Growth modulation in Swietenia macrophylla: Role of salicylic and Jasmonic acids in seedling development

Introduction

Swietenia macrophylla King, commonly referred to as Indian Mahogany, is one of the most commercially valuable timber tree species in neo-tropical forests, renowned for its rapid growth, fine timber quality, and adaptability to diverse environmental conditions (Lugo et al., 2002). Native to tropical regions of America, Mahogany has been extensively cultivated in India since its introduction in 1795 (Anoop et al., 2014) [2] . Its resilience to varying soil types, including alluvial, volcanic, and lateritic soils, coupled with its high demand in the timber industry, has made it a primary species for afforestation projects in regions like Kerala, Tamil Nadu, and Karnataka (Durai et al., 2017) [12] . Mahogany is often intercropped with agricultural crops, enhancing biodiversity and improving soil fertility through leaf litter decomposition. Its deep-rooting system helps reduce soil erosion, making it an excellent choice for sustainable land management (Reis et al., 2019) [27] . Furthermore, Mahogany plantations contribute to carbon sequestration, aiding in climate change mitigation efforts (Racelis et al., 2019) [24] Despite its adaptive traits, mahogany growth and productivity are often constrained by abiotic stressors such as drought and salinity, which are becoming increasingly prevalent due to climate change (Rahman et al., 2013) [25] . These stressors disrupt cellular homeostasis, with drought, in particular, leading to cellular dehydration, which triggers secondary stresses, such as osmotic and oxidative stress. These conditions elevate reactive oxygen species (ROS) levels, detrimental to membrane integrity and macromolecular functionality (Ghosh et al., 2014) [16] . Salinity compounds these challenges by restricting water uptake and nutrient availability, further inhibiting the plant's overall growth and yield (Deshmukh et al., 2017) [10] .

Plants have evolved sophisticated defense mechanisms to mitigate the adverse effects of abiotic stress. These mechanisms involve the perception and transduction of stress signals, regulating stress-responsive genes, and making metabolic adjustments (Zhang et al., 2022) [38] . According to Chaves et al. (2003) [6] , plants undergo several phases during stress response, including alarm, acclimation, resistance, and exhaustion, ultimately leading to recovery or death if stress is sustained. The plant hormone Abscisic Acid (ABA) is a critical mediator in drought tolerance, modulating gene expression and stress-responsive pathways. Drought-induced ABA synthesis activates SnRK2 protein kinases, pivotal in regulating the stomatal aperture and conserving water by reducing transpiration (Yoshida et al., 2002; Li et al., 2015) [36] . In addition to ABA, Jasmonic Acid (JA) and Salicylic Acid (SA) play vital roles in modulating plant responses to various abiotic stresses. JA is synthesized in plastids, peroxisomes, and the cytosol, is essential in stress tolerance (Ahmad et al 2016) [1] . During stress, JA signaling is regulated by the degradation of Jasmonate ZIM-domain (JAZ) proteins, enabling the transcription of genes involved in stress defense (Chini et al., 2007) [7] . The application of exogenous JA has shown promising results in enhancing salt tolerance, as demonstrated by Faghih et al. (2017) [14] , where 0.25 mM of Methyl Jasmonate (MeJA) improved the salt tolerance of strawberry seedlings under saline conditions (60 mM NaCl). Similarly, Salicylic Acid is instrumental in drought and salt stress tolerance. Elevated SA levels inhibit stomatal opening, reducing water loss under drought conditions (Sun et al., 2021) [32] . Exogenous application of SA has been shown to enhance plant resilience to salt stress. In mungbean, the application of 0.5 mM SA conferred significant tolerance to 100 mM NaCl, highlighting SA's potential in stress mitigation (Esther et al., 2023) [13] . However, the concentration of SA is crucial, as both excessively low and high levels can increase susceptibility to stress. In contrast, moderate concentrations (0.01 mM to 0.05 mM) offer optimal stress protection (Yuan et al., 2008) [37] . Understanding how these hormones modulate plant growth and stress responses in forestry species like mahogany is critical for developing effective strategies for plantation management under challenging environmental conditions. This study investigates the influence of exogenous applications of Salicylic Acid and Jasmonic Acid on the growth and development of mahogany seedlings, with a focus on key growth parameters such as plant height, biomass allocation, photosynthetic efficiency, and water use. By elucidating the role of these defense hormones in enhancing abiotic stress tolerance, this research aims to contribute to the optimization of forestry practices and the resilience of mahogany plantations amidst changing climatic conditions.

