1 Introduction

Environmental fluctuations in soil properties, water, and climate affect the vegetable crops directly or indirectly leading to yield losses, and in the biodiversity of the vegetable crops (Nair et al. 2019). The changes in climate and environment, affect the growth of the crops in many ways sometimes improving the stature of the crops or affecting the crop productivity (Tripathi et al. 2019). Biodiversity being critical to the crop plants has heritable and other changes that are observed clearly in their species (Tripathi et al. 2019). This genetic diversity is a major area of study which deals with the the extent of the genetic variation present within the population (Bhatt et al. 2021). The study of biodiversity helps in clearly understanding the nature of adaptation of the population to the habitat and plant-environmental interactions (Singh et al. 2019).

Genetic diversity most importantly deals with the variations or differences in the diversity ranges of the plants belonging to same species at the gene, allele, and nucleic acid levels (Takahashi et al. 2019). A well-known fact is that the DNA sequence of any organism is transcribed to the mRNA and is later translated to form functional proteins which are the final products useful in promoting positive effects on the plants toward any biotic and abiotic stresses (Gore et al. 2019, 2021a). This mechanism which deals with transcription and translation leading toward the production of the final product, i.e., amino acid is mostly affected by the genetic diversity carried out by that plant species (Kumari et al. 2021).

Therefore, these genetic diversity analyses on crop improvement, improved plant cultivars, desired plant traits, high potential, and large number of seeds have been the main factors studied carefully by observing the differences in the populations, i.e., within the populations (intrapopulation) or in-between (interpopulation) the populations (Singh et al. 2020; Pratap et al. 2021). Within the populations, the genetic diversity can be observed based on the number of variations within the same species that are compatible for reproduction. Also, this study can be carried out within the members of same species in a specific area (Gore et al. 2021b) whereas, in interpopulation studies for genetic diversity, the characters or variations are observed between two or more populations of same species. Sometimes these are affected by the natural disasters and the geographical areas (Tripathi et al. 2021a, b).

This genetic diversity is believed to be obtained from the evolutionary processes like natural selection, geneflow, and genetic drift in any species especially in plants (Carneiro et al. 2020). This mechanism of evolutionary process involves the adaptation of the plants toward environment and developing the ability to cope up the stresses caused by any biotic or abiotic factors which comes under natural selection (Dhakal et al. 2021). Also, this natural selection is achieved properly depending upon the time span and the intensity of the conditions. Followed by natural selection, the gene flow is observed within the species that are highly adapted to any area (Djanta et al. 2020). This gene flow involves in the exchange of genes from one organism to another which can be of same population or in-between same species which would improve the integrity between the species and maintain the genetic flow from one generation to another generation. This exchange of genes between same populations or different populations is achieved properly by cross-pollination or through proper breeding methods (Du et al. 2020).

The final evolutionary process involving genetic drift is obtained by the changes in the gene pool within the populations or between the populations either through natural factors or any artificial means which leads to the changes in the genetic diversity (Figueira et al. 2019). Even though, many factors are present in nature to promote genetic diversity in plants which is a major and important mechanism in promoting the hike in the genetic characters and adaptability in plants, sometimes certain mutations lead to the damage of the genetic diversity of any population belonging to a particular gene pool (Gondim-Tomaz et al. 2022).

Certain natural or induced mutations are known to alter the genetic diversity by causing changes, affecting the gene pool or populations by creating genetic variations (Hong and Gruda 2020). These mutations allow DNA repairs prior to the affect caused to the plant’s fitness and adaptability. These mutations that can be either natural or induced, effect the plants, and lead to major genetic variations. These changes can occur at different levels and may lead to different modifications sometimes making plants unadaptable to their existing conditions and ultimately leading to the loss of important characters in plants (Islam et al. 2019) (Fig. 1).

Fig. 1
A cluster bar graph for the number of genotypes versus the few vegetable crops observed for the loss of genetic diversity. The value for a century ago peaked for cabbage at 544. The value for a decade ago peaked for tomatoes at 80.

Loss of genetic diversity in vegetable crops over the years (Source: Khoury et al. 2022)

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The genetic diversity carried out in different crops has been deteriorating gradually due to few factors like limited gene flow, inbreeding, reduced population size, and the genetic drift (Jiang et al. 2021). This deterioration in the genetic diversity in the recent years, i.e., a decade ago was due to different environmental factors, biotic abiotic stresses, and sometimes due to mutations which create versions in the same gene. The number of cultivars that are high yielding and adaptable to different stresses are below 100 in the present era (a decade ago), compared to a century ago where all the vegetable cultivars are in between 250 and 400 which proves that the conservation of genetic diversity is very important to reduce the yield losses and improving resistance toward stresses.

Although, mutations create many problems in plants leading to the losses of genetic conservation and important characters, there are many other methods that help in promoting the genetic diversity in plants (Kaler and Purcell 2021). One of which includes the usage of molecular markers in assessing the diversity in plant species. These markers are highly known for their accuracy, variability, and their abundance suitable for high-throughput analysis.

