Prospect for Vegetative Bulbs Productions and Enlargement of Rose Onion (Allium cepa L.) Via Agro-Physiological Optimization in Tropical Climate

Published on: July 2025

Full paper is on Caraka Tani: Journal of Sustainable Agriculture DOI: http://dx.doi.org/10.20961/carakatani.v40i3.100515

Abstract

Rose onion (Allium cepa L.), a unique variety of red onion from Karnataka, India, is valued across Asia for its culinary significance. Recent onion supply dsiruptions following triggered an export ban from India, caused price surges and a decline in onion quality. This study evaluated two agro-physiological cultivation methods for rose onion under tropical Malaysian conditions to address Malaysia’s onion import dependency. The bulbification method aimed at optimising planting material production utilised open field conditions and resulted in enhanced pseudostem proliferation (p < 0.001) and increased bulb counts (p < 0.001). In contrast, the bulking method designed to maximise yield employed shaded environments and nutrient management achieving an 86% yield increase (10.78 t/ha) and higher economic returns (MYR 102,410/ha). Key physiological measurements, including chlorophyll content (+30.68%, p < 0.05) and water-use efficiency (+87.71%, p = 0.003) highlighted the adaptability of both methods to tropical climates. Economic analysis revealed that although pre-harvest costs were 65% higher for the bulking method, it generated MYR 63,715 in net profit representing a 450% increase in profitability and a Benefit-Cost Ratio (BCR) of 2.65. The findings support targeted cultivation strategies based on production objectives and emphasise the agronomic value of integrating physiological monitoring and cost-benefit analysis in tropical onion systems.

INTRODUCTION

The rose onion (Allium cepa L.) holds a position of considerable importance in Malaysia, being a favoured ingredient in many beloved national dishes, due to its distinctive flavour and aromatic qualities (Gayathri et al., 2016). However, Malaysia is significantly reliant on imported rose onion from Karnataka, India to satisfy the local demand. This import dependency is underscored by the Department of Statistics Malaysia's 2021 report, revealing that 622.2 thousand metric tonnes of red onion, shallot, and garlic, amounting to MYR 1,477.6 million, were imported, translating into an  import dependency ratio (IDR) of 100% for onion bulbs (DOSM, 2023). This heavy reliance increases national vulnerability especially during periods of supply disruption, such as India's export restrictions, which have historically led to price volatility and compromised onion quality within Malaysian markets (DOSM, 2023; Kai et al., 2019). Addressing this dependency is crucial not only for economic stability but also for ensuring food sovereignty and resilience in the face of climatic and geopolitical uncertainties.

In response to this, Malaysian authorities are actively promoting local onion cultivation to reduce import dependency and ensure a stable supply. In efforts to effectively support this national initiative, a thorough understanding of two vital agro-pysiological stages in the lifecycle of onions is essential, specifically the process of bulbification and bulking. Bulbification refers to the initiation of bulb formation, characterised by the morphological transformation of the swollen leaf bases into a nutrient-storing organ, while bulking refers to  the enlargement phase where carbohydrates and water accumulate in the developing bulb (Atif et al., 2020; Taylor et al., 2010). These stages are controlled by a complex network of environmental and genetic triggers, most notably photoperiodism which regulates expression of FLOWERING LOCUS T (AcFT) genes. AcFT1 promotes bulb initiation while AcFT4 suppresses it, ensuring proper developmental timing under varying light regimes (Lee et al., 2013).  Phytochromes and circadian clock genes perceive these light signals, coordinating hormonal cascades that reprogram the plant from leaf elongation to bulb swelling (Atif et al., 2020; Taylor et al., 2010). The photothermal requirements for these processes vary by cultivar but often intersect with key hormonal regulators such as gibberellins, abscisic acid, and cytokinins. These hormones modulate assimilate partitioning and cellular expansion, with cytokinin application showing promise in enhancing chlorophyll retention and stimulate vegetative vigor and bulb size (Acharya et al., 2020; Pagano et al., 2023) External factors such as light quality, temperature, and moisture availability determines not only the transition point but the rate and success of bulb formation. For instance, shading alters spectral ratios of red and far-red light and may either delay or promote bulb initiation depending on intensity and duration (Gao et al., 2021a). As the onions proceed to bulb development, the application of balanced fertilisers, particularly nitrogen, phosphorus, and potassium, supports the final growth stage, which demands a high nutrient input to maximize bulb size and quality. Generally, a temperature of 13-23°C is optimum for vegetative growth and 20-25°C for bulb development (Sharma & Chauhan, 2022). Therefore there is a need to optimise cultivation strategies that integrates physiological and environmental domains for a systems-level understanding of bulbification and bulking in onions especially in tropical settings like Malaysia.

To assess the suitability of Malaysia for onion cultivation, it is important to consider the climatic conditions in relation to the origin of the onion cultivars. The climate of Perak (4.4356° N, 100.8915° E) and Selangor (5.2101° N, 100.6124° E) state in Malaysia shares key similarities with Karnataka, India (15.3173° N, 75.7139° E), the origin of the imported rose onions. Both regions are located in the northern hemisphere, within comparable latitudes between the Equator and the Tropic of Cancer. Furthermore, while Karnataka experiences a subtropical climate with distinct seasons, and Selangor has a tropical rainforest climate with consistently high temperatures (25-32°C) and abundant rainfall. Crucially, the optimal temperature range for onion growth (20°C to 30°C) is largely consistent across both locations. This climatic compatibility suggests that cultivating Karnataka-origin rose onions in Selangor is indeed a viable prospect.

