Deciphering Seed Pre-Treatment and Soil Amendment Effects on the Germination and Early Growth of Radhachura/Peacock Flower (Caesalpinia pulcherrima L.)

Abstract

Seed pre-treatment is imperative for breaking the seed dormancy of some perennial species. The addition of soil amendments might be helpful in supporting seed germination and growth by available essential plant nutrients. This research investigated the effects of different pre-treatment and soil amendments on the germination, growth, and physiological performance of radhachura (Caesalpinia pulcherrima L.), an important ornamental and multipurpose woody shrub. Four pre-treatments and five soil amendments were applied in a CRD (Completely Randomized Design) arrangement to evaluate their individual and combined impacts under controlled nursery conditions. The ANOVA result revealed that seed germination indices of radhachura were mostly influenced by soil amendment rather than the seed pre-treatment. Among the soil amendments, vermicompost had a more profound impact on germination speed, Timson’s index and peak value, which had a similar effect to NPK application. Soil organic amendments positively affected growth, with vermicompost exerting the greatest influence on multiple germination traits that may support the early growth of radhachura, while biochar and compost maximized certain root and plant-length traits. Pearson correlations and PCA (first seven PCs explaining 76.2% variation) revealed the strong integration of late biomass, plant length, and root development, identifying vermicompost as key enhancers of multivariate vigor in radhachura seedlings. It might be concluded that C. pulcherrima L. species germination and growth was mostly influenced by soil amendment rather than seed pre-treatment. The study highlights that integrated nursery practices combining appropriate pre-treatment and soil amendments can enhance the germination success of radhachura.

1. Introduction

Radhachura, or Peacock Flower (C. pulcherrima), is a tropical ornamental species that is highly valued for its colorful bloom and medicinal uses [1,2]. This woody shrub, a member of the family Fabaceae, is commonly planted as a live fence and for landscape beautification in urban environments, including institutional and roadside plantations [3]. The genus Caesalpinia L. comprises about 150 pantropical species of trees, shrubs, and thorny climbers, many of which hold substantial economic, pharmacological, and ecological value [4]. In Bangladesh, twelve species of Caesalpinia have been recorded, among which C. pulcherrima is distinguished by its long pedicels and stamens (>5 cm), stipel-bearing leaflets, and bright red-to-yellow flowers [4].

Radhachura leaves, flowers, and seeds have medicinal uses, including treating fever, sores, cough, breathing difficulties, and chest pain, while its root can induce early-term abortion [1]. The plant is also used for treating swellings, earaches, rheumatic pain, and cardiovascular diseases, and it is also used for the treatment of bronchial organs (asthma and bronchitis), gastrointestinal organs (cholera, diarrhea and dysentery) and liver complaints [2]. It contains bioactive compounds attributed to a diverse range of secondary metabolites such as cassane diterpenoids, flavonoids, glycosides, and steroids, which contribute to its antimicrobial and therapeutic properties [1]. Besides its medicinal and esthetic significance, radhachura is a common option for home gardens, roadside plantings, and urban landscaping because of its eye-catching, vibrant blooms and feathery leaves [5]. Integumentary dormancy has been found among the species of the Fabaceae family; thus, it would be imperative to decipher the response of seed germination with pre-treatment and soil amendments.

Seed germination is a vital stage in a plant’s life cycle and is highly influenced by environmental factors such as soil texture, temperature, moisture content, and nutrient availability [6]. Proper soil management enhances germination efficiency and seedling establishment, particularly for radhachura [7,8]. The seeds of radhachura are hard-coated, exhibiting physical dormancy that restricts water absorption and gas exchange, thus delaying germination [4]. To overcome these constraints and optimize early growth, soil fertility and amendment practices play an essential role. The potential amendments include both organic and inorganic chemical as well as biologically active additions, improving soil structure, nutrient availability, water retention, and aeration [9]. Organic amendments such as cow dung, compost, vermicompost, and biochar improve soil microbial activity and nutrient supply, promoting seedling vigor and sustained plant development [10,11,12]. Cowdung and compost progressively supply macro- and micronutrients, increase soil porosity, and promote root growth [13,14]. Vermicompost enriches soil with beneficial microorganisms and growth hormones, whereas biochar enhances cation exchange capacity and moisture retention [11,14,15]. In contrast, inorganic fertilizers like NPK provide fast-acting nutrients that stimulate early growth and biomass accumulation but may cause leaching and acidity when overapplied [16,17,18]. The integration of organic and inorganic amendments enhances nutrient cycling, soil fertility, and plant productivity in Fabaceae species [19], highlighting their importance for successful radhachura germination and sustainable management in agroforestry and restoration systems [20,21,22,23].