In plant breeding studies which include both classical and DNA-based markers in which many markers are included, certain DNA-based markers like random amplified polymorphic DNA (RAPD), restriction fragment length polymorphism (RFLP), Simple Sequence Repeats (SSRs), and amplified fragment length polymorphism (AFLPs) are widely used in genetic diversity studies along with crop improvement practices involved in plant biotechnology (Konovsky et al. 2020). These molecular markers through marker-assisted selection help the plant breeders in selecting the plants with varied characters on the basis of genotype rather than the phenotype or the observable traits in order to obtain maximum variations along with comparisons between the plant cultivars within a particular plant species or in-between the species which can be highly useful in improving the growing conditions of crops (Li et al. 2021). Apart from molecular markers, certain plant breeding techniques like mutation breeding, mass-selection, cross-breeding, development of hybrids also assist in promoting proper growth conditions in vegetable crops along with improving the genetic variability among the plant species. Through these techniques or different breeding methods, the identification and the development of new varieties are possible which are having several advantages (Lin and Wu 2021).

This genetic diversity is observed in plants or created with the help of different hybridization methods, mutations, genetic polyploidy, certain soma clonal or germ clonal variations, genetic engineering methods, and molecular techniques. All the above plant breeding techniques mentioned for improving genetic diversity are very important and are easily accessible (Fig. 2). After these methods are conducted the suitable species showing high genetic diversity are selected for further trails in different areas of the country for evaluation under different environmental influences (Mozzoni and Chen 2019). Later, the fittest of all the seed materials is collected and multiplied and distributed as a variety having high genetic variations and improved characters. As an initial step, the seed material is collected from fields or experimental sites, proper breeding method is selected, then the superior germplasms are selected from evaluation process after completing multiple location trails and these germplasms are multiplied and distributed to the farmers and seed companies as a new cultivar obtained from the selected plant breeding methods. These plant breeding methods are accurate in certain crops which are having short life cycle but difficult in vegetable crops having large life cycle.

Fig. 2
A flow chart of the genetic variation creation by hybridization methods, mutation, polyploidy, somaclonal and germaclonal variations, genetic engineering, and molecular biology techniques. The steps followed are selection, evaluation, multiplication, and distribution.

Cause of genetic variation in different vegetable crops. (Source: Hoban et al. 2023)

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Many are the important vegetable crops around the world, with high nutritional values and many other benefits were termed as the powerhouses for human health. Such vegetables include okra, cauliflower, bitter gourd, beans, spinach, fenugreek, onions, ridge gourd, not only these vegetables but also many others are known to be highly nutritious with major health benefits. Okra contains less sodium and no cholesterol, similarly cauliflower contains zero carbohydrates and cholesterol so that now a days, this cauliflower is grated and used as a substitute for rice. Other vegetables mentioned like beans, spinach, and gourds contain many minerals and vitamins along with fiber useful in maintaining human body’s health and longevity (Shilpashree et al. 2021).

Vegetables are considered as major sources of carbohydrates for body building, protein sources acting as energy sources, minerals, and vitamins which aid as protective agents, fiber sources for regulatory purpose, etc. All these important properties of vegetables may not be present in one species, but they are distributed within different species depending upon their growth and the type of edible part. Therefore, including all the vegetables and fruits in diet helps not only in promoting strength to the muscles but also helps in development of bones, improves digestion, increases blood supplementation, supplies with minerals and essential vitamins, and aids in releasing fullness of antioxidants into the system (Silva e Souza et al. 2020).

To improve the nutrition of vegetables, yield, growth, and other parameters requires proper maintenance and increase of genetic variation among the vegetable species. The ability of the species to adapt to different environmental conditions must be improved as well as the yield attributes. In this chapter, our focus is on different methods useful for improving genetic variation along with studying and understanding different important vegetable crops based on their nutrition and study their genetic conditions which leads to variations so that these selected cultivars in different species can be checked for trial and error and are released as new varieties for better yields and usage by the farmers and consumers (Yu et al. 2021).

Although, many methods of plant breeding like mass selection, improving hybrid and synthetic varieties are used these days, the problem lies with the application and conducting these methods for creating genetic variation due to laborious methodologies and maximum life cycle of the vegetables. Therefore, the possible ways to reduce the time consumed for developing a cultivar suitable for growth and cultivation within less time are through gunshot/gene gun plant breeding method, known to make the work easy for the breeders and the farmers to obtain quality crop for improving the nutrition in humans. Therefore, a comprehensive study, on certain vegetable families is done to identify the research gap and develop the suitable methods to achieve it (Yu et al. 2022).

2 Genetic Diversity in Vegetables

Important vegetable crops are studied with respect to the genetic diversity assessment. The growing countries and gene banks of these vegetables help provide useful information to the breeders in recollecting the seed material (Table 1). Also, different markers used in these important vegetables are discussed (Table 1).

Table 1 Vegetables and their importance in genetic diversity studies
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2.1 Solanaceae

Solanaceae (night shade) family is a major fruit-producing family having more than 2000 species with different genetic variation in each of the species. The members of this family include tomato, potato, chilies, and eggplants which are the major members of this family. Half of the members of this family directly belong to the genus solanum (Gagnon et al. 2021).