Despite the promising climatic alignment, current knowledge and established best practices for commercial onion cultivation within Malaysia’s specific environmental conditions remain limited. While major global onion producers such as China and India predominantly favour seed-based propagation methods for their extensive operations (Manna, 2016; Peiwen et al., 1994), this research explores the viability of vegetative propagation using sets (immature onion bulbs) as a potentially more suitable approach for initiating a local industry in Malaysia. Set-based propagation and offers advantages in terms of faster crop establishment, shorter production cycles, and higher initial yields (Mubarak, 2021; H. M. Singh & Singh, 2018) and may be particularly beneficial in the Malaysian context. Furthermore, with consumer-grade onion bulbs readily available, utilising sets presents a practical starting point for establishing a local onion industry. Although onion research in Malaysia was conducted in the early 1980s,  the results of the study found that the cultivation of onions in the country was uneconomical due to high planting costs, less profitable farm sales prices and high pest and disease attack (Rozita et al., 2024). However, due to the current issues with inconsistent supply, high import costs, unstable world onion market prices and increasing domestic demand involving national food security and safety issues, Malaysian authorities are actively encouraging domestic onion cultivation making this research timely and relevant to national agricultural goals.

Current literature provides scant empirical evidence addressing the agronomic and economic feasibility of cultivating rose onions under the tropical hot and humid conditions. This context specific knowledge gap is critical, as Malaysia's soil and climatic environments are substantially different from the temperate or arid regions where most onion research has traditionally been situated. Consequently, the absence of adaptive cultivation models tailored to Malaysia’s tropical lowland agroecosystems limits both policy implementation and farmer adoption of onion production strategies. Recognising the critical need for locally adapted cultivation techniques, this research is dedicated to demonstrate two distinct methods of cultivating rose onions: one aimed at maximising planting material production (high bulb number) and the other at optimising marketable yield (high bulb weight) under tropical conditions. The outcome of this study lays a preliminary foundation for methodological advancements of rose onion cultivation in tropical horticulture which will contribute significantly to the ongoing national endeavour to promote local onion production and substantially reduce Malaysia’s current dependence on imports which can also be adopted by similar developing nations.

MATERIAL AND METHODS

Description of the study sites

The study was carried out for two seasons in two different locations. The bulbification method was conducted from September to November 2023 at the Malaysian Agricultural Research and Development Institute (MARDI) in Parit, Perak, Malaysia (4.4348° N, 100.8911° E) on Telemong series soil. On the other hand, the bulking method was conducted from December 2023 to February 2024 at Field 15, Faculty of Agriculture, University Putra Malaysia in Serdang, Selangor (3.0077 °N, 101.7026 °E) on Serdang series soil. 

Table 1. Monthly climate data for Parit, Perak and Serdang, Selangor during the study period

Data was collected from nearby weather station located in study site.

Plant material and growth conditions

Store-bought rose onion bulbs originating from Karnataka India, and commonly available in Malaysian markets, were used as planting material for this study. Selected bulbs measuring 25–35 mm in diameter and weighing 15–25 g were prepped by removing one-third of the top. The bulbs were then placed in a tray covered with plastic film to maintain high humidity for approximately 48 hours to initiate rooting and sprouting.

In both methods, the prepped bulbs were planted directly into 20cm raised beds (bed area) in a completely randomised layout. Each bed contained 18 bulbs, spaced 15 cm apart. The planted bulbs were covered with a thin layer of soil and mulched with dried rice straws to prevent desiccation.  

Molluscicide (Siputox®–Agricultural Chemicals) was applied on the seedbed prior to planting to control snail infestation. Biological control against fungi was conducted biweekly using organic fungicides Phyzoctium F-10 and Alterdew F-11. Additionally, wood vinegar was applied to the plant foliage as an organic pesticide at a 5% (v/v) concentration with a biweekly application frequency. Harvesting occurred 60–70 days after planting (DAP), once the plant necks had completely collapsed, indicating maturity. BBCH scale was used to identify crop stages (Meier et al., 2009)

Description of bulbification method regimes

Controlled-release fertiliser (CRF), NPK 23:9:13 was applied three days before planting at 50 g per plot. Liquid fertiliser was applied as a foliar spray containing NPK 10:30:30 (Upsara® - Zeenex) at a concentration of 0.5% (v/v) at 35 DAP (approximately 40 ml per plant). The seedbeds were irrigated by flooding, as the research plot was situated in an existing paddy field that retained water. A summary of these methods is provided in Figure 2A.