Pre-treatment of seeds is crucial for promoting germination and seedling establishment of some species. Radhachura produce seeds with hard, impermeable seed coverings that, by obstructing gas exchange and water absorption, result in physical dormancy and delayed or irregular germination in the wild [24]. Several pre-sowing methods have been investigated to overcome this restriction, such as chemical scarification (acid treatment with concentrated sulfuric acid), mechanical scarification (scratching or nicking the seed coat), and thermal treatments (soaking in hot or cold water) [25,26]. For instance, soaking radhachura seeds in hot water or treating them with sulfuric acid for a specified length has been shown to dramatically enhance germination percentage while decreasing mean germination time when compared to untreated or un-scarified seeds [27]. Other woody shrubs and leguminous trees, like Albizia lebbeck and Acacia nilotica, have shown similar favorable benefits from such treatments [28,29]. Effective species-dependent pre-treatment is essential for large-scale planting, nursery production, and ecological restoration projects since it not only guarantees consistent and quick germination but also increases seedling vigor [30,31]. Therefore, selecting appropriate pre-treatment methods based on species dormancy type and seed coat hardness is essential for the successful propagation and sustainable utilization of ornamental and multipurpose woody shrubs like radhachura.

This study was undertaken to examine the combined effects of seed pre-treatment methods and soil amendments on the germination and early growth of radhachura. The research specifically aimed to evaluate the impacts of different pre-treatment techniques, such as water soaking, hot-water soaking, and sulfuric acid treatment, with or without scarification, on seed germination indices; assess the influence of various soil amendments, both organic (compost, biochar, cow dung, vermicompost) and inorganic (NPK fertilizer), on seedling emergence and growth performance; and analyze both above- and below-ground physiological traits, including shoot and root growth, leaf number, and biomass allocation, in response to these treatments. Ultimately, this study seeks to optimize effective propagation and soil management practices for radhachura, contributing to its improvement in agroforestry, medicinal, and ornamental horticultural applications.

2. Materials and Methods

2.1. Plant Material and Experimental Condition

The radhachura seeds were collected from the Agroforestry Research Field, Department of Agroforestry and Environment, Hajee Mohammad Danesh Science and Technology University (HSTU), Dinajpur, Bangladesh, where the experiment was also conducted. The site is geographically located at 25°13′ N latitude and 88°23′ E longitude, approximately 37.5 m above sea level, and belongs to the Level Barind Tract (AEZ-25) based on its geological and environmental features. The study was carried out over a three-month period, from 28 February to 27 May 2025, to evaluate the effects of seed pre-treatments and soil amendments on the germination of Radhachura.

2.2. Seed and Soil Collection Material

Seeds used in this study were collected from mature radhachura trees growing on the HSTU campus during the late fall or winter season. After collection, the pods were sun-dried for a week to facilitate seed separation, and the extracted seeds were stored in airtight zip-lock bags prior to pre-treatment and sowing in pots. Soil used for pot filling was obtained from the experimental fields of the HSTU research farm. The collected soil was sieved thoroughly to remove debris and homogenized before being placed into pots measuring 18 cm height × 15 cm diameter. Each pot was filled uniformly to maintain consistency across all treatments. Different soil amendments were incorporated at specific ratios according to treatment requirements and mixed thoroughly before the seeds were placed for germination assessment.