2.1.1 Eggplant

Eggplant (S. melongena L.) was mostly grown and widely consumed plant in Asia, America, and Africa whose origin was believed to be Indo-Burmese. This eggplant is strictly sub-tropical with more than 30 wild relatives (Jarret et al. 2019). These plants are well known for their nutritional aspects like high vitamin A and C, improving eyesight also helps in protecting cells against any damage. Along with this, the polyphenols present in brinjals help in curing diabetes. Also, there are certain articles in brinjal stating that the constant consumption of brinjals is known to reduce the heart diseases and have cancer fighting properties since these brinjals are rich of higher antioxidants (McCarthy et al. 2019).

Many are the disadvantages associated with the loss of the genetic diversity in different vegetable crops which include loss in the shape, size, color, glossiness, and taste of the fruit making these parameters as very important attributes in vegetable crop production. Most of the brinjal fruit yield is affected by the lack of proper wild relatives promoting high-yield conditions in the cultivars. Also, the plant height and the number of fruits per plants can be increased by certain crosses between elite cultivars. The fruit weight, length, and color can be obtained by repeated crosses and selecting improved hybrid cultivar. The most important parameters in brinjals called ascorbic acid content and phenolic contents were gradually raised by genetic manipulations and gene expression analysis in the cultivars that are highly tolerant to environmental stresses (Montechiari et al. 2020).

Along with these plant breeding techniques like crosses between normal cultivar and elite cultivars, certain markers were also used to understand the genetic diversity in a group of eggplant cultivars so that the high-yielding and elite genotypes can be selected and analyzed for further increase in the nutritional values. Certain molecular markers like RAPDs, SSRs, and ISSRs were used in the studies for identification and the development of elite hybrid genotype suitable for achieving all the parameters up to a certain level (Rifkin et al. 2021).

2.1.2 Tomato

Tomato (Solanum lycopersicon L.) was a very useful and wonderful crop with many health benefits originally consumed in South America and was introduced in Europe and North America late in eighteenth century due to its several health benefits. A total of one large tomato contains maximum amount of vitamin C providing a day’s nutrients up to 40%. These red colored tomatoes which are bright in color and full contain maximum amount of lycopene due to which the antioxidant properties are maximum in case of tomatoes. Along with this, these tomatoes help in making bones strong and are proven to be good for skin by improving the skin’s complexion. Most importantly the rich antioxidant nature in this vegetable helps in fighting several cancers especially, skin cancer (Rodríguez et al. 2020).

There are many yield parameters to be achieved in tomatoes, along with the improvement in the genetic diversity, not many wild relatives are present in tomatoes but a few in which the fruit quality and taste greatly differ. The height of the plant, has been a major concern in the past but due to certain crosses between two different cultivars, the average height of 191 CMS was achieved and was known to be the best plant height in the variety PAN 1286 which was followed by 186 CMS in the variety Kundan. Along with this, certain yield attributes like total fruit yield and the number of primary branches were also achieved by certain crosses between elite cultivars from which the hybrids were selected and checked for improvement (Stiefkens et al. 2020).

It was also known, that the hybrids obtained by different crosses were known to be improved and can achieve more than one yield attributes. The important and the main compound found in tomatoes is beta-carotene which triggers vitamin A and was known to be less in many tomato varieties until the genetic manipulation studies and cloning achieved improved beta-carotene content in few cultivars of tomatoes making them highly nutritious and useful for consumption. Another important compound, Lycopene which was not up to a maximum level in most of the cultivars but differs from cultivar to cultivar based on the environmental factors and lycopene-producing genes. This can be totally altered and achieved by the gene expression analysis and RNA interference techniques. Also, the diversity studies in tomatoes can be achieved by using certain molecular markers like RAPDs, ISSRs, SSRs, AFLPs, and SNPs making the study easy and understandable with respect to genetic diversity in old and new cultivars of tomatoes (Van de Peer et al. 2021).

2.1.3 Potatoes

Potatoes (Solanum tuberosum L.) are a root vegetables having high amounts of starch originated in the Americas. Their wild relatives are present in south part of America. These potatoes were often mistaken for their starch content but lesser known for their health benefits and several nutritional factors. The starch content in the potatoes helps in maintaining the weight by making the stomach full for longer time after consumption. Apart from these, potatoes were known to contain several antioxidants like flavonoids, carotenoids, and phenolic compounds making them very useful for consumption. The dietary fiber in the potato starch is also useful in promoting digestion. With lesser-known evidence, this vegetable was also known to control diabetes and other related diseases (Zenil-Ferguson et al. 2019).

Many are the yield attributes in potatoes for observing or improving the genetic diversity in this vegetable. Improving the height of the plant for proper photosynthesis and disease escape is a major milestone crossed by two varieties called Asterix which is around 62 CMS and Diamont which is around 60 CMS. These varieties can be used along with several other cultivars in improving the plant height. Also, the size of the tuber, quality, and the starch content can be obtained by selecting proper potato cultivars and carefully manipulating them at their genetic level. Along with these factors, the protein content, organic acids content were also achieved in potatoes. Plant breeding does not have much significance in potato crossing as this vegetable is highly self-pollinated. But certain crosses between tomatoes and potatoes were advantageous as they contain nutrients from both the plants and are highly recommended for consumption (Mabuza et al. 2022).

