Onion (Allium cepa) and garlic (Allium sativum) are important vegetables, condiment and spice crops. The onion and garlic stand as one of the most ancient vegetables, and their presence is documented in numerous ancient scriptures (Basak, 1987; Kahn, 1996; Khandagale and Gawande 2019). By the time of the Middle Ages, it had cemented its place as a culinary staple in many cuisines worldwide, ensuring a year-round demand. Remarkably, the onion ranks as the third most crucial horticultural crop, trailing only behind the potato and tomato, and is cultivated commercially in over 170 countries across the globe (Teshika et al., 2018). The global onion production was 107 million tonnes in 2021 from 5.78 million hectares area. India is a leader in onion production, with 31.6 million tonnes harvested from an area of 19.41 hundred thousand hectares (Kale et al., 2024). Garlic, a ubiquitous Allium species with a long culinary history, is a globally significant crop according to the Food and Agriculture Organisation (FAOSTAT, 2021). Global production reached 28 million metric tonnes from 16.36 million hectares, with China dominating the landscape. China cultivated over half the global garlic area (8.3 million hectares), producing a staggering 207.57 million tonnes. Despite China’s dominance, other players like India (29.17 million tonnes) contribute a significant share of global production.

In plants, physiological disorders refer to abnormalities or irregularities in the normal physiological processes that can affect their growth, development, and overall health. These disorders are often non-infectious and result from internal factors, environmental conditions, or a combination of both. Unlike diseases caused by pathogens, physiological disorders do not involve infectious agents such as bacteria, viruses, or fungi. Identifying the specific cause of a physiological disorder in plants is crucial for implementing appropriate corrective measures. Physiological disorders in onions and garlic can arise from various factors, including environmental conditions, cultural practices, and genetic factors. Here are some common physiological disorders that may affect these Allium crops: premature bolting, doubles, greening, sprouting, watery scale, thick neck, rubberization, premature sprouting, etc. ( Figure 1 ). However, there is no single study that reviewed the progress of research in physiological disorders in onion and garlic.

Physiological disorders constitute a significant, yet often overlooked, economic threat to onion and garlic production. Studies report yield losses attributable to physiological disorders ranging from 10% to 50% in onion. The economic woes associated with physiological disorders extend beyond the field, as these disorders can increase susceptibility to spoilage during storage, leading to additional post-harvest losses. In onion, the losses due to physiological disorders vary along with the season and the varietal response/traits. In India onions are grown in three seasons viz; Kharif: planting in July-August, harvested in October-December. 2. Late Kharif: planted in October-November, harvested in January-February, Rabi: planted in December-January, harvested in March-May. In the Kharif season, the highest yield losses contributed by physiological disorders are due to post-harvest sprouting (54%) and double bulbs (up to 17%) (Annual report, ICAR-DOGR 2004; Annual report, ICAR-DOGR 2022). Whereas, in the late-Kharif season, the bolting (up to 23%) and double bulb (up to 25%) contribute more in losses (Annual report, ICAR-DOGR 2022). Comparatively, the post-monsoon (Rabi) onion production witnessed lesser yield losses due to physiological disorders.

Bibliometrics is a quantitative method used to analyze various aspects of scientific or academic literature (Ellegaard and Wallin, 2015; Wei et al., 2022, 2023). It involves the statistical analysis of publications, citations, and other bibliographic data to understand patterns, trends, and relationships within a particular field or body of literature. Further, this analysis is visualised using different graphical tools. In the present review, we conducted a bibliometric analysis of literature from 1946 to April 2024 on physiological disorders in onion and garlic retrieved from the SCOPUS database.

Scopus is one of the most comprehensive scientific literature databases for bibliometric analysis (Baas et al., 2020). In the present analysis, we retrieved the relevant literature related to physiological disorders in onion and garlic from the Scopus database. The review of the literature was done using different keywords like ‘onion sprouting,’ ‘premature bolting,’ ‘watery scales,’ ‘premature sprouting,’ etc., using Boolean operators such as ‘AND’ and ‘OR.’ The searched terms should be present in the following fields: title, abstract, and keywords. A total of 308 documents published between 1946 to 2024 were retrieved. The data was exported from Scopus in CSV format for further bibliometric analysis. Then, these articles are analysed using Bibliometrix-Biblioshiny (Aria and Cuccurullo, 2017) and the VOSviewer version 1.6.20 (Van Eck and Waltman, 2010). These tools are widely used in bibliometric analysis and visualization due to their ease of use and clarity in representation. The detailed search strategy and data filtering process are shown in Figure 2 .