2.3. Experimental Design and Treatments

The experiment was conducted using a Completely Randomized Design (CRD) with three replications to evaluate the combined effects of seed pre-treatments and soil amendment mixtures on seed performance. Four seed pre-treatments and five different soil mixtures were applied as treatments. The seed pre-treatments included un-scarified (control) seeds soaked in water for 24 h (T₁); scarified seeds (with sandpaper) soaked in water for 24 h (T₂); hot-water treatment for 2 min followed by seed soaking in water for 24 h with scarification (T₃) [32]; and sulfuric acid (H₂SO₄) soaking for 2 min followed by seed soaking in water for 24 h with scarification (T₄) [33]. The soil amendment treatments comprised five different soil mixtures: soil and cow dung (3:1) (F₁) [34,35]; soil and compost (3:1) (F₂) [36]; soil and vermicompost (3:1) (F₃) [37]; soil amended with biochar (40 g/kg soil) (F₄) [38]; and soil supplemented with NPK fertilizer (4 g/kg soil) (F₅) [39]. For each treatment combination, 4 seeds were sown per pot at a depth of 2 cm, with three pots per combination; the pot was considered the experimental unit in all statistical analyses. Pots were kept in a nursery under a transparent polythene roof without side walls to provide shade and protection from rainfall, irrigated as needed to maintain adequate moisture, and grown in the amended soil mixtures without any additional fertilization to allow accurate assessment of the seed pre-treatments and soil amendments. Both seed and soil amendments were factorially combined to examine their interaction effects under controlled conditions.

2.4. Data Collection and Measurement Parameters

Following planting, observations were made on various germination and growth parameters, including germination percentage (GP), peak germination time (PGT), germination speed (GS), Timson’s index (TI), peak value (PV), plant height (PH), leaf number (LN), plant length (PL), root length (RL), ratio of plant height to root length (PH:RL), chlorophyll content (CC), shoot fresh weight (SFW), root fresh weight (RFW), shoot dry weight (SDW), root dry weight (RDW), ratio of shoot dry weight to root dry weight (SDW:RDW), leaf area index (LAI), specific leaf area (SLA).

2.5. Analysis of Germination Indices

Germination percentage (GP) refers to the percentage of seeds that successfully sprout within a given time frame. GP was calculated using the following formula [40]:

$$\mathrm{G}\mathrm{P}={\displaystyle \frac{\mathrm{N}\mathrm{g}}{\mathrm{N}\mathrm{t}}}\times 100$$

(1)

where Ng is the number of germinated seeds and Nt is the total number of seeds.

Peak germination time (PGT) refers to the time in which the highest frequency of germinated seeds are observed and need not be unique. PGT was calculated using the following formula [41]:

$$\mathrm{T}\mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}=\{\mathrm{T}\mathrm{i}:\mathrm{N}\mathrm{i}=\mathrm{N}\mathrm{m}\mathrm{a}\mathrm{x}\}$$

(2)

where Ti is the time from the start of the experiment to the ith interval, Ni is the number of seeds germinated in the ith time interval (not the accumulated number, but the number corresponding to the ith interval) and Nmax is the maximum number of seeds germinated per interval.

Germination speed (GS) is the rate of germination in terms of the total number of seeds that germinate in a time interval. GS was calculated using the following formula [42,43]:

$$\mathrm{G}\mathrm{S}=\sum _{\mathrm{i}=1}^{\mathrm{k}}\mathrm{N}\mathrm{i}/\mathrm{T}\mathrm{i}$$

(3)

where Ti is the time from the start of the experiment to the ith interval, Ni is the number of seeds germinated in the ith time interval (not the accumulated number, but the number corresponding to the ith interval), and k is the total number of time intervals.

Timson’s index (TI) is the progressive total of cumulative germination percentage recorded at specific intervals for a set period and is estimated in terms of cumulative germination percentage (Gi) as the following formula [44,45]:

$$\sum \mathrm{K}=\sum _{\mathrm{i}-1}^{\mathrm{k}}\mathrm{G}\mathrm{i}$$

(4)

where Gi is the cumulative germination percentage in the time interval i and k is the total number of time intervals.