Certain molecular markers were used in potatoes for genetic diversity studies like AFLPs and RAPDs useful in observing and understanding the diversity between different cultivars of potatoes and their improvement aspects (Serbessa et al. 2021).

Apart from these three major vegetables belonging to Solanaceae family, there were several other vegetable plants having similar antioxidant properties and health benefits similar to the above vegetable crops. This family also includes cherries, bell peppers, chilies, cape gooseberries, etc. (Fig. 3), all of them are having similar advantages but differ in taste and size of the plants and fruits as well (Deanna et al. 2022).

Fig. 3
4 photos of vegetables like eggplants, tomatoes, cloves of garlic, and onions.

Important vegetables under genetic diversity studies. (a) Eggplant, (b) tomatoes, (c) garlic, (d) onions. (Photo by: Bandi Arpitha Shankar)

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2.2 Fabaceae

This family is one of the major classes of vegetable families having highest economic importance not only because of the vegetable itself but also due to the presence of atmospheric nitrogen-fixing bacteria near the roots of these vegetables. This family is known to be highly nutritious and known to contain trees, shrubs, and plants. All the members of Fabaceae family contain pods as economic parts along with their immense nutrition. Certainly, this family provides maximum yield due to which all the classes of people can afford the vegetables belonging to this family. The major important vegetables in this family were green peas and peanuts (Araújo et al. 2019).

2.2.1 Green Peas

These are the major vegetables in Fabaceae family occupying the maximum portion for cultivation and production along with yields and nutrition. Green peas are a good source of vitamin C and E along with Zinc and many other antioxidants which are useful in reducing diabetes, cancers, and many heart diseases. The maximum protein content in the pods of green peas and their seeds adds many nutritional benefits long with maximum fiber content promoting easy digestion (Artemov and Egorova 2021).

Also, in green peas which are a major vegetable in the family of Fabaceae, certain yield parameters are difficult to achieve like plant height, number of pods, their lengths, and their sizes. Although there are not many wild relatives for green peas, one major wild relative species called P. fulvum L. is present and is known to possess many important characteristics like maximum number of pods and their sizes. Therefore, the plant height and other characteristics related to the pods in green peas can be achieved by carefully pollinating between two different cultivars of elite genotypes so that the hybrid obtained will be full of useful characters. A method called Sonication was highly recommended in increasing the amylose and protein contents in green peas. Also, many molecular techniques emerged in alternating the genes in green peas for better yields. In green peas, only SSR markers were used because they are highly codominant, polymorphic, and were highly useful in genetic diversity studies because of large populations in Fabaceae family (Chapman and Wieczorek 2021).

2.2.2 Peanuts

Peanuts (Arachis hypogea L.) which is popularly known as ground nut has several health benefits like all the other nuts. These peanuts are high in proteins, fiber, and rich in good cholesterol useful for body functioning and have several advantages. These good fats help increase in metabolism and provide multiminerals like magnesium, zinc, iron, etc. (Kholina et al. 2021).

The number of pods underground is a very important attribute in achieving maximum yields. Also, the pod test weight along with filling is the major measure required for attaining maximum yields in peanuts. These parameters like yields, test weight, and pod filling can be increased by maintaining proper soil conditions along with the crossing of high-yielding varieties with normal cultivars to achieve better hybrids (Sandanov et al. 2021). These hybrids were known to contain qualities from both the parent cultivars which promote grain filling and maximum yields. Above all these parameters, the oil content in the pods is very much important in achieving a successful cultivar to be released. Sometimes this oil production or oil content in groundnuts depends upon the type of cultivars used and seasons. The alterations in oil production can be achieved by many plant breeding methods and identification of oil-producing gene and its incorporation (Selyutina et al. 2019).

Many molecular markers like RFLP, SSRs, AFLPs, DArT, and DAF were used for the clear understanding of genetic diversity and the study of different parameters finally improving the yields. Therefore, ground nuts which are the major sources of amino acids, minerals, proteins, and fats also provide oil compared to other nuts due to all these important outputs, these were called as poor man’s almonds (Trofimova 2020).

Although, many vegetable crops in Fabaceae family, the green peas and groundnuts, have gained much importance due to their small life cycles and adaptive nature. Rest of the other members in this family include hyacinth bean, fava bean, and cowpeas. By proper understanding, the nature, and the availability of wild relatives with respect to resistance against many diseases, insects, and changes in the environments, the yields, quality, and the quantity of the pods can be achieved fairly (Xue et al. 2020).

2.3 Cucurbitaceae

The members of this family are water rich foods and mostly melons. Higher benefits were that these vegetables can be consumed, raw, cooked, pickled, and stored or canned. The mix of these veggies with herbs enhances the taste of food (Guo et al. 2020). These are rich in electrolytes and contain high amounts of vitamin C and beta-carotene making them a good source of vitamins and the precursors that trigger vitamin A. These vegetables are the natural water sources because 80–90% of the vegetable pulp contains water and useful compounds. Also, these vegetables contain a good amount of lignans, terpenes, and flavonoids making them very useful as they were highly anti-inflammatory (Xu et al. 2021).

