficiency. J Agrometeorol. 2020;22(3):285-294.
3. Bhatt P, Singh VK. Effect of humic acid on soil properties and crop production: a review. Indian J Agric Sci. 2022;92(12):1423-1430.
4. Canellas LP, Olivares FL, Okorokova AL, Façanha RA. Humic acids isolated from earthworm compost enhance root elongation, lateral root emergence and plasma membrane H⁺-ATPase activity in maize roots. Plant Physiol. 2002;130:1951-1957.
5. Chadha KL. Handbook of horticulture. New Delhi (India): Indian Council of Agricultural Research; 2001.
6. Gomez KA, Gomez AA. Statistical procedures for agricultural research. New York (USA): John Wiley and Sons; 1984.
7. Guruprasad MK. Preliminary studies on leaf litter quantity, composition and nutrient release in rubber plantation [thesis]. Kerala (India): Kerala Agricultural University; 1992.
8. Vellayani (India): College of Agriculture; 2017.
9. Hartmann HT, Kester DE, Davies FT, Geneve RL. Plant propagation: principles and practices. 8th ed. New Delhi (India): Pearson Education Inc.; 2011.
10. Hussein SA, Noori A, Kanber H. Effect of stratification period with different agricultural media and humic acid fertilization on seed germination and seedling growth of apricot. Plant Cell Biotechnol Mol Biol. 2020;21:23-29.
11. Khan MR. Effect of humic acid and seed size on germination of mango seed. Pure Appl Biol. 2015;7(1):315-320.
12. Nainar P, Muthulakshmi S. Effect of growing media on seed germination and seedling growth of acid lime. Int J Curr Microbiol Appl Sci. 2021;10(8):110-122.
13. Nardi S, Pizzeghello D, Muscolo A, Vianello A. Physiological effects of humic substances on higher plants. Soil Biol Biochem. 2002;34(11):1527-1537.
14. Prisa D, Caro S. Alternative substrates in the cultivation of ornamental and vegetable plants. Zenodo. 2023. doi:10.5281/zenodo.8265397.
15. Singh R, Kumar A, Sharma P. Citrus cultivation: techniques and practices. J Hortic Sci. 2019;14(2):4553.
16. Swift MJ, Heal OW, Anderson JM. Decomposition in terrestrial ecosystems. Oxford (UK): Blackwell Scientific Publications; 1979.
17. Williams CE, Lipscomb MV, Johnson WC, Nilsen ET. Influence of leaf litter and soil moisture on early establishment of seedlings. Am Midl Nat. 1990;124:142-152.