Physiological and morphological performance of acid lime (Citrus aurantifolia Swingle) seedlings as influenced by leaf litter amendments and humic acid concentrations under nursery conditions

Introduction

Acid lime (Citrus aurantifolia Swingle) is an economically important citrus crop cultivated extensively in tropical and subtropical regions of India. The success of commercial orcharding largely depends on the availability of uniform, vigorous, and physiologically efficient planting material. However, conventional nursery practices often result in seedlings with weak root systems, poor biomass accumulation, and low field establishment potential (Naik, 1949) [5] . Improving early seedling growth through sustainable and cost-effective nutrient management strategies is therefore essential. In recent years, the use of organic residues and biostimulants has gained considerable attention in nursery production systems. Leaf litter amendments contribute to soil organic carbon, improve physical properties, and release essential nutrients during decomposition, thereby enhancing the rhizosphere environment. The nutrient composition and decomposition rate of leaf litter vary with botanical origin, which may influence seedling growth responses differently. Mango, sapota, and karonda leaf litters are commonly available organic residues in citrusgrowing regions, yet their comparative effectiveness in citrus nurseries has not been adequately evaluated. Humic acid, a major component of soil organic matter, is known to stimulate plant growth by enhancing nutrient availability, improving root membrane permeability, and promoting root proliferation (Nardi et al., 2002) [6] . Several studies have reported that humic substances positively influence root architecture and biomass accumulation, leading to improved shoot growth and physiological efficiency in fruit crops (Canellas et al., 2002; Zandonadi et al., 2007) [3,8] . Despite these documented benefits, information on the interactive effects of leaf litter substrates and humic acid concentrations on citrus seedling physiology remains limited. The present study was therefore undertaken to evaluate the morphological and physiological responses of acid lime seedlings to different leaf litter amendments in combination with humic acid seed priming, to identify an optimum nursery management practice for producing high-quality planting material.

Materials and Methods

Experimental site and plant material

The experiment was conducted at the nursery of Jawaharlal Nehru Krishi Vishwa Vidyalaya (JNKVV), Jabalpur, Madhya Pradesh. Seeds of acid lime cv. Kagzi were used for the study.

Treatments and experimental design

The experiment consisted of thirteen treatments laid out in a Randomized Block Design with three replications. The treatments included soil alone as control and soil amended with mango, sapota, or karonda leaf scrap at 100 g or 200 g per kg of soil, combined with humic acid seed priming at two concentrations (1500 ppm and 2000 ppm). Leaf scrap materials were shade-dried, powdered, and thoroughly mixed with the potting soil before sowing.

Seed priming and nursery management

Seeds were primed in respective humic acid solutions for the prescribed duration, shade-dried, and sown in nursery containers. Uniform cultural practices were followed throughout the experimental period.

Observations recorded

Morphological parameters such as plant height, number of leaves, stem girth, and fresh and dry weights of shoots and roots were recorded at 150 DAS. Physiological parameters, including Leaf Area Index (LAI) and Leaf Area Duration (LAD), were calculated using standard growth analysis procedures (Watson, 1952) [7] . Derived indices such as root-shoot ratio, Seedling Vigour Index I and II, and survival percentage were computed using established formulae.

Statistical analysis

The data were subjected to analysis of variance (ANOVA) and treatment means were compared at a 5 per cent level of significance.

Results and Discussion

Plant Height

Plant height of acid lime seedlings was significantly influenced by the combined application of leaf litter substrates and humic acid concentrations at 150 DAS. The treatment comprising soil amended with mango leaf scrap (100 g) along with humic acid seed priming at 2000 ppm recorded the maximum plant height (11.01 cm), which was significantly superior to the control treatment (6.07 cm). Other treatments receiving humic acid and leaf litter also showed improved growth, though to a lesser extent. The enhanced plant height observed under mango leaf scrap combined with humic acid may be attributed to improved nutrient availability and stimulation of cell elongation processes. Humic acid is known to enhance nitrogen uptake and activate metabolic pathways associated with vegetative growth, while decomposing mango leaf litter likely supplied essential nutrients in a sustained manner. Similar improvements in seedling height following humic acid application have been reported in citrus and other perennial fruit crops (Nardi et al., 2002; Mosa et al., 2020) [6,4] .

Number of Leaves

The number of leaves per seedling varied significantly among treatments. Seedlings grown in soil amended with mango leaf scrap (100 g) and humic acid at 2000 ppm produced the highest number of leaves (13.22), nearly double that recorded in the control (6.78). Increased leaf production was also observed in other humic acid-treated combinations, though mango leaf scrap proved most effective. An increase in leaf number reflects improved meristematic activity and nutrient status of the seedlings. Humic substances have been reported to enhance chlorophyll synthesis and nitrogen assimilation, which support leaf initiation and expansion. In addition, mango leaf litter may have improved soil microbial activity, thereby facilitating better nutrient mineralization and uptake. Comparable findings were reported by Adelani (2023) [1] , who observed increased leaf production in citrus seedlings grown with organic leaf litter amendments. [3] .