Peak value (PV) is the accumulated number of seeds germinated at the point on the germination curve at which the rate of germination starts to decrease. It is computed as the maximum quotient obtained by dividing successive cumulative germination values by the relevant incubation time. PV was calculated using the following formula [46]:

$$\mathrm{P}\mathrm{V}=\max \left({\displaystyle \frac{\mathrm{G}1}{\mathrm{T}1}},{\displaystyle \frac{\mathrm{G}2}{\mathrm{T}2}},\dots \dots \dots ,{\displaystyle \frac{\mathrm{G}\mathrm{k}}{\mathrm{T}\mathrm{k}}}\right)$$

(5)

where Ti is the time from the start of the experiment to the ith interval, Gi is the cumulative germination percentage in the ith time interval, and k is the total number of time intervals.

2.6. Growth and Physiological Measurements

Plant height (PH) was measured from the base to the apical point of the plant using a centimeter-scale at 15-day intervals up to 90 DAS. The number of leaves (LN) was recorded manually at the same intervals by counting all visible leaves except immature ones at the shoot apex. Root length (RL) was measured from the base to the tip of the longest root after carefully washing adhering soil particles, while plant length (PL) was determined as the sum of PH and RL. The ratio of plant height to root length (PH:RL) was calculated using the mean PH and RL of each treatment. Chlorophyll content (CC) was assessed using a handheld Soil–Plant Analysis Development (SPAD) chlorophyll meter (SPAD-502, Konica Minolta, Osaka, Japan), which measures light transmittance at 660 and 940 nm to estimate relative chlorophyll concentration non-destructively. Shoot fresh weight (SFW) and root fresh weight (RFW) were measured immediately after carefully uprooting plants at 90 DAS using a precision balance. Shoots and roots were then oven-dried at 65 °C for 48 h to obtain shoot dry weight (SDW) and root dry weight (RDW). The ratio of shoot to root dry weight (SDW:RDW) was determined to assess biomass partitioning between above- and below-ground parts. Leaf area index (LAI) was quantified using a leaf area index meter (Model: LLAM-A10, Labtron Equipment Ltd., Camberley, Surrey, UK), and specific leaf area (SLA) was calculated as the ratio of total leaf area to leaf dry weight, following standard procedures [47].

2.7. Statistical Analysis

Germination indices were calculated in RStudio (version 4.2.3; 2023) using the germinationmetrics packages, which implement standard indices such as germination percentage, peak germination time, germination speed, Timson’s index, and peak values from cumulative germination counts. All subsequent statistical analyses were performed in Minitab 18 (version 21.4.3.0). A two-factor factorial ANOVA was used for each response variable, with seed pre-treatment and soil amendment as fixed factors and their interaction term included in the model. For plant height and leaf number, which were recorded at 15-day intervals, separate factorial ANOVAs were conducted at each time point. The Tukey HSD All-Pairwise Comparisons Test was employed to determine the statistical significance (p < 0.05) of the differences among the mean values. Significant differences were indicated by different letters in the table and figures. Graphs were prepared in Microsoft Excel (Microsoft Corporation, Redmond, WA, USA).

3. Results

3.1. Effects of Seed Pre-Treatments and Soil Amendments on the Germination Indices of Radhachura

Germination percentage (GP) was significantly affected by seed pre-treatment, whereas peak germination time (PGT), germination speed (GS), Timson’s index (TI) and peak value (PV) were significantly affected by soil amendments, as reflected in the key germination indices (Tables S1 and S2). Germination percentage (GP) varied significantly under different pre-treatments (Figure 1A; p < 0.029), with the highest GP observed in both scarified and non-scarified treatments, i.e., T₁ (93.33%) and T₂ (93.33%), followed by T₄ (83.33%), and it was the lowest in the seeds immersed in hot water after scarification, i.e., T₃ (71.67%). Among soil amendments, vermicompost amended soil (F₃) showed the maximum GP (91.67%) and F₅ (NPK) the minimum (81.25%) (Figure 1A). Peak germination time (PGT) differed notably by soil amendment (p < 0.01), being longest in biochar treatment F₄ (7.00 DAS) and shortest in F₃ (4.33 DAS) (Figure 1B). Germination speed (GS) also varied significantly (p = 0.016), with the highest rates in F₃ (20.47% day⁻¹) and F₅ (18.78% day⁻¹) (Figure 1C, Table S2). The Timson’s index (TI) also showed a similar trend to GS (Figure 1D, Table S3). Peak value (PV) also responded significantly to soil amendments (p < 0.01), recording the highest in F₅ (16.88% day⁻¹) and F₃ (15.65% day⁻¹), and the lowest in F₄ (10.28% day⁻¹) (Figure 1E).