Description of bulking method regimes

Store-bought ice was applied at 1 kg per bed daily for three consecutive days to enhance bulb formation, and shading was provided using a pongee cloth installed 2 m above the ground. Controlled-release fertiliser (CRF), NPK 23:9:13 was applied three days before planting at 50g per plot. Liquid fertiliser NPK 16:16:16 + TE (Biogreen, Yi Nong) was applied through foliar application at 0.1% (v/v) and were conducted weekly at 7, 14, 21, and 28 DAP, alongside compost tea prepared with fish emulsion, molasses, microbes, and compost. Liquid fertiliser NPK 10:30:30 (Upsara® - Zeenex) was sprayed at 40 DAP at 0.5% (v/v). A synthetic growth regulator derived from cytokinin namely forchlorfenuron (C₁₂H₁₀ClN₃O, from Dunedin) was applied at the rate of 0.1% (v/v) at 21 DAP and 0.05% (v/v) at 40 DAP to promote bulb formation. Plants were irrigated twice daily for 15 minutes using a drip irrigation system. A summary of these methods is provided in Figure 2B.

Measurements

The light readings at the planting site were recorded at 30 DAP using a spectrometer (LI-180; LI-COR Inc., USA) between 9.00 to 10.00 am. The spectrometer was positioned 110 cm above the ground, and three measurements were taken from three central points of the planting site.

Progressive measurements were taken every fortnightly. Plant height (cm) was measured from the ground level to the tip of the longest leaf using a measuring tape. The number of leaves and pseudostems were manually counted.

Leaf to pseudostem ratio was calculated by dividing the total number of leaves by the total number of pseudostems for each plant, using the formula:

The total chlorophyll content was measured on a healthy leaf using the Soil Plant Analysis Development (SPAD) chlorophyll meter (Chlorophyll meter SPAD-502 Plus, Konica Minolta Inc., Japan). Readings were taken on three locations along the midpoints of the leaf, and the average SPAD index was recorded. Measurements were taken every fortnightly until harvest.

Leaf gas exchange, including assimilation rate and stomatal conductance, was measured during the vegetative stage at 30 DAP using the LI-6800 photosynthesis system (LI-COR Inc., USA). Five plants were randomly selected, and measurements were taken at the midpoint of a fully expanded leaf. The leaf width was measured and input into the LI-6800 for accurate calculation. The light intensity was set at 1,500 μmol m^-2 s^-1, composed of 10% blue and 90% red light. Temperature and humidity were maintained at 27–30°C and 60%, respectively. The flow rate was set at 600 μmol s^-1, with a valve pressure of 0.1 kPa, fan speed set to 10,000 rpm, and CO₂ concentration in the leaf chamber was maintained at 400 ppm.

Water use efficiency was calculated as the ratio of assimilation rate to stomatal conductance, using the following formula:

Carboxylation efficiency was calculated as the ratio of assimilation rate to intercellular CO₂, using the following formula:

Yield data were collected at the time of harvest. The number of bulbs per clump were manually counted. The fresh clump weight (g) and fresh bulb weight (g) were measured using an electric weighing scale (FX-1200i, A&D, CO. Ltd., Japan). Bulb diameter (mm) was measured at the equatorial position using a vernier calliper. Dry clump weight (g) and dry bulb weight (g) were recorded after two weeks of curing, using an electronic weighing scale (FX-1200i, A&D, CO. Ltd., Japan).

Moisture loss percentage was calculated using the following formula.

Bulb grading was performed according to (Gayathri et al., 2016). The bulb number potential per hectare, yield potential per hectare, projected plantable area per hectare, and projected revenue per hectare (in Malaysian Ringgit) were estimated based on the bulb yield obtained per square metre. The selling price for rose onions in the bulbification method was set at a wholesale price of MYR 6.25 per kg, whereas the selling price for the bulking method was set at the market price of MYR 9.50 per kg as reported on the Federal Agricultural Marketing Authority (FAMA) website (FAMA, 2024).

Economic analysis

The economic analysis was conducted on December 4, 2024, using an exchange rate of 1 Malaysian Ringgit (MYR) = 0.23 United States Dollars (USD). Pre-harvest costs included land preparation, labour, bulb inputs, fertiliser, and pesticides. Land preparation including shading in the bulking method, and labour costs were based on (Mohamed Hafeifi et al., 2024). Bulb input costs were obtained from a wholesale distributor website (Like Best (M) Sdn Bhd, 2024). The harvesting cost was calculated based on a labour cost of RM90 per day for a total of 14 days, while the curing structure cost was set at RM1,000, as mentioned by (Mohamed Hafeifi et al., 2024). Yield potential for each method was measured in tons per hectare (t/ha), and total revenue was calculated based on yield estimates and current market onion prices. Total production costs were determined by summing pre-harvest and post-harvest costs, representing the total investment required. Profitability for each method was evaluated using three key metrics: Net Return, Net Return per MYR Investment, and the Benefit-Cost Ratio (BCR). Net return was calculated by subtracting total costs from total revenue, while net return per MYR investment assessed the economic efficiency of resource use. The BCR was applied to evaluate cost-effectiveness, with a ratio exceeding 2.0 indicating strong economic viability. To facilitate comparative analysis, percentage changes between the bulbification and bulking methods were calculated.