Materials and Methods

Location of the experimental site

This study was conducted at the College of Forestry, Kerala Agricultural University, Vellanikkara, Kerala, India (13° 32'N, 76° 26'E), from July 2023 to September 2024, under the Department of Forest Biology and Tree Improvement. The site experiences a warm, humid climate with an average annual rainfall of 2100-2500 mm, temperatures ranging from 24.4 °C to 42.8 °C, and relative humidity of 80-100%.

Treatments and Experimental Design

The experiment was conducted using a Completely Randomized Design (CRD) with six treatments comprising different concentrations of Salicylic Acid (SA) and Jasmonic Acid (JA) (Table 1). Each treatment had three replications, with five plants per replication. Six-month-old mahogany seedlings were subjected to foliar applications of the defense hormones, administered twice with a two-day interval, between 9:00 AM and 11:00 AM to maximize absorption and minimize evaporation. A control group was included to establish a baseline for comparison. The study aimed to identify the optimal concentrations of SA and JA that promote enhanced growth and development in mahogany seedlings.

The volume index of each seedling was calculated for both initial and final observations using the formula:

Where d is the collar diameter (cm) and h is the seedling height (cm).

Sturdiness quotient

The sturdiness quotient was determined as the indicator of the strength of the seedlings to adverse climatic conditions (Ritchie, 1984).

Seedling quality index

Dickson et al., (1960) [11] gave the formula to calculate seedling quality based on morphological traits and seedling biomass.

Physiological observations

Physiological parameters such as photosynthetic rate (µmol CO₂ m⁻² s⁻¹) and transpiration rate (mmol H₂O m⁻² s⁻¹) were measured on the third leaf from the tip using a portable IRGA (LI-6400) system. Total chlorophyll content (mg g⁻¹ FW) was estimated using the DMSO method developed by Hiscox and Israelstam (1979) [18] , and relative water content (RWC %) was determined following the method described by Barrs (1968) [4] . The morphological and physiological data were analyzed using ANOVA via the General Linear Model (GLM) procedure in R (v4.3.1) to assess treatment significance. Least Square Difference (LSD) tests compared treatment means across growth stages, and Duncan's Multiple Range Test (DMRT) identified significant variations. Hierarchical cluster analysis, performed using Average Linkage with Euclidean Distance, grouped treatments into clusters, with dendrograms and interclass distances visualized via the 'agricolae' package. This integrated approach comprehensively evaluated the effects of JA and SA on seedling growth and physiology.

Results and Discussion

Evaluating morphological and physiological parameters in mahogany seedlings over 150 days revealed the impact of JA and SA on growth dynamics. Optimal concentrations of JA and SA significantly enhanced seedling height, collar diameter, photosynthetic efficiency, and chlorophyll content, contributing to improved vigor and stress tolerance. These findings align with studies showing that exogenous application of SA and JA boosts plant growth by enhancing nutrient uptake, optimizing photosynthesis, and activating stress response mechanisms (Rivas-San Vicente & Plasencia, 2011; Monte, 2023) [29,22] .