Overall, germination indices of C. pulcherrima were profoundly influenced by soil amendments rather than the seed pre-treatments, particularly vermicompost (F₃), which consistently improved germination efficiency.

3.2. Effects of Seed Pre-Treatment and Soil Amendments on the Growth-Related Traits of Radhachura

Seed pre-treatments caused significant plant height (PH) variation only at 15 DAS (Figure 1A, Tables S3 and S4), with no further effects. Soil amendments significantly increased PH at 15, 30, 45, and 75 DAS (Figure 1B), where F₃ yielded the highest PH (8.27 cm at 15 DAS, 19.79 cm at 90 DAS), F₁ the peak at 45–60 DAS (15.14 cm), and F₂/F₅ the lowest (F₂: 6.03 cm at 15 DAS; F₅: 14.83 cm at 75 DAS). Pre-treatments and amendments had non-significant effects on leaf number (LN), leaf area index (LAI), and specific leaf area (SLA) (Figure S1; Tables S3 and S4).

Soil amendments significantly increased root length at 90 DAS (RL90) (p = 0.004; F₂: 34.40 cm; F₅ lowest: 19.18 cm; Figure S1A; Table S3), plant length at 90 DAS (PL) (p = 0.017; F₂: 50.94 cm; F₅: 35.35 cm), ratio of plant height to root length at 90 DAS (PH:RL) (p = 0.002; F₅: 0.96; F₂: 0.54), and ratio of shoot dry weight to root dry weight at 90 DAS (SDW:RDW) (p = 0.040; F₄: 6.83; F₂: 2.97), with non-significant pre-treatment or interaction effects except for chlorophyll content at 90 DAS (CC) (pre-treatment p < 0.001, interaction p < 0.001; T₂: 26.40 µmol m⁻²; T₂F₄: 30.90 µmol m⁻²) (Figure 2 and Figure S1; Tables S3 and S4). Root fresh weight at 90 DAS (RFW), shoot dry weight at 90 DAS (SDW), and root dry weight at 90 DAS (RDW) showed no significant main or interaction effects (p > 0.05), although trends favored F₁–F₄ over F₅ (SFW F₁: 2.10 g; SDW F₄: 0.55 g) (Figure S1E,F; Tables S3 and S4).

3.3. Physicochemical Analysis of the Soil Amendments

The physicochemical analysis of the soil amendments revealed marked differences in nutrient composition compared with the base soil. Vermicompost exhibited the highest soil organic carbon (SOC) content (21.74%), followed by cowdung (12.46%) and compost (9.28%), whereas biochar and unamended soil contained comparatively lower SOC (8.16% and 1.66%, respectively) (

Table 1. Physicochemical analysis of the soil amendments used in this study.

Variables	SOC (%)	N (%)	P (%)	K (%)	S (%)
Cowdung	12.46	0.75	0.55	1.32	0.43
Compost	9.28	1.54	0.52	2.65	0.38
Vermicompost	21.74	0.84	0.56	1.32	0.06
Biochar	8.16	0.70	0.25	2.32	0.10
Soil	1.66	0.10	0.36	0.43	0.16

). Total nitrogen was greatest in compost (1.54%), with moderate levels in vermicompost (0.84%), cowdung (0.75%), and biochar (0.70%), while the soil alone had very low nitrogen (0.10%) (Table 1). Available phosphorus ranged from 0.25% in biochar to 0.56% in vermicompost, with cowdung and compost also providing relatively high P compared with soil. Potassium content was highest in compost (2.65%), followed by biochar (2.32%), with cowdung and vermicompost each at 1.32%, all exceeding the K level in soil (0.43%) (Table 1). Sulfur content was moderate in cowdung (0.43%) and compost (0.38%), lower in soil (0.16%) and biochar (0.10%), and minimal in vermicompost (0.06%), indicating that each amendment contributed a distinct nutrient profile likely to differentially influence seedling growth and soil fertility (Table 1).