Statistical analysis

The Shapiro-Wilk test was conducted to evaluate data normality, and Levene’s test was used to assess the homogeneity of variances. For data that were normally distributed with equal variances, Student’s t-test was utilized to compare means. When the assumption of normality was violated, the Mann-Whitney U test was applied. For non-normal data with unequal variances, the Brunner-Munzel test was utilised, while Welch’s t-test was employed for normally distributed data with unequal variances. Statistical analyses, calculations, and graph generation were performed using Microsoft Excel 365 (Microsoft Corporation, Redmond, WA, USA) and GraphPad Prism 9.2.0 (GraphPad Software, USA). Diagrams were created using Microsoft PowerPoint 365 (Microsoft Corporation, USA).

RESULTS AND DISCUSSION

Environmental Compatibility

This study demonstrates the successful cultivation of the rose onion cultivar in the tropical climate of Perak (4.4356° N, 100.8915° E) and Selangor (5.2101° N, 100.6124° E) state in Malaysia, using imported bulbs originating from the state of Karnataka, India (15.3173° N, 75.7139° E) (Fig. 1A and 1B). Both regions are located within the northern hemisphere, between the Equator (0°) and the Tropic of Cancer (23.43602° N). Perak and Selangor exhibit a tropical rainforest climate characterised by consistently high temperatures (25°C to 32°C) and rainfall influenced by maritime factors. Karnataka, in contrast, experiences a subtropical climate with distinct seasons, including warm, dry summers (15°C to 35°C), mild winters, and a monsoon season from June to September. Nevertheless, as the optimal growth temperature for onions typically falls within the range of 20°C to 30°C (Immanuelraj et al., 2014), the climatic compatibility in Perak and Selangor aligns with the bulbs of Karnataka origin thus providing suitable growth conditions for this particular onion cultivar.

Figure 1. A. Map of Southeast Asia highlighting Selangor state, Malaysia and Karnataka state, India. Karnataka, India is marked with a red dashed circle, while Perak and Selangor, Malaysia is highlighted with a blue dashed circle. The dashed black line indicates the Equator, while the solid black line represents the Tropic of Cancer. The geographical proximity and similar latitudinal positions of Karnataka and Selangor, within the Southeast Asia region, suggest that these two places share similarities in climate patterns. B. The timeline and the location of the onion cultivation for the two seasons. Note. From Map of Asia, In Wikipedia, 2007, (https://en.m.wikipedia.org/wiki/File:Map_of_Asia.svg). CC-BY 4.0.

Malaysia's favourable growth temperatures position it as a promising location for cultivating rose onions. During the first season, the bulbs were planted in a deeply moist soil bed in MARDI Parit, Perak, with the expectation that the combination of aerobic and anaerobic conditions would enhance nutrient absorption and promote larger bulb formation. However, instead of achieving the desired marketable size, the cultivation method resulted in a high number of smaller bulbs, leading to the designation of this method as the Bulbification method. To mitigate the issue of excessive pseudostem formation and its associated nutrient competition, a subsequent cultivation trial in Selangor incorporated the application of the plant growth regulator forchlorfenuron (cytokinin) to regulate pseudostem development. The rationale behind this approach was to optimise nutrient allocation by reducing the number of pseudostems, thereby facilitating the growth of larger individual bulbs (Bennett et al., 2012). This bulking cultivation method was implemented by integrating shading, ice application, and frequent foliar and compost tea treatments to encourage bulb enlargement. The key methodological aspects of this approach are summarised in Figure 2.

The bulbification method, conducted under full sunlight, had a high relative humidity (RH) of 89.7% and an average temperature of 27.9°C which were due to the standing water body in the paddy field.  PPFD measurements were 1288 µmol m-2 s-1 and solar radiation was 1042 (mW/m2)​. In contrast, the bulking method, performed under pongee cloth shade, resulted in a lower RH of 66.2% and a higher temperature of 32°C, with a 30% reduction in light intensity. Despite that, the light was more pronounced in the bulking method, with PPFD measurements of 352.4 µmol m^-2 s^-1 and solar radiation of 412.5 (mW/m2), particularly in the far-red and UV spectra, which decreased by 45% and 47%, respectively which had been shown to mitigate heat stress, enhance light interception, and improve carbon utilisation, contributing to better crop performance (Gunadi & Sulastrini, 2013; Hadid & Febriana, 2022).

Figure 2. Cultivation practices and key differences between the Bulbification and Bulking methods. A. The bulbification method focusing on producing high number of bulbs, along with its methods and associated environmental conditions. B. The bulking method focusing on producing high bulb weight, along with its methods and corresponding environmental conditions.

Vegetative growth

Plants in the bulbification method exhibited yellowing on the leaf tip compared to those in the bulking method, which retained a green, healthy appearance at 45 DAP (Figures 3A and 3B). This pattern was consistent with differences in plant height (Figure 3C). At 45 DAP, plants in the bulbification method showed a decline in height, while plants in the bulking method exhibited steady growth, reaching an average height of 36 cm, a 93% increase compared to the bulbification method (p < 0.001).