Exogenous application of JA and SA significantly enhanced seedling height in mahogany. JA at 0.6 mM and SA at 0.8 mM resulted in 158.87% and 139.62% growth after 150 days, respectively (Fig. 1). In contrast, the control groups showed lower growth, with JA and SA controls increasing by 74.13% and 69.38%, respectively (Table . 2,3). This growth enhancement is attributed to the role of JA and SA in regulating plant growth through mechanisms such as cell elongation, division, and nutrient allocation (Rohwer & Erwin, 2008) [30] . However, higher concentrations (1.0 mM) inhibited growth, underscoring the importance of precise hormone application to avoid disrupting metabolic processes (Wang et al., 2020) [34] . These findings highlight the effectiveness of optimal hormone concentrations in promoting seedling growth.

Defense hormones significantly influenced collar diameter in mahogany seedlings. JA at 0.6 mM (T5) caused the greatest increase (180.53%, 11.53 mm), while SA at 0.8 mM led to a 146.93% rise (10.63 mm). Controls showed lower growth (75.24% for JA, 74.02% for SA). Higher and lower concentrations inhibited growth, indicating a critical range for optimal development (Table . [29,39,34] .

Number of leaves

JA and SA treatments significantly increased the number of leaves in mahogany seedlings, with optimal concentrations of 0.4 mM JA (221.70%, 39 leaves) and 0.8 mM SA (210.69%, 41 leaves) promoting the highest leaf development (Table . 2, 3). This increase is attributed to the crosstalk between defense hormones and auxin, which stimulates cell division and leaf formation (Pauwels & Goossens, 2011) [23] . Enhanced photosynthetic performance further supports growth by providing energy and resources (Khan et al., 2015) [19] .

Biomass accumulation

Biomass accumulation is a critical factor for assessing plant growth and overall health, particularly in terms of fresh and dry weight in Mahogany seedlings. After 150 days, the optimal hormone concentrations for biomass accumulation were observed with specific treatments of Jasmonic Acid (JA) and Salicylic Acid (SA). JA at 0.6 mM (T4) resulted in the highest fresh weight of 38.71 g and dry weight of 18.32 g, reflecting substantial biomass accumulation (Fig 1). SA at 0.8 mM (T5) also showed significant growth with a fresh weight of 35.32 g and dry weight of 15.19 g, outperforming both control and other concentrations. However, excessive hormone concentrations, like 1.0 mM of either JA or SA, led to reduced biomass accumulation, highlighting a threshold beyond which hormone application may be counterproductive. This suggests that hormone-induced growth is most effective within an optimal concentration range, promoting cell division and elongation, which is essential for biomass accumulation (Pauwels & Goossens, 2011) [23] .

Seedling quality parameters

Assessing seedling quality is crucial for evaluating their outplanting potential and overall survival. Total seedling height is often considered a key factor among various quality indicators, correlating with the plant's overall vigor.

Seedlings with larger shoot mass typically have a higher photosynthetic capacity and better growth potential. However, shoot mass must be balanced with root mass to ensure optimal seedling performance. Larger root masses promote healthier growth and improved survival rates, as roots are crucial for nutrient and water uptake. It is also important to recognize that root mass does not always indicate root fibrosity; a seedling with numerous fine roots may have a similar mass to one with a larger taproot. The [17,31] .

Volume index

Height and collar diameter are key indicators of volume increment, crucial for assessing growth and biomass accumulation in mahogany seedlings. After 150 days, optimal growth was observed with specific JA and SA concentrations. JA at 0.6 mM (T4) resulted in the highest volume at 155.48 cm³, while SA at 0.8 mM (T5) yielded 111.17 cm³. Control and higher hormone concentrations (1.0 mM) showed minimal growth, indicating that excessive hormone levels may hinder development (Fig 2). JA and SA promote cell division and elongation, especially in the apical meristem and cambium, facilitating vertical growth and collar expansion through enhanced auxin transport (Pauwels & Goossens, 2011) [23] .