It suggests that the daily intake of cucumber adds 10% of nutritional value required. Recent studies suggested that the peels of cucumbers or all the members in that family were highly nutritious than the fruit inside. The major yield attributes of cucumber involve the size and weight of the fruit which depends upon the environmental conditions and the type of variety. Many other parameters like fruit length, color, and bitterness also were affected due to external environment (Osipowski et al. 2020). Although there are not many wild relatives in cucumbers except Cucumis hystix L. which is the one and only major relative found in Asia. The yield improvement, fruit size, and color improvement can be done by cross-pollinating the cucumbers between two different varieties till the characters were obtained in the hybrid. These hybrids can be checked in multiple yield trails to observe that characters and release. Though, cucumbers do not contain many wild relatives, their genetic diversity is not much so that sincere efforts were to be done to maintain the genetic diversity between different cultivars which is possible only through plant breeding techniques and the use of molecular markers (Yano et al. 2020).

SCARs, SSRs, and RAPDs were the useful and important markers in cucumbers to understand the relation between different cultivars with respect to their qualitative and quantitative characters so that the genetic diversity can be studied and maintained. Many other members of Cucurbitaceae family like Melons, Watermelon, Zucchini, Gourds, etc., were very fast growing and contain nutritional values like cucumbers (Qin et al. 2021).

2.4 Alliaceae

This is the major vegetable family not only in India but almost entire world. The major vegetables of this group are onions and garlic. Most of the dishes, vegetarian or nonvegetarian, need onions and garlic for that fresh taste, aroma, and texture (Fig. 3). Not only for cooking purpose but also onions and garlics have many health benefits compared to any other vegetables (Alekseeva et al. 2021).

2.4.1 Onions

Onion (Allium cepa L.) is the major vegetable of the family Alliaceae having many important health benefits like reducing cholesterol and breaking down blood clots by which the heart diseases were reduced. This is mainly due to the presence of organon sulfur compounds in the onion bulbs which give pungent smell and taste. Also, these onions were available in three forms red, white, and yellow onions. Out of all the present onions, the red onions were known to be having high amounts of oregano sulfur compounds and were highly nutritious. Also, an important compound called Quercetin present in the bulbs of onions was known to have high amount of antioxidant properties due to which many cancers and arthritis were reduced (Bahadirli and Ayanoglu 2020).

The major yield parameters observed in onions were plant height, size of the bulb, bulb length, bulb yield, and the color of the bulbs. Out of all these, the plant height and the color of the bulbs depend upon the variety used for crossing. The size of the bulb, its length, and yield are the major growth parameters in onions which can be achieved by plant breeding techniques by crossing the elite cultivars having good yield parameters and nutritional values. Since, onion is both self- and cross-pollinated crop, the crosses between them are very successful and show cent percent transfer of characters to the hybrid (Ustariz et al. 2022).

Out of all the markers, only SNP markers were used in onions due to their polymorphic nature and were linked to disease resistance in many onion cultivars acting as a major and important tool in identifying certain disease resistant genes. Certain genetic manipulations were also possible in onions by cloning and gene expression techniques (Ha et al. 2021).

2.4.2 Garlic

Garlic (Allium sativum L.) is a useful herb highly useful in treating heart diseases and several cancers. Also, this acts in improving blood system and protecting human body from several damages. The major compound present in garlic is allicin which is believed to be highly useful in promoting nutritional qualities in humans by improving the antioxidant properties in human system after consumptions. It was believed and studied that daily intake of at least 3–4 garlic bulbs improves heart health and reduces cancerous growth in humans (Fig. 3) (Herrera-Arroyo et al. 2020).

The major parameters in garlic are the plant height, number of leaves, number of bulb layers, number of bulbs, pyruvic acid, bulb moisture, etc., out of all these attributes, the plant height is not of major importance, but the number of leaves and bulb layers increase the yields as the bulbs were the economic part of garlic. Also, the bulb moisture is to be maintained because the freshness lies in the moisture of the bulbs which enhances the taste and aroma of the cuisine (Jiao et al. 2020).

The only marker that studies different cultivars which helps in improving the genetic diversity in garlic was SSR markers through which the comparison between the cultivars was possible along with the understanding of genetic diversity studies with respect to other yield attributes in garlic because of a smaller number of wild relatives in garlic (Salgotra and Stewart Jr. 2020). Not only garlic and onions but also other vegetables like leeks and chives share the same family which were used for improving the aroma and the taste of the dish (Poljuha et al. 2021).

2.5 Malvaceae

This a diverse family group also called Mallow family which is highly important for its nutritional properties. Also, the members of this family were distributed worldwide due to their economic importance. The members of this family are the source of natural fibers that helps in improving gut health. Out of all the members in Malvaceae okra called okra has gained much importance (Chandran et al. 2022).