Pseudostem (or neck) number per plant (Figure 3D), a key determinant of the final bulb count at harvest, was consistently higher in the bulbification method compared to the bulking method, with significant differences observed at all time points (p < 0.001). At 30 DAP, the pseudostem number in the bulbification method was 44% higher than in the bulking method (Figure 3D, Welch’s t-test, p < 0.001) which showed the potential of this method to mass-produce bulbs to become planting materials. Similar pseudostem proliferation under stress-prone or resource-limited conditions has been reported in onions, where vegetative propagation tends to be prioritised over sink development (Khokhar, 2017). High pseudostem counts are associated with early bulb initiation and accelerated physiological aging which often reduce biomass accumulation per bulb (Brewster & Salter, 1980) In contrast, the resource optimised environment in the bulking method tend to promote fewer but more photosynthetically efficient shoot axes, sustaining leaf growth longer into the bulbing phase (Dutta et al., 2024).

Leaf number per plant (Figure 3E) was significantly higher in the bulbification method at 15 and 30 DAP (p = 0.006 and p < 0.001, respectively). However, leaf number in the bulbification method declined significantly at 45 DAP, whereas plants in the bulking method maintained steady growth in leaf number.

The leaf-to-pseudostem ratio (Figure 3F), defined as the number of leaves divided by pseudostem number, suggests that an optimal leaf number per pseudostem is necessary for efficient assimilate translocation from leaves to bulbs. The leaf-to-psuedostem ratio (Figure 3F) displayed a progressive and notable increase under the bulking method as the plants matured, consistent with the effects of cytokinin application, which was known to promote leaf appearance, leaf expansion, and overall growth of plants (di Benedetto et al., 2020). This reflects the plant's capacity to meet the assimilated demands of bulb enlargement occurring subterraneously. From an agronomic standpoint, the leaf-to-pseudostem ratio may serve as a phenotypic marker of productivity. The higher ratio recorded in the bulking treatment indicates more efficient assimilate translocation and greater yield potential. This observation aligns with previous research in cereals and tuber crops, where similar morphological ratios were linked to the harvest index and sink strength (Lopes & Reynolds, 2012; Zhang et al., 2014).

The vegetative advantage observed in the bulking method can be attributed to the integrated use of multiple physiological enhancers. Weekly foliar applications of a balanced macro and micronutrient blend (NPK 16:16:16 + TE) improved nutrient uptake efficiency by delivering elements directly to metabolically active leaf surfaces. Foliar feeding has been shown to promote rapid nutrient assimilation, particularly under intensive systems or where soil uptake may be constrained (Ishfaq et al., 2022). In parallel, the application of compost tea prepared from organic fish emulsion, molasses, microbial inoculants and compost further supported plant vigor by improving leaf nutrient status and stimulating beneficial microbial colonization as reported in (Garg & Rakshit, 2024). Shading installation using pongee cloth modulated canopy microclimate, reducing light and temperature stress and maintaining higher relative humidity around the foliage. A study by (Gao et al., 2021b) reported that moderate shading with blue and white photo-selective nets can optimise light quality, improve physiological performance, enhance antioxidant defense, and increase yield and quality in green onion cultivation under high light and temperature conditions. The application of forchlorfenuron (CPPU), a synthetic cytokinin, likely contributed to cell expansion, delayed senescence, and enhanced vegetative mass through increased mitotic activity (Farhadi & Alizadeh Salteh, 2017).

Figure 3. Visual Appearance at 45 DAP and Growth Parameters of Rose Onion in Bulbification and Bulking Methods. A, B. Images of rose onion plants at 45 DAP: A. Bulbification method. B. Bulking method. White scale bar = 2 cm. C. Progressive growth of plant height at 15 DAP, 30 DAP, and 45 DAP. D. Progressive growth of pseudostem number at 15 DAP, 30 DAP, and 45 DAP. E. Progressive growth of leaf number at 15 DAP, 30 DAP, and 45 DAP. F. Leaf/pseudostem ratio at 15 DAP, 30 DAP, and 45 DAP. Statistical tests: C. Brunner-Munzel test for 15 DAP, Welch's t-test for 30 DAP, and Student’s t-test for 45 DAP, p-values as shown. D. Welch's t-test for 15, 30, and 45 DAP, p-values as shown. E. Mann-Whitney U test for 15 DAP, Brunner-Munzel test for 30 DAP, and Student’s t-test for 45 DAP, p-values as shown. F. Student’s t-test for 15 DAP, Mann-Whitney U test for 30 DAP, and Welch's t-test for 45 DAP, p-values as shown. Sample size: Bulbification n= 48 and Bulking n= 36

Physiological Responses

The physiological responses of rose onion plants varied significantly under different cultivation methods, particularly in chlorophyll content, stomatal conductance, and water-use efficiency (WUE) (Figure 4). Total chlorophyll content (Figure 4A) significantly increased by 30.68% in the bulking method at 51.06 compared to the bulbification method at 61.7 (Student’s t-test, p < 0.05) indicating enhanced capacity for light capture and energy conversion, which are critical for photosynthetic productivity. This increase aligns with plant adaptations to lower light conditions where pigment concentration rises to maximise light harvesting (Simkin et al., 2022). The shading used in the bulking method likely moderated light intensity and reduced photoinhibition, consistent with reports demonstrating that controlled shading improves chlorophyll retention and photosynthetic efficiency in onions and other crops (Gao et al., 2021a). However, excessive shading can suppress photosynthesis, emphasising the need to optimize light levels (Dolatkhahi et al., 2013; Mauro et al., 2011).