Sturdiness Quotient

The sturdiness quotient, reflecting seedling stability and stress resilience, is influenced by JA and SA concentrations. JA enhances sturdiness by promoting lignin biosynthesis and cell wall fortification, with the highest quotient at 0.6 mM due to optimal structural reinforcement (Wasternack & Hause, 2013) [35] . Higher concentrations (1.0 mM) overstimulate stress pathways, reducing sturdiness (Fig. 3). Similarly, SA strengthens cell wall integrity, with the highest sturdiness at 0.8 mM, balancing growth and defense (Vlot et al., 2009) [33] . Excessive SA (1.0 mM) reduces sturdiness, likely due to toxicity.

Seedling quality index

The seedling quality index, which reflects plant vigor and survival potential, is influenced by JA and SA concentrations. JA enhances seedling quality by improving nutrient uptake and photosynthetic efficiency, with the highest index at 0.6 mM, where growth and stress responses were optimally balanced (Pauwels & Goossens, 2011) [23] . Higher concentrations (1.0 mM) triggered stress responses, reducing quality. SA also improved seedling vigor, with the highest index at 0.8 mM (Khan et al., 2015) [19] , while excessive SA (1.0 mM) led to diminished returns due to stress-induced growth inhibition.

Effect of exogenous application of JA and SA on the physiological parameters of the mahogany seedlings

Exogenous applications of JA and SA significantly enhanced various physiological parameters in mahogany seedlings, promoting growth and metabolic activity. Photosynthesis in mahogany seedlings was significantly enhanced with JA and SA application. JA at 0.6 mM (T4) achieved peak rates of 3.78 µmol CO₂ m²/s by day 120, while the control group (T1) declined from 2.55 to 2.14 µmol CO₂ m²/s. SA at 1.0 mM (T6) reached 4.50 µmol CO₂ m²/s on day 30, with lower concentrations showing moderate improvements. However, a decrease in rates towards the end of the evaluation period was likely due to heat stress and reduced hormone efficacy after 120 days (Table 4,5). JA and SA significantly enhanced the transpiration rate of mahogany seedlings, which is vital for water balance and gas exchange. JA at 0.6 mM (T4) peaked at 0.178 mmol H₂O m²/s at 30 days and 0.145 mmol H₂O m²/s at 120 days, while the control group (T1) decreased from 0.086 to 0.078 mmol H₂O m²/s. SA at 0.8 mM (T5) reached 0.177 mmol H₂O m²/s at 30 days and 0.142 mmol H₂O m²/s at 120 days.

Both hormones promoted stomatal opening and modulated aquaporin expression, enhancing water transport and improving water regulation, growth, and stress resilience in seedlings (Curticăpean, 2020).

Both hormones also improved Relative Water Content (RWC), with SA at 0.8 mM showing the highest RWC of 86.68% at 30 days, which gradually decreased over time (Wang et al., 2020; Khoshbakht, 2015) [34] . Lastly, total chlorophyll content was significantly higher in JA at 0.6 mM and SA at 0.8 mM, contributing to enhanced photosynthetic efficiency and plant health (Rehman, 2023; Farhangi-Abriz et al., 2018). These findings highlight the potential of JA and SA in improving physiological performance, hydration, and stress resilience in mahogany seedlings.

Conclusion

The application of JA and SA significantly enhanced the growth of mahogany seedlings, highlighting their potential as effective growth regulators. The study identified optimal concentrations for maximizing growth benefits while minimizing adverse effects. JA performed best at 0.6 mM, while SA exhibited peak effectiveness at 0.8 mM. Both lower and higher concentrations of these hormones negatively impacted growth parameters, underscoring the importance of precise optimization. Furthermore, these optimal concentrations can be applied to seedlings experiencing abiotic stress to alleviate stress effects and enhance overall growth performance. These findings

Acknowledgement

International Journal of Advanced Biochemistry Research https://www.biochemjournal.com provide valuable insights for improving the cultivation and management of forestry species under stress conditions.

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