Effect of stem girdling on rooting and sprouting performance in different mulberry (Morus spp.) Varieties

Introduction

Mulberry (Morus spp.) serves as the exclusive food source for the silkworm (Bombyx mori L.) and represents a critical component of the sericulture industry worldwide (Hariwal et al., 2024) [3] . Efficient vegetative propagation of superior mulberry genotypes is essential for maintaining leaf quality and yield consistency required for commercial silk production. Whilst mulberry can be propagated through seeds, cutting propagation remains the preferred method for preserving desirable genetic characteristics, as sexual reproduction leads to heterozygous offspring that do not breed true to type (Koyuncu and Senel, 2003) [6] . Stem girdling, the process of removing a ring of bark to interrupt phloem transport, has been extensively documented as an effective technique for enhancing rooting in stem cuttings (Haw, 1995) [5] . The physiological mechanism underlying this phenomenon involves the accumulation of photosynthates, endogenous auxins and rooting cofactors above the girdled zone, creating favorable conditions for adventitious root initiation (Druege et al., 2019) [2] . According to Hartmann and Kester (1975) [4] , girdling blocks the downward translocation of carbohydrates and growth-promoting substances, resulting in increased starch and sugar concentrations that provide energy and carbon skeletons necessary for root development. The formation of adventitious roots (AR) is a complex developmental process regulated primarily by auxin signaling pathways. Recent molecular studies have demonstrated that auxin accumulates at the base of cut stems within hours of excision, triggering founder cell initiation in pericycle cell layers and subsequent root primordia development (Druege et al., 2019) [2] . Additionally, girdling-induced stress responses may enhance ethylene biosynthesis and programmed cell death in epidermal tissues, facilitating root emergence (Druege et al., 2019) [2] . Despite the recognized benefits of girdling in various fruit and ornamental species, the systematic evaluation of this technique across diverse mulberry genotypes remains limited. Varietal differences in rooting competence significantly impact propagation success, with some genotypes exhibiting recalcitrant rooting behavior that limits their commercial deployment (Bharathi et al., 2024) [1] . Understanding genotype-specific responses to girdling treatments is therefore crucial for optimizing propagation protocols in mulberry improvement programs. The present paper was designed to evaluate the effect of stem girdling on rooting and sprouting parameters across seven commercially important mulberry varieties, with cuttings prepared at different intervals following girdling treatment. The specific objectives were to (1) assess varietal differences in sprouting percentage and temporal progression; (2) evaluate shoot growth parameters including height, number, and leaf production; and (3) quantify root biomass accumulation and distribution patterns.

Materials and Methods

Experimental Location and Plant Material

The study was conducted at the Department of Sericulture, Gandhi Krishi Vignana Kendra (GKVK), Bangalore, India. Seven mulberry varieties were selected from the germplasm collection: V 1 , S 36 , S 13 , S 54 , M5, MR2, and Mysore Local. These varieties represent diverse genetic backgrounds and are widely cultivated in Karnataka.

Girdling Treatment and Cutting Preparation

Three identical, healthy shoots per variety were selected for both girdled (treatment) and non-girdled (control) conditions. Girdling was performed using a grafting knife to remove a 0.5-1.0 inch ring of bark at the base of selected shoots, ensuring complete phloem disruption without damaging the underlying xylem tissue (Haw, 1995) [5] . Control shoots remained unmanipulated on the parent plant. Cuttings were prepared from girdled and control shoots at two distinct intervals: 7 days after girdling (7 DAG) and 15 days after girdling (15 DAG). For each treatment combination, five cuttings approximately 15-20 cm in length were prepared using secateurs and planted in polyethylene bags (30 × 15 cm) containing a standardized potting mixture of soil, sand, and compost (1:1:1). Cuttings were labeled with identification tags and maintained under nursery conditions with regular irrigation.

Experimental Design and Data Collection

The experiment followed a factorial design with three factors: variety (7 levels), girdling treatment (2 levels: girdled vs. control) and cutting preparation time (

Results

Sprouting Percentage

Effect of girdling on sprouting percentage (%) in different mulberry varieties at 7 days after girdling (7 DAG) is indicated in Table 1. The results indicate that significant variation in sprouting percentage was observed among varieties, girdling treatments and cutting preparation times (Table 1). For cuttings prepared at 7 DAG, variety MR2 exhibited the highest sprouting rates at 15 DAP (80% in girdled vs. 100% in control), while M5 showed delayed sprouting with 0% emergence at 15 DAP in both treatments. By 30 DAP, V 1 girdled cuttings achieved 100% sprouting compared to 80% in controls, indicating enhanced bud break following girdling treatment.

Root Growth Parameters 3.3.1 Maximum Root Length

Root system development, measured as maximum root length at 90 DAP, revealed substantial treatment and varietal effects (Table 7). Variety V 1 (15 DAG) produced the longest roots in girdled treatments (18.26 ± 1.58 cm) compared to controls (9.9 ± 0.22 cm), representing an 84.4% increase. Similarly, S 36 girdled cuttings achieved 11.5 ± 0.81 cm versus 8.64 ± 0.47 cm in controls. However, variety S 13 showed reduced root length in girdled treatments (4.4 ± 0.10 cm) compared to controls (0 cm), suggesting differential genotypic sensitivity to girdling-induced physiological changes.