Despite a non-significant trend toward higher carbon assimilation in the bulbification method (Figure 4B, Student’s t-test, p = 0.752), stomatal conductance was significantly greater by 48.64% compared to the bulking method (Figure 4C, Welch’s t-test, p = 0.007). Elevated stomatal conductance under the bulbification method (0.50 mol m-² s^-1) reflects increased stomatal opening typical of plants in high-light environments (Shen et al., 2021). However, this increase did not translate into improved carbon assimilation or carboxylation efficiency (Figure 4E, Student’s t-test, p=0.934) suggesting stomatal regulation inefficiencies or physiological stress potentially caused by nutrient limitations or microclimatic factors (Kumari et al., 2022) resulting in reduced resource-use efficiency. Wang et al., (2021)noted that optimal nitrogen availability increased not just stomatal conductance but also the entire biochemical pathway for photosynthesis, aligning with the idea that environmental and nutrient conditions critically influence stomatal function and photosynthesis.

Water-use efficiency (WUE) was markedly higher (87.71%) in the bulking method (Figure 4D, Mann-Whitney U test, p = 0.003) indicating more effective carbon fixation per unit water loss. This improvement likely results from better stomatal control and reduced transpiration, consistent with research showing shading mitigates photoinhibition and improves water conservation in water-stressed plants (Montanaro et al., 2009; Shen et al., 2021). Carboxylation efficiency remained stable across both methods (Figure 4E, Student’s t-test, p=0.934), indicating that biochemical CO₂ fixation capacity via Rubisco was not a limiting factor (Cocon & Luis, 2024). Thus, differences in photosynthetic performance mainly arise from stomatal and environmental regulation rather than enzymatic activity. Although leaf temperature remained stable across both methods in this study (Figure 4F, Mann Whitney U test, p = 0.467), earlier research indicates that shading can reduce leaf temperatures in heat-sensitive crops but may have minimal effects on heat-tolerant species (Alves et al., 2022; Mensah et al., 2022) 

The use of foliar biostimulants such as compost tea and cytokinin enhanced chlorophyll content, stomatal conductance, and WUE. Previous studies have demonstrated that cytokinins increase chlorophyll accumulation and stomatal activity in wheat and maize (Islam et al., 2021; Lazova & Yonova, 2010), while compost tea improves plant health and photosynthetic efficiency (Pane et al., 2014). These results suggest that combining shading with foliar biostimulants may be an effective strategy for optimising rose onion cultivation. The findings suggest that the bulking method enhances chlorophyll retention, improves WUE and maintains photosynthetic efficiency making it a potentially advantageous for cultivation in tropical climates. However, the greater stomatal conductance observed in the bulbification method indicates potential benefits for rapid gas exchange and CO₂ uptake in high-light environments but at the cost of WUE efficiency. These results highlight the value of incorporating physiological trait monitoring into agronomic management to optimise tropical onion production.

Figure 4. Physiological Parameters of Rose Onion at 36 DAP: Comparison Between Bulbification and Bulking Methods. A. Total chlorophyll content at 45 DAP. B. Assimilation rate. C. Stomatal conductance. D. Water-use efficiency. E. Carboxylation efficiency. F. Leaf temperature. Statistical tests: A, B, E. Student’s t-test, p-values as shown; C. Welch's t-test, p-values as shown; D, F. Mann-Whitney U test, p-values as shown. Sample sizes: Bulbification (n = 24) and Bulking (n = 10), except for A. Total chlorophyll content, where Bulbification (n = 48) and Bulking (n = 36).

Bulbs Productivity

The results of this study reveal a significant trade-off between bulb number and individual bulb size, influenced by the chosen cultivation method (Figure 5). The bulbification method produced 33.7% more bulbs per plant (7.5 bulbs/set), supporting it’s suitability for generating planting material (Figure 5A, Welch’s t-test, p < 0.001). In contrast, the bulking method yielded larger bulbs (7g/bulb), with a 55.4% greater equatorial diameter of 20mm (Figure 5B, p < 0.001), a 223.8% increase in fresh bulb weight (Figure 5E, p < 0.01) and a 192.1% increase in dry bulb weight (Figure 5F, p < 0.001). These findings align with the well-documented inverse relationship in bulbous crops, where increased bulb number is typically associated with reduced individual size (Deng et al., 2020). This trade-off is attributed to constraints in assimilate partitioning. Simultaneous bulb formation in the bulbification method appears to have limited the carbon allocation to each sink, thereby restricting individual biomass accumulation (Lambers & Oliveira, 2019). In contrast, the bulking method seems to have promoted assimilate consolidation into fewer sinks, enabling greater bulb enlargement. Comparable results in potato and onioncitrus suggest that reduced sink competition, coupled with sustained photosynthetic activity favors increased bulb mass (Paul & Foyer, 2001).