Okra commonly called Lady’s finger (Abelmoschus esculentus L.) was one of the major crops in the Malvaceae family due to the presence of mucilage, fibers, and vitamins. This vegetable is highly economical as it is easy to produce and fast growing (Abd El-Fattah et al. 2020).

The major yield parameters to be achieved in okra were the plant height, number of branches per plant, fruit length, total number of fruits, and total yield per plant. All these parameters widely depend upon the cultivars selected and their crosses. Not many wild relatives were present in okra but a few called Abelmoschus ficulneus L. and Abelmoschus tuberculatus L. which have high-yielding capacity but contain many hairs on their surface making them uncomfortable for consumption. Also, these wild relatives were not adaptable to diverse environmental conditions. Since okra is highly self-pollinated, careful cross-pollinations were also possible under controlled conditions to improve many yield parameters which come under development using plant breeding techniques (Li et al. 2019a, b).

Also, molecular markers like RAPD and SSRs were the most used markers in okra-promoting genetic diversity studies in different cultivars useful for improving yields and other parameters. Not only okra but also other members of this family including hibiscus, roselle, and cotton have different economic importance but were not suitable for consumption. These members produce a very large, colorful, attractive colors which attract bees for crossing. Also, cotton which is one of the important members in Malvaceae is providing fiber and oils for economic use (Xiong et al. 2020).

2.6 Umbelliferae

This family was also recognized as Parsley family because of its important members. All the members belonging to this family were distributed as vegetables, leafy vegetables, etc. This family has gained much importance not only in India but also in other parts of the world. The vegetables belonging to this family can be eaten raw or cooked. Also, these vegetables were used as flavoring agents and coloring agents apart from cooking (Altameme and Ibraheam 2019).

Carrot (Daucus carota L.) is one of the major and very important vegetable in this family compared to other members due to its high nutritional factors and healthy properties. It was known that these carrots were rich in vitamin A and B-carotene which help improving eyesight. Also, there were certain evidence that these carrots help in reducing blood sugar and reduces cholesterol as well maintain weight and health. The bone health will be maintained by consuming carrots on daily basis as these were known to contain maximum calcium and adequate amounts of vitamin K (Esfandani-Bozchaloyi et al. 2019).

Although many species nearly 300–3000 were present in the family umbelliferae, carrot does not have many wild relatives; the only wild relative was named after the cultivated variety Daucus carota L. (wild carrot) whose improved characteristics were released as cultivated variety. Also, there are a few yield parameters in carrots like plant height, number of leaves, length of carrot, diameter of carrot, softness, and sweetness. The above parameters were achieved mainly by using popular cultivars and were also influenced by external environmental conditions. Cross-pollination was also possible in this family mostly by bees and insects due to their protandrous nature. Therefore, the advantage of cross-pollination avoiding halting self-pollination provides scope for improving many yield characters by using certain plant breeding methods. These methods aim at developing a vigorous hybrid having all the required features and resistance against several stresses (Firat and Güzel 2019).

Also, certain genetic manipulations in carrots help in improving calcium production by altering the gene expression levels of that gene that produces calcium in developing carrots. Also, the maximum genetic diversity analysis and genetic mapping works in carrots will be done by using certain markers like AFLP, RAPD, and Transposon Display (TD) markers. Therefore, carrots, the vegetables with immense nutrients and vitamin A are very much useful for consumption very often. Not only carrots but also there were few other vegetables involved in this family having maximum health benefits and very nutritious like parsley, celery, fennel, and coriander (Hossein-Pour et al. 2019).

2.7 Brassicaceae

The mustard or Cruciferae family comprises of many important vegetables that were both economical and nutritious because they can be consumed fresh either by cooking or raw, useful in making oils and condiments. These members in this family are easy to grow with less investment. These vegetables were not only famous in India but also around the world. This family is mainly known for preparing rice-like items instead of consuming rice which lowers the intake of carbohydrates like cabbage rice and cauliflower rice (Akhatar et al. 2021).

On an average, there were a total of 37 species under Brassicaceae family. All the members of this family were unique and important in their own way and provide maximum nourishment on regular consumption. Cabbage helps in reducing cholesterol and blood sugars along with providing maximum nutrients that help in protecting lining of intestines. Both of cauliflowers and broccoli have maximum nutrients that are highly beneficial for eyes. But comparatively broccoli has maximum amounts of vitamin K and C (Faisal et al. 2021).

The proper development of curd and heads in cauliflower and cabbage respectively is very important parameter along with their maximum sizes. The proper maintenance of genetic resources in brassica species was done through in situ and ex situ conservation. In situ conservation helps in the conservation of wild relatives and parent genotypes, whereas the ex situ conservation promotes the increase of diversity and maintains the link between the conserved and nonconserved species. These two methods of maintaining genetic diversity help in maintaining the genetic resources. Also, the vegetative propagation in almost all the members of this family not much improvement in characters was observed through plant breeding techniques. But genetic manipulations in these members improved their resistance against certain stresses including both biotic and abiotic. Therefore, apart from cabbage, cauliflower, and broccoli, there were few other vegetables having highest nutrients like radish, mustard, and kale (Jiang et al. 2021).