Root Biomass

Fresh and dry root weights followed patterns similar to root length, with V 1 and S 36 showing significant girdling benefits. V 1 girdled cuttings produced 2.56 ± 0.01 g fresh root weight compared to 0.79 ± 0.03 g in controls, while dry root weight increased threefold (0.3 ± 0.00 g vs. 0.1 ± 0.00 g). The root-toshoot biomass ratios (fresh weight basis) indicated balanced allocation patterns, with V 1 showing ratios of 1.97 (girdled) versus 2.08 (control), suggesting that girdling enhanced both root and shoot development proportionally.

Discussion

The present study demonstrates that stem girdling significantly enhances vegetative propagation efficiency in mulberry, though the magnitude of response varies considerably among varieties. These findings align with established horticultural principles regarding girdling-induced accumulation of assimilates and growth regulators above the treated zone (Hartmann and Kester, 1975; Haw, 1995) [4,5] .

Physiological Mechanisms of Girdling-Enhanced Rooting

The improved rooting performance observed in girdled cuttings can be attributed to multiple physiological mechanisms. Primarily, phloem girdling disrupts the downward translocation of photosynthates, resulting in carbohydrate accumulation above the girdle that provides essential energy and carbon skeletons for root primordia development (Stoltz and Hess, 1966) [8] . This accumulation creates a favorable source-sink relationship that supports the metabolically demanding process of adventitious root formation. Furthermore, girdling-induced interruption of basipetal auxin transport likely results in elevated auxin concentrations at the cutting base, directly stimulating root initiation. Druege et al.

(2019) [2] demonstrated that auxin accumulates specifically in pericycle cell layers at the base of cut stems, triggering founder cell differentiation and subsequent root primordia development.

The temporal dynamics of auxin accumulation, peaking within hours of wounding and maintaining elevated levels through root emergence, correspond with the enhanced sprouting and rooting observed in our 7 DAG and 15 DAG treatments. The interaction between auxin and ethylene signaling pathways may also contribute to girdling effectiveness. Wounding during girdling and cutting preparation induces ethylene biosynthesis, which in combination with elevated auxin promotes programmed cell death in epidermal tissues overlying root primordia, facilitating root emergence (Druege et al., 2019) [2] . This synergistic hormonal interaction likely explains the earlier and more uniform sprouting observed in girdled treatments.

Varietal Differences in Girdling Response

The significant variation in girdling response among the seven tested varieties reflects inherent genetic differences in rooting competence and physiological plasticity. Variety V 1 consistently demonstrated superior performance across multiple parameters, including sprouting percentage, shoot height, and root biomass. This variety is recognized for its vigorous growth characteristics and adaptability to diverse agro-climatic conditions (Hariwal et al., 2024) [3] , traits that apparently extend to superior adventitious rooting capacity. Conversely, variety S 13 exhibited more modest responses to girdling, with lower overall sprouting percentages and reduced root biomass accumulation. Such varietal differences in rooting recalcitrance have been documented in other woody species and may relate to differences in endogenous auxin levels, rooting cofactor concentrations, or vascular tissue organization (Bharathi et al., 2024) [1] . The observation that S 13 control cuttings failed to root entirely while girdled treatments achieved limited success suggests that this variety may require additional hormonal supplementation or modified environmental conditions for optimal propagation. The differential performance between 7 DAG and 15 DAG cuttings indicates that the interval between girdling and cutting preparation significantly impacts propagation success. The 15 DAG treatments generally outperformed 7 DAG treatments, suggesting that sufficient time is required for adequate accumulation of carbohydrates and growth regulators. However, excessive delays may lead to tissue senescence or pathogen infection, highlighting the importance of optimal timing in commercial applications.

Implications for Sericulture Industry

These findings have direct practical implications for mulberry propagation in sericulture operations. The demonstration that girdling can enhance rooting in otherwise difficult-to-root varieties expands the range of genotypes available for commercial cultivation. For varieties like V 1 and S 36 , which showed strong girdling responses, this technique could reduce reliance on synthetic rooting hormones and improve propagation uniformity. The superior shoot growth observed in girdled cuttings, particularly for variety MR2, suggests that girdling affects not only root development but also subsequent shoot vigor through improved water and nutrient uptake capacity. This "carry-over" effect on plant establishment could significantly reduce the nursery phase duration and accelerate field planting schedules. However, the labor-intensive nature of manual girdling may limit large-scale application. Mechanization of girdling operations or development of chemical alternatives that mimic phloem disruption effects could enhance commercial viability (Liang et al., 2021) [7] . Additionally, integration of girdling with other propagation enhancements, such as controlled environment rooting systems or mycorrhizal inoculation, warrants investigation for maximizing propagation efficiency.

Conclusion

Stem girdling represents an effective technique for enhancing vegetative propagation in mulberry, with significant positive effects on sprouting percentage, shoot development, and root biomass accumulation. However, the magnitude response varies substantially among varieties, with V 1 , S 36 , and MR2 showing the strongest girdling benefits. The optimal interval between girdling and cutting preparation appears to be approximately 15 days, allowing sufficient time for assimilate accumulation while maintaining tissue viability. These findings support the integration of girdling into commercial mulberry propagation protocols, particularly for varieties with inherent rooting difficulties. Future research should focus on mechanization of girdling operations, optimization of timing for specific varieties, and elucidation of molecular mechanisms underlying differential genotypic responses. The development of improved propagation systems based on these findings will contribute to sustainable intensification of sericulture production through enhanced availability of superior planting material.

References

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