Environmental and nutritional management contributed to the enhanced bulb biomass under the bulking method. Shading and regular foliar applications of compost tea and cytokinin were central to this method. Shading is known to delay senescence therefore increasing assimilate availability (Xie et al., 2023) These benefits, however, may not be universal. (Mettananda & and Fordham, 1999) found that shading reduced bulb size and yield in certain onion cultivars, highlighting genotype-specific responses. Similarly, (Kurosaki & and Yumoto, 2003) observed yield declines in soybean under shading, contrasting with the benefits seen in tropical crops like rose onion, which may tolerate moderate shade more favourably (Bhatt et al., 2002). Cytokinin application is thought to delay senescence, enhance sink strength, and promote cell expansion (Rademacher, 2015). Yet, excessive cytokinin use can disrupt hormonal balance and reduce stress resilience, as reported by (Wang et al., 2015) in Arabidopsis. The addition of compost tea also contributed to the improved bulb biomass in the bulking method, consistent with reports linking it to enhanced soil fertility and productivity (Hatungimana et al., 2024). Nonetheless, (Scheuerell & Mahaffee, 2006) cautioned that compost tea efficacy depends on microbial composition and environmental conditions, suggesting variability across systems. The increased biomass accumulation in the bulking method further supports research indicating that CO₂ assimilation and stomatal conductance influence resource allocation to bulbs (Merkebu, 2021). 

The marketability of onion bulbs depends on size distribution, post-harvest stability, and consumer preferences. The distribution of bulb sizes between the two methods is shown in Figure 6. The bulbification method produced bulbs exclusively in the < 25 mm size category (100%), making them suitable as planting material rather than direct sale. In contrast, the bulking method produced a wider range of bulb sizes, with 2.41% exceeding 30 mm in diameter, making it more suitable for commercial markets aligning with market preferences for larger, uniform bulbs which are valued for their culinary applications and fructooligosaccharide content (Azeem et al., 2024). However, contrasting evidence shows that under certain market contexts, higher bulb counts even at the expense of size can be economically viable. Smaller sized onions are also reported to contain higher phenolic compounds and simple sugars, which may provide nutritional advantages in niche markets (Major et al., 2023) Increasing local production could reduce import dependency, stabilise prices and offer fresher, nutritionally superior alternatives to imported onions (Berni et al., 2019; Khan et al., 2022).   

Figure 5. Yield Analysis of Rose Onion at Harvest: Comparison Between Bulbification and Bulking Methods. A. Bulb number per plant. B. Bulb equatorial diameter. C. Fresh clump weight. D. Dry clump weight. E. Fresh bulb weight. F. Dry bulb weight. Statistical tests: A. Welch's t-test, p-values as shown; B. Student’s t-test, p-values as shown; C, D, E, F. Brunner-Munzel test, p-values as shown. Sample sizes: Bulbification (n = 48) and Bulking (n = 17), except for B. Bulb equatorial diameter, where Bulbification (n = 15) and Bulking (n = 17).

 Figure 6. Bulb grading comparison between Bulbification and Bulking methods

Projection and feasibility

The economic viability of onion production depends on multiple interacting factors, including yield potential, input costs, and market pricing dynamics. Statistical analysis revealed significant differences between the bulbification and bulking methods across key parameters, namely the bulb number potential per hectare, yield potential per hectare, projected plantable area and projected revenue per hectare (Figure 7). The bulbification method increased bulb numbers per hectare by 33% (Figure 7A, p < 0.001), while the bulking method produced a 68.3% higher yield per hectare (Figure 7B, p = 0.024), leading to a 180.6% rise in projected revenue (Figure 7D, p < 0.001). The higher bulb number potential in the bulbification method also translated into a significantly larger projected plantable area, covering 33% more planting area compared to the bulking method (Figure 7C, p = 0.0007). This significant improvement highlights the economic benefits of producing larger, marketable bulbs under the bulking method, which contributes to greater revenue generation per unit area. In contrast, while the bulbification method produced a higher bulb number and a larger projected plantable area, its revenue potential was constrained by the dominance of smaller bulbs, which are typically suited for use as planting material rather than for direct market sale.

The economic advantage of the bulking method was primarily due to its higher bulb weight and marketability, resulting in a net return of MYR 63,715/ha (Table 1), a 450% increase over the bulbification method. Larger bulbs typically command higher prices in wholesale and retail markets (Hussaini & Amans, 2000), reinforcing the importance of producing market-preferred sizes. However, the bulbification method remains valuable for seed stock production, as it produced 33% more plantable bulbs per hectare, supporting a larger cultivation area in subsequent cycles. This aligns with studies suggesting that bulb seed-based onion farming can be economically sustainable, particularly for propagation-focused producers (Astuti et al., 2023). 

The economic analysis of the bulbification and bulking methods shows significant disparities in production costs, yield output, and overall profitability (Table 2). Despite its higher profitability, the bulking method required significantly greater input costs, with land preparation costs amounting to MYR 13,000/ha due to the addition of shading installation and fertiliser expenses of MYR 9,865/ha increasing by 160% and 157%, respectively. These factors contributed to a 65% rise in total pre-harvest costs, making cost management critical. Although compost tea was used to enhance productivity, the reliance on purchased inputs raised costs. As studies suggest, integrating 50:50 organic and inorganic fertiliser regimes or producing compost tea from farm wastes could reduce input costs while maintaining yield (Singh & Ram, 2014) enhancing the long-term feasibility of the bulking approach.