3.4. Pearson Correlations Coefficient Relationship Among Growth, Biomass, and Physiological Traits

Across developmental stages, plant height exhibited strong positive temporal correlations, with particularly high coefficients among later measurements (PH60-PH90, r = 0.67–0.85, p < 0.001), indicating a stable ranking of treatments for vertical growth over time. Leaf number showed a comparable pattern (LN45-LN75, r = 0.38, p = 0.003; LN60-LN90, r = 0.45, p < 0.001), whereas cross-trait associations between height and leaf number were generally weak or non-significant, suggesting partially independent regulation of elongation and leaf production (Figure 3). Root length at 90 DAS (RL90) was strongly and positively correlated with plant length (PL, r = 0.96, p < 0.001) and later plant height measurements, while the ratio of plant height to root length (PH:RL) was negatively associated with both traits (PH:RL-PL, r = −0.66, p < 0.001), pointing to treatments that combine taller shoots with disproportionately longer roots (Figure 3). Plant length was further associated with shoot fresh and dry weight (PL-SFW, r = 0.52, p < 0.001; PL-SDW, r = 0.48, p < 0.001) and root dry weight (PL-RDW, r = 0.47, p < 0.001), highlighting tight coupling between the reproductive architecture and whole-plant biomass (Figure 3).

Biomass-related traits were highly inter-correlated: SFW, RFW, SDW, and RDW all showed strong positive relationships (SFW-SDW, r = 0.65, p < 0.001; SDW-RDW, r = 0.41, p = 0.001). Leaf area index (LAI) was moderately correlated with shoot biomass (LAI-SFW and LAI-SDW, r ≈ 0.40, p ≤ 0.003), consistent with a larger canopy supporting greater dry matter accumulation (Figure 3). Collectively, these patterns point to a suite of positively associated shoot and root traits, with plant length, biomass components, and LAI emerging as integrative indicators of plant vigor.

3.5. Principal Component Analysis (PCA) Among Growth, Biomass, and Physiological Traits

Principal component analysis (PCA) showed the first seven PCs with eigenvalues greater than one, collectively explaining 76.2% of total phenotypic variation among the 23 traits (Figure 4A–C). PC1 (29.6% variance) was dominated by late biomass and growth traits, with the highest loadings on PH90 (0.301), PH75/PH60 (0.285), PL (0.295), SFW (0.316), and SDW (0.313) (Figure 4B,D). PC2 (14.6%; cumulative 44.3%) highlighted early shoot elongation (PH30/PH15: 0.405/0.354; PH45: 0.322) and PH:RL (0.354), contrasting with negative root loadings (RL90: −0.307) (Figure 4B,D). PC3 (9.9%; cumulative 54.1%) emphasized root traits (RFW: 0.319; RL90/PL: 0.295/0.260), with score plots (Figure 4B–D) revealing clusters along vigor gradients, where high PC1-PC2 scores indicate integrated shoot–root performance.

4. Discussion

The present study analyzed the impact of various seed pre-treatments and soil amendments on the germination, growth, and biomass allocation of radhachura. The results indicate that both factors, individually, significantly influence seedling emergence and early growth performance.