3 Genetic Diversity in Leafy Vegetables

Different leafy vegetables are adapted and grown in different countries depending upon their usage and are exported to other countries. These leafy vegetables are not only tasty but also provide many health benefits as discussed in Table 2. Also, different markers used in these leafy vegetables help in identifying the associated DNA of the particular gene.

Table 2 Leafy vegetables and their importance in genetic diversity studies
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Along with normal vegetables, leafy vegetables also play a very important role in promoting health and improved nourishment in humans. These leafy vegetables were fully packed with important minerals, vitamins, and very less in carbs and calories which is a major advantage. The genetic diversity studies in leafy vegetables help in improving the phytonutrient content, stress resistance, and leaf yield-related traits in many leafy vegetables. These were known to reduce the risk of obesity and blood pressure by keeping heart diseases at bay. Not only these benefits but also many are the important uses of consuming green leafy vegetables daily (Ali et al. 2019).

Although there are many major green leafy vegetables available for consumption as they provide high levels of nutrients, minerals, and vitamins especially vitamin A, zinc, iron, and many more, this study deals with two major leafy vegetable families like Chenopodiaceae and Lamiaceae (Balaguera and Beleño 2019).

3.1 Chenopodiaceae

This is one of the major green leafy vegetables family as it involves important crops like spinach, beets, and chard (Fig. 4). These green leafy vegetables help in maintaining body’s immunity by providing major minerals and vitamins (Hossein-Jafari et al. 2019).

Fig. 4
2 photos of leafy vegetables like coriander and spinach.

Important leafy vegetables under genetic diversity studies. (a) Coriander and (b) spinach. (Photo by: Bandi Arpitha Shankar)

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Spinach contains full of nutrients and vitamins like A, K, and C which aids in promoting health benefits. Also, this vegetable is fully packed with fiber which assists in improved digestion. Along with these, the major important health benefits of consuming spinach involve prevention of cancer, weight loss, reduction in blood sugars, and have anti-inflammatory properties (Morillo et al. 2020). Not only spinach but also the other two vegetables belonging to this family aid in improving health by consuming them often. Beets contain high amounts of vitamin A and K whereas, chard contains beta carotene and well known to play major roles in formation of many organs and helps in their functioning (Pereira et al. 2019).

These leafy vegetables were generally cross-pollinated because pollen shed before the female flowers open causing a hindrance to self-pollination. But most of the pollinations were needed only to develop the seeds and fruits which are not so necessary in breeding of spinach, chard, and beets. Therefore, through traditional breeding method strategies, these plants were grown from the old root stocks and the farm conditions were properly maintained for growth of the leaves (Romero et al. 2019).

This family contains 20 wild and cultivated species which were very near and does not have many differences within them. Also, for proper understanding and studying the genetic diversity, SSR markers were used. There was certain evidence that the genetic manipulations were conducted in these plants in order to improve the resistance against herbicides, diseases, and insects along with certain abiotic factors and also aim at improving photosynthesis in these plants. Therefore, Chenopodiaceae which was also known as goose foot family is well known for its nutrients and importantly provides immunity by consuming on a regular basis (Yegizbayeva et al. 2021).

3.2 Lamiaceae

This is one of the important leafy vegetables family having many types which were used as flavoring agents and condiments. The extracts of these leaves were highly used in making medicated creams and balms used in treating skin allergies and rashes. Also, this family can grow even under severe drought conditions. Certain members of this family like thyme and rosemary were used as dried leafy additives in western dishes which add flavor and aroma. Mint is one of the most used leafy vegetables in almost all the special dishes in Indian dishes. This family is not only famous in India for its flavor and medicinal properties but also used in each part of the world due to their health benefits (Alekseeva et al. 2021).

This family includes mints, basil, thyme, rosemary, oregano, and sage where all these have equal importance and were unique in their nature of providing health benefits to humans (Fig. 4). It was also suggested that the regular consumption of these leaves in small amounts releases the stress and detoxifies the human body making it very healthy. The mint and basil leaves were highly recommended for stress relieving by improving sleep quality taken in form of tea (Balakrishnan et al. 2021).

Whereas, rosemary is wonderful source of antioxidants which helps in improving immune system; also intake of one tea spoon grounded thyme acts as a wonderful source of calcium, potassium, and iron which are vital for human body. Along with this, oregano contains maximum amounts of antibodies which helps in promoting good health. Also, sage contains many anticancer properties also known to improve heart and brain health when consumed as tea (Bernardes et al. 2021).