The outcome bibliometric analysis is summarised in Supplementary File 1 . The annual scientific production associated with physiological disorders in onion and garlic showed a steady increase over the last three decades, and most articles were published in the year 2016. However, the small number of articles published per year suggests the limited importance given by researchers to this area. India, the UK, and the USA are the top three countries with scientific production, but in the list of most cited countries, the UK is on the top, followed by the USA, Pakistan, and India, suggesting the importance of physiological disorders of onion and garlic in these countries. Though, China is one of the major producer of onion garlic, research output on physiological disorders is found limited. It might be due to that we restricted literature retrival to the English language only and some of the publications may not be present in the Socpus database. These major physiological disorders are largely influenced by environmental interactions. In South Asian countries, particularly India, multiseasonal onion cultivation contributes to a higher incidence of these disorders compared to China and other temperate regions, where onion cultivation is limited to a single season with long-day onion varieties. This may explain the increased research focus on these disorders in short-day onion regions.

We further analysed the data for corresponding author countries and multiple and single-country publications. Both multiple-country publications (MCP) and single-country publications (SCP) are the highest in India. Whereas the USA, Pakistan, and Korea have almost similar MCPs. MCP tends to have an impact in terms of citations and collaborations, diverse expertise compared to SCP. Based on the data retrieved from Scopus, the most relevant source of scientific literature related to physiological disorders is found to be a journal named ‘Acta Horticulturae’ with the highest number of publications. This analysis showed that Cranfield University is the most relevant affiliation from which the highest number of research on physiological disorders of onion and garlic has been published. A word cloud generated from Biblioshiny provides an intuitive approach to the visualization of key themes in scientific literature to get insights into the research landscape of the field of interest. The size of a word is correlated to its frequency of occurrence. Figure 3 depicts different bibliometric analyses.

Bolting is premature flowering in the onion crop cultivated for bulb production. Premature flowering, i.e., seed stalks/scapes, are developed before the onion bulbs complete their normal life cycle ( Figure 5 ). This transition in the plant life cycle is controlled by several endogenous and environmental signals such as temperature, photoperiod, nutrient management, varietal characteristics etc. It is an undesirable character as the further development of onion bulbs stops, and bulbs of such plants are light in weight, fibrous, and have a lower shelf life. It adversely affects the yield, quality, and, ultimately, income of the onion grower. The problem of premature floral stalk emergence poses a significant problem impacting the production and quality of onions. Estimates suggest potential damage of up to 30%, which varies with the climatic conditions and the planting date (Macías and y Grijalva, 2005). Due to the inferior quality of bolted bulbs, their storage life is shorter. In India, 5-80% bolting is reported in early Rabi and late Kharif seasons, which leads to an annual loss of approximately 11 to 50 hundred thousand tonnes of onion bulbs (Gorrepati et al., 2017).

It is well known that bolting negatively affects the yield and market value of onions. Researchers have studied these effects of bolting on onion in terms of TSS, sugar content, weight, pungency, phenolics content, storage life, etc. Kwon et al. (2016) studied the impact of premature bolting and its removal at various lengths on quality and growth. They observed that there was a reduction in weight without affecting the diameter of the bulb. Further, there was no change in the TSS, total sugar, pungency, and phenolic content. The author suggested that as biochemical properties are similar in both bolted and normal bulbs, they can be used in processing to prepare dehydrated powder, flakes, rings, etc. In contrast, Gorrepati et al. (2020) observed a significant difference in the TSS of bolted and non-bolted onions of four different varieties. However, the moisture and flavonoid levels did not vary significantly.

Due to environmental factors, bulbil formation in flowers instead of seed development in onions is observed in certain genotypes. Though it is a natural phenomenon in Allium x proliferum (also called as Egyptian onion, tree onion, walking onions), these bulbils can be utilized for the propagation of vegetatively proppagted onion (Kamenetsky and Rabinowitch, 2006). However, it is considered undesirable in onion seed production plots as it reduces the yield of the seeds, which have to compete with these bulblets or topsets. We observed this anomaly in a few onion flowers in seed production plots where bulbils are observed instead of seed development ( Figure 7 ). Hasanov and Akparov (2018) observed that hot weather conditions resulted in this disorder in few flowers. Propagation with these bulbils or topsets has some disadvantages, such as lower yield and difficulty in early establishment crops, i.e., more mortality.

Sprouting can alter the flavour and texture of the onion, making it less desirable for the market. In storage, significant loss occurs due to the dormancy break and eventual sprouting of onions (Adamicki, 2005). Onions are harvested after the completion of the bulbing phase, i.e., after neck fall and senescence of foliage. At this stage, bulbs are in endo-dormancy. After some time in storage, the bulbs start to break dormancy, and the emergence of sprouts can be observed ( Figure 8 ). This dormancy period varies with genotype. This dormancy period can be extended by cold temperature storage and the use of growth regulators (Forney et al., 2022). Several factors are involved in the sprouting of onions such as genotype, soil moisture at the time of harvest, fertilizer management, physiological maturity at harvest, curing, etc. (Miedema, 1994; Sorensen and Grevsen, 2001; 2004; Gorrepati et al., 2017). Geisseler et al. (2022) reviewed the role and requirement of nitrogen fertilizer in onion crop development and concluded that excess N fertilizer led to higher sprouting and bulb rotting.