These results indicate that the additional expenditures on inputs such as fertilisers and land preparation are economically justified by the substantial increase in profitability. The net return per MYR investment under the bulking method also rose to 1.65, reflecting a 250% improvement. Furthermore, the bulking method’s economic feasibility was further supported by its higher benefit-cost ratio (BCR) of 2.65, indicating that its financial returns significantly exceeded production expenses. This is consistent with studies showing that high-input farming systems can yield high returns, with large farms achieving cost-benefit ratios of up to 1:2.22 (Piyush et al., 2024) 

The projected revenue in both cultivation methods, MYR 36,250 per hectare for the bulbification method and MYR 102,400 per hectare for the bulking method demonstrates the lucrative income for farmers wanting to venture into rose onion farming in Malaysia. As Malaysia remains a major importer of the rose onion, there is already an established market for the produce. The bulbification method incurs significantly lower pre-harvest costs of MYR 21,684/ha compared to the bulking method at MYR 35,717/ha, making it more accessible for smallholder farmers with limited financial resources. Its higher bulb count supports propagation and decentralized seed system development, contributing to long-term supply chain sustainability (Astuti et al., 2023). On the other hand, although the bulking method has higher pre-harvest costs, its efficient net return per investment ratio (1.65) delivers significantly higher financial returns, making it ideal for-profit focused farmers.

Successful commercialisation of onion cultivation depends on the broader supply chain infrastructure. Onion production in the tropical regions frequently suffer from inadequate storage, fragmented marketing networks and price volatility linked to import dependency (Khan et al., 2022). By expanding local production, particularly through high yield, high value systems using the bulking method, domestic markets can reduce dependency on imports, stabilise prices and offer consumers fresher, more nutrient dense produce (Berni et al., 2019; Khan et al., 2022). Locally grown onions avoid long distance transport therefore preserving flavor and bioactive compound integrity. This transition supports national food security and resilience. Global disruptions due to trade restrictions, climate shocks, or geopolitical tensions can threaten onion imports (Kuma & Alemu, 2015). Domestic cultivation provides a buffer, reducing vulnerability and enhancing local self-sufficiency (Yeshiwas et al., 2023). However, to realize these benefits, enabling policies and infrastructure such as cold storage, market information systems and cooperative aggregation is essential. Successful value chain participation by smallholders also depends on access to affordable inputs, storage facilities, reliable market information and training. Strengthening linkages between producers, aggregators, and retailers potentially through cooperative models or digital platforms will be essential to fully realise the economic potential of improved cultivation methods (Nourbakhsh & Cramer, 2022)

Figure 7. Economic Parameters of Rose Onion at Harvest: Comparison Between Bulbification and Bulking Methods. A. Bulb number potential per hectare. B. Yield potential per hectare. C. Projected plantable area. D. Projected revenue per hectare. Statistical tests: A. Welch’s t-test, p-values as shown; B, C, D. Brunner-Munzel test, p-values as shown (ns p > 0.05, p < 0.05, *p < 0.01, ***p < 0.001). Sample sizes: Bulbification (n = 48), Bulking (n = 17).

Table 2. Economic comparison of Bulbification and Bulking methods: Production costs, yield, revenue, and profitability

_{Note: The wholesale price for rose onion in the bulbification method was set at MYR 6250/t, while the market price in the bulking method was set at MYR 9500/t, based on FAMA prices as of December 5, 2024.}

This study provides valuable preliminary insights into rose onion cultivation under tropical Malaysian conditions. However, several methodological limitations warrant consideration. The study was conducted over a limited number of growing seasons and locations, potentially restricting the generalisability of the results across Malaysia’s diverse agroecological zones. Although physiological and yield parameters were assessed, molecular analyses of gene expression associated with bulbification and bulking were not performed, which could provide mechanistic understanding of the observed differences. Future research incorporating multi-location trials, comprehensive environmental monitoring, and molecular approaches would enhance the robustness and applicability of these findings.

CONCLUSION

This study establishes the first successful method for cultivating rose onion under tropical conditions using bulb-based propagation. It allows growers to choose between bulbification for propagation (7.54 bulbs/set, 3 g) or bulking for market production (5 bulbs/set, 7g), based on production goals. By tailoring agronomic practices such as shading and foliar fertilisation to local conditions, distinct outcomes were achieved. The findings support sustainable horticulture by enhancing local production, reducing import reliance, and improving food security. Future research should focus on varietal adaptability, resource efficiency, and economic viability to support wider adoption and align with climate resilience objectives.

Prospect for Vegetative Bulbs Productions and Enlargement of Rose Onion (Allium cepa L.) Via Agro-Physiological Optimization in Tropical Climate

Mahaletchumy Krishnamoorthy, Afiq Kamaruzali, Liang Su Toh, Nurul Aaliyah Mohd Sahud, Khairunnisa Najwa Khairulazlan, Mashitah Jusoh, Azzami Adam Muhamad Mujab, Khalisanni Khalid, Andrew Fleming, Nazmin Yaapar.

Caraka Tani: Journal of Sustainable Agriculture, 40(3), 404-424, 2025

DOI: http://dx.doi.org/10.20961/carakatani.v40i3.100515

Keywords: bulbification; bulking; forchlorfenuron; red onion; yield improvement.

Published on: July 2025

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License