Seed germination indices are key indicators of seed vigor and establishment potential, particularly in woody ornamentals. Pre-treatments and soil amendments differentially affected these indices, reflecting complex physiological responses to dormancy-breaking and nutrient conditions. Soft mechanical scarification maintained high germination percentage (GP) in line with the good natural viability of radhachura, and likely enhanced water uptake and embryo activation without severe seed coat damage [48]. Hot water and concentrated acid treatments, by contrast, risk thermal or chemical injury to embryos and seed coats, a concern previously noted for hard-coated seeds. Soil amendments did not markedly change GP, but vermicompost tended to increase emergence, consistent with evidence that organic inputs improve seedling establishment via enhanced soil moisture retention and structure [49]. Peak germination time and germination speed were sensitive to amendment type, indicating that nutrient status and microbial activity strongly influence the rate and uniformity of germination [50]. Vermicompost appeared to promote faster and more synchronized germination, likely through improved nutrient availability and rhizosphere activity, whereas biochar tended to delay and spread-out germination, suggesting that certain biochar properties may transiently alter water uptake or nutrient dynamics [51,52]. Timson’s index and peak value, which integrate the speed and extent of germination, were highest under chemical fertilizer and lowest under biochar, indicating that readily available nutrients can boost early metabolic activation, while some biochar formulations may temporarily restrict nutrient availability [53]. Overall, these patterns suggest that dormancy-breaking in radhachura is best achieved with untreated or gently scarified seeds, combined with organic amendments such as vermicompost to enhance germination speed and uniformity [54]. In contrast, harsh hot water or acid scarification should be used cautiously to avoid injury. These findings align with previous work showing that appropriate seed pre-treatment and sustainable soil management jointly improve germination and nursery performance in tree species [55,56].

Plant height is a widely used indicator of vigor and early growth potential in nursery and field conditions [57,58]. In radhachura, scarification promoted faster initial elongation, supporting the view that enhanced water imbibition after gentle seed coat disruption accelerates early growth [55,56]. However, differences among pre-treatments tended to diminish over time, suggesting that long-term height performance is more strongly determined by inherent plant physiology and environmental conditions than by dormancy-breaking alone [59,60]. Soil amendments significantly influenced height at multiple stages, emphasizing the importance of nutrient availability and organic matter for sustained growth. Vermicompost supported the greatest height growth, consistent with its role in enhancing mineralization, microbial activity, and nutrient supply, while cowdung provided intermediate but steady growth, in line with its slow-release nutrient profile [13,61,62]. NPK and compost were less effective under these conditions, possibly due to the mismatch between nutrient release patterns and seedling demand [63,64]. These results support combining gentle pre-treatments with nutrient-rich organic amendments to maximize early height growth and establishment, echoing previous reports on the benefits of integrating scarification with organics in tree nursery systems [65,66].

Leaf number is a significant morphological parameter which has a direct relation with photosynthesis capacity and biomass yield potentiality of plants [67,68]. In the present research work in radhachura, there was non-significant variation in leaf number were observed in response to seed pre-treatment and soil amendments. Previous studies suggests that extensive scarification stress or seed coat damage can suppress even emergence or seedling stress, resulting in suppressed leaf initiation [69,70]. However, studies reported of positive effect of soil organic amendments like biochar and vermicompost in enhancing soil structure, water holding capacity, and nutrient levels, thereby boosting vegetative growth [71,72,73]. Vermicompost is well-known for its beneficial microbial action and balanced provision of nutrients with the best support for profuse leaf growth [74]. In this study, the leaf numbers were monitored for a shorter period, but long-term observation for 12 months or more might assist in predicting the effects of soil amendments on the leaf characteristics. Researchers suggest that untreated or modestly treated seeds with good-quality organic amendments can be utilized to optimize leaf initiation and growth [21,75].

Shoot and root dry weights integrate the cumulative effects of pre-treatment and soil environment on early seedling performance. Although treatment differences were not always statistically significant at 90 DAS, trends indicated that biochar and cowdung produced higher biomass than other amendments, while NPK was the least favorable. The porous structure of biochar can improve water and nutrient retention and support long-term soil conditioning, whereas cowdung contributes organic matter and a gradual nutrient supply [76,77]. Seed pre-treatments had limited direct impact on biomass, but non-scarified and hot-water-treated seeds tended to produce slightly higher shoot and root dry weights than seeds subjected to harsh acid scarification, reinforcing the need for gentle dormancy-breaking [78]. Soil amendments also influenced shoot:root allocation, with biochar and vermicompost promoting higher shoot-to-root dry weight ratios (SDW:RDW), which may reflect improved photosynthetic efficiency and water use under enhanced soil physical and biological conditions [79,80]. Cumulatively, these findings highlight the utility of organic soil amendments such as cowdung, compost, vermicompost, and biochar in sustainable seedling production. When coupled with sensible seed pre-treatment, these inputs enhance root and shoot balance, a pre-requisite for healthy seedling establishment and field performance [81,82].