The exact period when dormancy breaks in onion is still not completely understood. During dormancy break or sprouting, several biochemical and molecular changes occur in the bulb, such as growth regulator balance, modification of cell wall, carbohydrate mobilization, fructan redistribution, etc. Ohanenye et al. (2019) reported that fructan redistribution could be considered a marker for predicting onion bulb dormancy break. Recently few studies investigated the sprouting in onion at transcriptome and transcription factor level. Another study by Kleman et al. (2024) revealed that regardless of maleic hydrazide use the levels of fructose and glucose were linked to the degree of sprouting, with the highest levels occurring at the onset of sprouting in spring. Additionally, dry matter content was significantly correlated with the proportion of sprouted bulbs in a sample. Alamar et al. (2020) concluded that ethylene modulates the expression of several genes, such as ACO, EIN4, and EIL3 to extend the dormancy. Puccio et al. (2022) demonstrated the key role of AcWRKY32 in onion bulb dormancy release. These studies are encouraging the researchers to extend the dormancy of onion to control the losses due to sprouting.

Watery scale is an economically important physiological disorder observed in stored onion. The onion exhibits sensitivity to elevated carbon dioxide levels, whether found within its internal scales or in the external atmosphere during storage in controlled atmosphere (CA) storage facilities. This higher CO₂ level led to the development of a physiological disorder called watery scale. Adamicki et al. (1977) found that CO₂ of 10% in CA storage resulted in the incidence of watery scales. Further, the same disorder was reported in all bulbs stored in polythene bags containing 10% CO₂. Microscopic studies revealed the damage to the cell walls, which might be due to the activity of hydrolytic enzymes. The content of free amino acids was also found on the higher side in bulbs having this disorder than in normal bulbs. Therefore, proper storage condition needs to be provided to increase the shelf life of onions in storage. Hoftun (1993) demonstrated that the majority of gas exchange in onion bulbs occurs through the neck, and the watery scales incidence was noted when the CO₂ levels exceeded 13% and oxygen (O₂) levels fell below 4% in the internal atmosphere. The observation suggests that onions exhibit a higher sensitivity to elevated CO₂ levels than to reduced O₂ levels.

The symptoms and causes of watery scale disorder were described in detail by Solberg (2015) based on visual observation and biochemical analysis; he grouped the symptoms as follows: leathery scale: thick, dark scales between outer dry and inner fleshy scales; translucent scale: glassy, firm fleshy scales. Watery scale disorder is found to be linked to disruptions in the gaseous exchange of bulbs, which are characterized by elevated internal CO₂ and/or reduced internal O₂ levels. Higher humidity is also reported to be correlated with the watery scale of onion (Solberg Øivind and B⊘e, 1999). Further, it is also noted that watery scale is also a one of the symtoms of bacterial soft rot caused by bacterial pathogen Pectobacterium carotovorum subsp. Carotovorum. Similarly, injury also may cause the translucent scale, thus it is a complex disorder (Shock et al., 2003).

Bulb splitting in onions, also known as bulb doubling or twinning, refers to a phenomenon where a single onion bulb divides into two or more separate bulbs. This occurrence can result in multiple bulbs growing from a single root, each developing into a distinct onion ( Figure 9 ). While some factors, such as specific environmental conditions or nutrient imbalances, may contribute to bulb splitting, it is generally considered an undesirable trait in commercial onion cultivation. Growers typically aim for single, well-formed bulbs, which are more marketable and easier to manage.

The incidence of this disorder is reported to be influenced by the genetics of the plant, climatic conditions, nutrient management, depth of planting, spacing between the plants, etc (Hassan, 1984; Thompson, 1934; Rabinowitch, 1979). The higher doubling was observed in plants developing from the larger sets than smaller ones, irrespective of short-day or long-day cultivars (Thompson, 1934; Rabinowitch, 1979). The application of higher nitrogenous fertilizer late in the growing season may accelerate bulb splitting (Brewster and Butler 1989). In 1984, Hassan noted that higher nitrogen application correlated with an increase in bulb doubling in onions, whereas potassium application resulted in the least occurrence of double bulbs. The higher nitrogenous fertiliser doses increased the count of double bulbs (Syed et al., 2000). Mechanical injury to developing bulbs during intercultural operations may lead to the emergence of outgrowth, and it might be one of the reasons for bulb splitting and doubling in onion.