This family has in total of 18 species which are highly related to each other and does not have much difference from cultivar to cultivar. Their growth includes obtaining crops from old root stocks and rarely waiting for seed setting. This family was usually affected by changes in the genetic diversity which leads to the loss of certain species due to which certain breeding strategies like MAS (Marker-Assisted Selection) were used to improve the quality and the quantity of the plants. Also, certain genetic engineering methods like, genome editing, targeted gene knockouts and gene transfers, random T-DNA insertions, and RNAi-based knockdowns help in improving the medicinal properties of these plants making them more beneficial for humans (Shahhoseini et al. 2023). Therefore, the members of this family contain majorly leafy vegetables which were consumed dry and used in almost every dish because of their flavor and their medicinal properties. It is always important to protect genetic diversity due to which the healthy plant populations are maintained from which different pests and disease resistant genes can be selected in leafy vegetables within these species to promote natural resistance toward pathogens, increases growth in the plant species indirectly improving the population size, adaptation toward new or changed environmental conditions, and finally increasing the economic benefits by the consumption of leafy vegetables. This mechanism helps in promoting adaptability of the plants toward the changes in the environment (Ha et al. 2021).

4 Measures of Genetic Diversity in Vegetable Crops

A well-known fact that genetic diversity is the amount of variation present in the species that helps in improving the growth of the species and their nature. This can differ from species to species and cultivar to cultivar. This genetic diversity can be studied by comparing different cultivars belonging to same species. Different vegetable crops having significant importance were known to be influenced by genetic diversity which helps in improvement of characters of the cultivars which promotes maximum production reducing the losses caused by biotic and abiotic stresses. This genetic diversity occurs due to the contrasting between alleles of genes in different plants belonging to same cultivars which results in the changes in the phenotypes (Willett et al. 2019).

This genetic diversity can be induced through three different methods like mutations, recombination, and introgression of the genes that cause genetic variations. In general, mutations sometimes cause changes in the genes referred to as alleles would cause changes in the phenotypic characters in plants like height, shape, color, and size of the vegetable. These changes caused by mutations would not only affect the vegetable crops but also fruits along with leafy vegetables. In case of leafy vegetables, the production is the major factor along with the color of the leaves and maximum growth of the leaves (Cai et al. 2020).

Secondly, genetic recombination also results in certain genetic variation. Usually, genetic recombination refers to recombination of broken pieces of DNA which results in new combinations so that genetic diversity was created and reflects in the physical parameters of the plants. There was certain evidence that this genetic recombination causes increase in the genetic diversity of the plant belonging to the same or different cultivars belonging to one species (Durand et al. 2022).

Finally, immigration of the genes also results in the genetic diversity where, the immigration of genes generally refers to the gene escape caused by the interbreeding between populations or species which results in new gene combinations. This causes changes in the genetic composition of the population resulting in maximum genetic diversity. Therefore, the above methods would promote genetic diversity in vegetable crop populations resulting in changes of different plant parameters like plant height, color of the vegetables, number of branches, yield and resistance against both biotic and abiotic stresses (Gioushy et al. 2020).

5 Threats to Genetic Diversity in Vegetable Crops

There were certain threats in genetic diversity caused by changes in environment, cropping patters, and many others. These threats would result in the loss of the genetic diversity due to the loss of individual genes which were produced by recombination or new combinations affecting the crop growth (Bhat et al. 2022).

These changes were the result of high-yielding varieties produced by severe plant breeding methods and intensive crosses between populations for producing elite cultivars, sometimes the land races were replaced by the high-yielding cultivars which result in the loss of primitive genes that have very good qualities. Also, certain factors like changes and the loss of the habitat, reducing area under forests, certain pathogens, harmful insects, obnoxious weeds, environmental pollutions, and most importantly global warming imposing threat to the natural environment and plants (Akin-Idowu et al. 2021).

6 Conclusions and Prospects

Often vegetables and leafy vegetable crops are a source of nutrition, livelihood, and health in the regions facing shortage of food for sustainability of food security and chain supply. Before improving genetic diversity, the maintenance of wild relatives has become an important source of genes useful in promoting growth and yields in plants and conferring resistance against certain diseases and pests. Genetic diversity on the other hand improves the number of genes through mutations, genetic recombination, or gene escape where these measures promote the increase in the new combinations of genes helping in the improvement of plants and their yield. Since, this genetic diversity aids in promoting new genes, new characters in plants were observed in their morphological parameters. This is the best way possible through intense breeding programs and genetic manipulations.

Many vegetable crops which are important in improving human nutrition have been discussed for their health benefits and modes of improving genetic diversity. Also, several markers are used to study the genetic diversity in vegetable plants for improving their yields. Although, many improvements in vegetable crops were leading to the losses of genetic diversity caused by high recombination, high-yielding varieties, and elite hybrids which spoil the genetic nature of the vegetable crops thereby reducing the resistance against different biotic and abiotic stresses.

The future prospect of genetic diversity includes the improvement of genetic diversity of the less important vegetables and nuts through the use of easy and efficient methods to create genetic variation, selection of the elite varieties and the use of tissue culture in improving genetic diversity.

Along with these, developing cost-effective and less time-consuming methods to study the genetic diversity of different vegetable plants is important. The maintenance of wild relatives is an important source of genes which promotes growth in their populations. Also, enhancing genetic diversity through forward and reverse genetic methods has great relevance. The proper research in novel recombination and enhancement in the introgression and recombination can help in improving the genetic diversity. Importantly, adaption of the advanced biotechnological and genomics methods can increase genetic diversity not only in vegetable crops but also in fruits, flowering plants, oil seed crops, ornamentals etc. can.