Biomass Accumulation (Shoot and Root)

Fresh and dry biomass accumulation of both shoots and roots was significantly influenced by the treatments. The highest fresh shoot weight (260.33 mg) and dry root weight (63.33 mg) were recorded in the mango leaf scrap (100 g) + humic acid (2000 ppm) treatment. In contrast, the control treatment recorded substantially lower dry root weight (9.23 mg).

Greater biomass accumulation under integrated treatments suggests improved photosynthate production and efficient partitioning towards both above-and below-ground parts. Humic acid application has been reported to stimulate root growth by enhancing lateral root formation and root hair development, thereby increasing the absorptive surface area (Zandonadi et al., 2007) [8] . Enhanced root growth, in turn, supports shoot biomass accumulation through improved water and nutrient uptake.

Leaf Area Index (LAI)

Leaf Area Index differed significantly among treatments at 150 DAS. The maximum LAI (2.09) was observed in seedlings grown with mango leaf scrap (100 g) combined with humic acid at 2000 ppm, while the control recorded the lowest value (0.71).

Higher LAI indicates better canopy development and greater photosynthetic surface area. The increase in LAI may be attributed to enhanced leaf expansion and increased leaf number under humic acid treatment. Humic substances are known to improve chlorophyll content and photosynthetic efficiency, resulting in larger and more functional leaves. The gradual nutrient release from mango leaf litter might have further contributed to sustained leaf growth, as also reported by Adelani et al. (2020) [2] in organic residue-amended systems.

Leaf Area Duration (LAD)

Leaf Area Duration, which represents the persistence of photosynthetically active leaf area over time, was significantly higher in seedlings treated with mango leaf scrap (100 g) and humic acid at 2000 ppm (32.43 cm² days) compared to the control (11.10 cm² days). Higher LAD values indicate prolonged photosynthetic activity and better assimilate production. The improved LAD observed in the present study suggests delayed senescence and sustained leaf functionality under integrated organic and humic acid treatments. This may be due to improved nutrient availability and enhanced physiological stability of leaves, leading to extended photosynthetic duration. Similar trends have been reported in fruit seedlings receiving humic substances and organic amendments (Mosa et al., 2020) [4] .

Root-Shoot Ratio

The root-shoot ratio was significantly influenced by the treatments, with the highest value (2.45) recorded in seedlings treated with mango leaf scrap (100 g) and humic acid at 2000 ppm. The control recorded a comparatively lower ratio (1.01), indicating relatively poor root development.

A higher root-shoot ratio reflects a well-developed root system capable of supporting shoot growth and improving seedling resilience under field conditions. Humic acid-induced stimulation of root growth, combined with improved soil physical conditions due to leaf litter decomposition, likely contributed to this balanced growth pattern. These findings align with previous studies reporting enhanced root development under humic substance application (Canellas et al., 2002; Zandonadi et al., 2007) [3,8] .

Seedling Vigour Indices and Survival Percentage

Seedling Vigour Index I and II were significantly higher in treatments receiving humic acid and leaf litter amendments, with the maximum values recorded in mango leaf scrap (100 g) + humic acid (2000 ppm). This treatment also recorded the highest survival percentage (81.00%), whereas the control showed the lowest survival (58.33%). Improved vigour indices and survival percentage are a cumulative outcome of enhanced morphological growth, physiological efficiency, and root system development. The integrated application of organic leaf litter and humic acid likely improved overall seedling health and adaptability, resulting in better survival. Similar improvements in seedling vigour under humic acid application have been reported in citrus and other perennial crops (Nardi et al., 2002) [6] .

Conclusion

The results of the present investigation clearly indicate that soil amendment with mango leaf scrap at 100 g combined with humic acid seed priming at 2000 ppm creates a favourable physiological and nutritional environment for acid lime seedlings. This integrated treatment significantly improves morphological growth, root system development, photosynthetic efficiency, and survival percentage under nursery conditions. Adoption of this practice may help in producing high-quality planting material for acid lime cultivation. Further studies under different soil types and environmental conditions are suggested to validate these findings.

References

1. Adelani DO. Effects of leaf litters of nitrogen-fixing trees and watering regimes on the early growth of Citrus tangelo JW seedlings. Ethiopian Journal of Environmental Studies and Management. 2023;16(2):213-234.
2. Adelani DO, Ogunsanwo JA, Awobona TA. Influence of leaf litters of selected nitrogen-fixing Albizia species on growth performance of Zingiber officinale Rosc. Journal of Forestry Research and Management. 2020;17(1):45-55.
3. 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. Journal of Plant Physiology. 2002;130:1951-1957.
4. Mosa WFA, El-Megeed NAA, Ali MM. Impact of humic acid applications on vegetative growth, nutrient uptake and physiological traits of fruit trees. Horticulturae. 2020;6:507507.
5. Naik KC. South Indian fruits and their culture. Madras: P. Varadachary and Co.; 1949.
6. Nardi S, Pizzeghello D, Muscolo A, Vianello A. Physiological effects of humic substances on higher plants. Soil Biology and Biochemistry. 2002;34(11):1527-1537.
7. Watson DJ. The physiological basis of variation in yield. Advances in Agronomy. 1952;4:101-145.
8. Zandonadi DB, Canellas LP, Façanha AR. Indoleacetic and humic acids induce lateral root development through a common signalling pathway. Planta. 2007;225:1583-1595.