This type of bulb deformity is found worldwide, and its symptoms are: the initial indication noticed is the division of the basal plate, and then several small bulbs emerge from the divided basal plate of the affected bulb (https://www.vegetables.bayer.com/za/en-za/resources/disease-guides/onions/bulb-splitting.html). Such bulbs are not marketable and lead to economic losses for growers. Irregular irrigation practices increase the occurrence of this type of bulb deformity. The repeated over-irrigation followed by a dry spell increases the percentage of split bulbs. Such splitting of the basal plate provides passage for entry to another pathogenic organism, which further deteriorates the bulbs, aggravating this disorder. Proper land preparation and regular and precise water and nutrient management can help in reducing the incidence of this disorder.

A “single-centred onion” usually refers to an onion bulb that has only one central growth point from which the layers of the onion develop ( Figure 10 ). Though this is not a physiological disorder in general, it can affect the use of onions for specific purposes. The single-centred onion bulbs are preferred by onion ring industries and restaurants for fresh use. Several factors affect the formation of a single-centred onion, such as genetics, nutrition, water management, stress, etc. The heritability of single-centred traits in different populations of short-day and intermediate-day type onion was studied and suggested that improvement can be made through selection (Wall et al., 1996; Cramer, 2006). This trait is found to be varied with the cultivars, and three years of evaluations of yellow cultivars showed that the single centre bulbs were in the range of 1 to 74% (Shock et al., 2005). Water stress at early growth stages (3 and 7, 3, 5-leaf stages) significantly reduced the single-centered bulbs (40%, 32%, and 18%, respectively) (Pelter et al., 2004). Similarly, Shock et al. (2007) also reported that onions proved susceptible to developing multiple centres under water stress, particularly during the 4 to 6-leaf stages of growth. Still, the physiological and molecular mechanism behind the single-centeredness in onion is not completely understood.

The thick neck in an onion occurs when the bulbs fail to mature, and there is continuous production of leaves; this leads to the thickening of the neck (pseudostem). It is also called incomplete bulbing (Brewster et al., 1987). It was observed that the application of nitrogenous fertilizer late in the season increases the pseudostem diameter and eventually produces thick-necked bulbs (Brewster and Butler 1989). The thick-necked bulbs deteriorate in storage rapidly due to sprouting and rotting. Previous literature searches revealed the correlation between thick neck and N application Tekeste et al., 2018; Khan et al., 2019). Some farmers are completely avoiding the use of urea to avoid the development of neck thickness and doubles (Palaniappan and Thangasamy, 2015). However, in addition to higher N application, neck thickness is also influenced by variety, location, transplant size, and season of planting (Brewster et al., 1987; Mettananda and Fordham, 1999).

Premature sprouting is a physiological anomaly wherein cloves, post-initiation, persistently develop shoots in the field, generating leaves instead of forming bulbs. These sprouts emerge through the leaf sheath, causing cloves to split and diminishing the market quality of the bulbs ( Figure 11 ). The varietal difference in premature sprouting of garlic bulbs in the field before harvest was studied, and found that white varieties are more susceptible to sprouting than purple varieties. Further, higher plant density resulted in less tendency towards sprouting (Garcia, 1980). Islam et al. (2015) evaluated garlic germplasm for premature sprouting under mulched conditions at multiple locations and found that premature sprouting ranged from 0.07% to 42%. Another study of screening for premature sprouting of garlic in Bangladesh reported 0.4% to 42% sprouting (Sarker et al., 2017). Such a varietal difference in the sprouting of garlic indicates a wide genetic variation for this trait. Generally, excessive soil moisture and more nitrogen are key factors responsible for the premature sprouting of garlic in the field. Further, delays in harvesting and irrigation after a longer dry period sometimes led to the splitting of garlic bulbs.

Rubberization is one of the physiological disorders in garlic bulbs. It is called rubberization/rubberification because the clove becomes elastic as a rubber. When you apply pressure with your finger, it leads to dent formation, and after the release of pressure, it comes to normal. These affected bulbs acquire a spongy texture after drying (Satyagopal et al., 2014). A correlation study of different factors influencing rubberization in garlic demonstrated that thrips infestation and higher doses of nitrogen were the main causes of rubberization (Selvaraj et al., 1994).

The physiological disorders in onion and garlic deteriorate the quality of produce. Therefore, these various disorders need to be managed with the help of adopting good agricultural practices, selecting suitable varieties, planting proper seasons, and following recommended post-harvest management. The detail recommendation for control of these disorders are mentioned in Table 1 . Therefore, agriculture extension workers and farmers need to know the reason for, symptoms of, and prevention of these different physiological disorders occurring in Alliums. Bibliometric analysis revealed that among these disorders, bolting and sprouting were more given attention by researchers while other disorders seem to be neglected. Thus, the present review emphasizes that physiological disorders must be given equal importance to understand the physiological and molecular factors behind them and devise a proper cultivation practice to reduce the economic losses of growers.