Chlorophyll content at 90 DAS responded mainly to seed pre-treatments and their interaction with soil amendments, rather than to amendments alone. Scarified and hot-water-treated seeds showed higher chlorophyll levels than untreated seeds, indicating that effective dormancy-breaking not only improves emergence but can enhance later photosynthetic capacity, likely via more uniform and vigorous seedling establishment [83,84]. Improved chlorophyll content may also reflect more efficient water and nutrient uptake by seedlings with better early root development, consistent with reports that high germination power is often associated with increased chlorophyll accumulation and biomass [85,86]. Among soil amendments, biochar and cowdung tended to support higher chlorophyll content than NPK, suggesting that organic inputs indirectly promote pigment maintenance through improved soil porosity, microbial activity, and nutrient cycling [87]. It is also a sign of the synergistic effect of dormancy-breaking treatment and organic soil management on seedling physiological quality [88]. Therefore, for optimal photosynthetic capacity and good seedling vigor, rightful pre-treatment in scarification could be supplemented with amendments in the soil such as biochar. Such an integrated practice is in line with sustainable nursery culture and facilitates healthy field establishment of the plant [73,89].

Leaf area index (LAI) and specific leaf area (SLA) are important markers of photosynthetic potential, resource use, and stress tolerance [90,91]. In radhachura, treatment effects on LAI and SLA were modest but showed consistent trends: cowdung-amended soils and un-scarified seeds tended to support larger leaf area, while harsh scarification and exclusive reliance on NPK coincided with lower LAI, echoing reports that organic matter enhances water holding capacity and nutrient supply to support leaf expansion [71,72,87]. Higher SLA under scarification and NPK suggests the development of thinner, fast-growing leaves often associated with nutrient-rich or more stressful environments, whereas lower SLA under biochar points to thicker, more structurally robust foliage [92,93,94]. These leaf trait patterns reinforce the advantage of organic amendments like cowdung, vermicompost, and biochar for building resilient leaf structures, while excessive dependence on mineral fertilizer may favor rapid but potentially less stress-tolerant foliage [95].

Pearson correlations revealed strong temporal stability in plant height (PH60-PH90) and high integration among biomass traits (SFW-SDW), with plant length tightly coupled to root length (RL90-PL) and shoot biomass, indicating that vigorous seedlings tend to maintain a balanced root–shoot allocation under the different soil amendment regimes. These patterns are consistent with previous reports in woody seedlings, where organic amendments such as vermicompost enhance coordinated growth through improved soil nutrient and water status. Principal component analysis (PCA) further supported these relationships, with PC1 (29.6%) largely driven by late plant height, plant length and biomass variables, and PC2/PC3 distinguishing early shoot and root traits, thereby clearly separating treatment combinations in the multivariate space. In this framework, vermicompost and gentle scarification emerged as key contributors to higher composite vigor scores, in line with biostimulant-like effects of organic inputs reported for other tree and agroforestry species.

Overall, this study demonstrates that an integration of suitable seed pre-treatments and organic soil amendments can be employed to enhance early growth performance in radhachura (Figure 5).

5. Conclusions

In conclusion, this study demonstrates that soil amendments have a stronger influence than seed pre-treatments on the germination, growth, and physiological performance of radhachura. While seed scarification, particularly mechanical scarification, enhanced germination percentage and vigor, hot-water scarification negatively affected germination and growth responses. Soil amendments had a more substantial overall impact on seedling development: vermicompost proved most effective, likely due to its high nutrient availability and favorable microbial composition, followed by biochar, which improved soil structure and moisture retention. These findings emphasize that appropriate combinations of seed pre-treatment and soil amendments can improve the germination behavior and early growth of radhachura.