Keywords

1 Introduction

Biological diversity, or biodiversity, is the variety of life on earth. It provides a better guarantee for survival, development, and evolution. Biodiversity represents a feature of sustainable development. In a larger sense, biodiversity is the natural heritage of the planet. A deep relationship has evolved between humans and plants extending far back the evolutionary periods. A reflection question may bring much light to this relationship: What would happen if we did not have plants, or if all the plants on earth suddenly disappear? How long would it take for a human to die off? As we know, humans definitely cannot survive anymore without plants, and life on earth would be impossible. Thus, plants are the source of life on earth. Being aware of this reality, human efforts were addressed over long periods to domesticate plants and to create new ones with breeding programs. Therefore, humans took seeds from natural habitats and grew them in different places. Earlier reports noted that domestication activities stand back about 100,000 years ago, when people from South West Asia started to domesticate crops such wheat, barley, and peas. About 4000 BCE, other crops started to be domesticated such as grapes, figs, dates, and olives (Forrester 2020). From a genetic point of view, plant domestication may be defined as “a genetic modification of a wild species to create a new form of a plant altered to meet human needs” (Doebley 2004). That is to say, those wild relative species are eventually modified, throughout domestication, into genetically different crop species (Schaal 2019). Year after year, generation after generation, the best plants, i.e., that better meet human needs and preferences, are selected. This winnowing caused a considerable reduction in population sizes, defined as “genetic bottleneck.” This is accompanied by a reduction in genetic diversity throughout the genome (Doebley 1989). Hence, genes controlling desirable phenotypes would be filtered out creating a drastic loss of diversity (Doebley 2004). This may explain the low genetic diversity generally found in domesticated crops when compared to their wild ancestors (Byrne et al. 2020).

The domesticated crops adapt to specific local agroclimatic conditions, often known as “landraces” (Byrne et al. 2020). When these landraces are cultivated for more than a century in such region, they are called autochthonous, and allochthonous when they are introduced to another region due to migration, (Marone et al. 2021). Therefore, landraces constitute a genetic reservoir of selected genes depending on physiological, biochemical, and morphological local adaptation. In addition, landraces may provide data tracing the geographic origin as well as the demographic trajectories of a given species. Hence, the efficient conservation, utilization, and improvement of plant landraces are a prerequisite. Unfortunately, plant landraces face, generally, serious threats as a result of the abandonment of traditional agricultural practices and the absence of national institutions responsible for their conservation (Marone et al. 2021).

Tunisia is a crossroad of numerous civilizations accumulated over 3000 years of history given its geographical position. In geopolitical terms, Tunisia has a strategic position enabling it to be part of several areas, Euro-Mediterranean, Maghreb, Arab-Muslim, and African, resulting in a rich cultural mosaic. The country is characterized by the diversity of its natural landscapes and ecosystems. No less than 3100 plant taxa have been identified, including 200 endemic species common to neighboring countries in the region. The agricultural sector is fundamental to the stability of the country representing 12% of its gross domestic product (GDP) and employing approx. 16% of Tunisia’s labor force (USAID 2018; Knaepen 2021). The different civilizations that the country has known have left their signatures on the early agricultural practices such as the domestication and cultivation of different crop plants. Hence, the domestication of major fruit trees in Tunisia dated back to antiquity, and arboriculture played a fundamental role in the existing biodiversity (Fuller and Stevens 2019).

2 Fruit Tree Genetic Resources in Tunisia

In Tunisia, arboriculture occupies a surface of 2.155 million ha. As in other countries of northern Africa, the main cultivations are the date palm, olive, grapevine, fig, and pomegranate regarded as the “first wave” of domesticated fruit crops (Santoro et al. 2020).

2.1 Olives (Olea europaea L.)

The olive tree is rooted to Tunisia’s history and life. Olives appear to be established early in Tunisia, in the eleventh century BC, most probably by the Phoenicians, founders of Carthage, and then spread by Carthaginians who planted olives where and when they could (Driss 1966; Ennaifer 1973) (Fig. 1a). The oldest olive tree in the Arabic world and in the Mediterranean region is located in the North of Tunisia, the region of Cape Bon (Haouaria). This olive tree, named “Zaytounet Echraff,” is about 2500 years ago. A youngster comes to steal the show called “Zaytounet Lakarit,” about 900 years old, situated in south-eastern Tunisia in “La douiret” of “Tataouine” (Jemaà 2019) (Fig. 1b, c).

Fig. 1
3 photos. A depicts mosaic pattern with the Museum of Tunisia is given at the center. B and C depict the Olive trees. B shows a stone inscribed in a foreign language in the foreground.

The olive tree lies at the core of Tunisia’s history. (a) “Mosaic of Lord Julius”: a roman mosaic found on the archaeological site of “Carthage” and actually deposited at “Le Bardo” Museum of Tunisia. It shows a scene of the picking of olives in the Roman era (source: https://en.m.wikipedia.org/wiki/Mosaic_of_Dominus_Julius_Carthage). (b) The millennial “Zaytounet Echraff” and (c) “Zaytounet Lakarit”: among the oldest olive trees in the world, located in the North of Tunisia and South of Tunisia, respectively (source: Jemaà 2019)

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The genetic characterization of local olive germplasm started since 1983. Studies conducted based on morphological traits, biochemical compounds, and molecular markers highlighted the rich genetic resources of Tunisian olive germplasm as a reservoir for selection of varieties (Trigui et al. 2002). Actually, the Tunisian olive germplasm is characterized by a huge genetic diversity with over 200 varieties distributed in the different regions of the country with 86 million olive trees registered in 2022 (Saddoud et al. 2022). Some of these trees are known as oil varieties, and others are devoted to table. Despite the rich variability of Tunisian olive germplasm, two olive accessions dominate the plantations: “Chetoui” in the North and “Chemlali” in the Centre and the South (Tekaya et al. 2022). On the other hand, some varieties are characterized by special acid composition and oil contents such as “Zarrazi,” “Chemlali Jerba,” “Toffahi,” and “Limouni” but are unfortunately not conserved (Trigui et al. 2002).

2.2 Date Palm (Phoenix dactylifera L.)

Date palms were introduced in Tunisia years BC by the Phoenicians. Several strategies have been concerted to modernize Tunisian oases, given their multidisciplinary potentialities. The date palms growing in southern Tunisia count more than 5 million on more than 40,000 ha of oases (oases of Djérid and Néfzaoua) (FAOSTAT 2020). In recent decades, Tunisia occupies an important competitive position in world date production. Studies conducted to identify the genetic resources of date germplasm showed a considerable genetic variability. The number of date trees cataloged in “le palmier dattier en Tunisie. Le patrimoine génétique” (Rhouma 1994, 2005) was around 300 varieties in 2005. In this revue, the authors noticed the disappearance of some accessions (around 30) previously described as rare genotypes (Rhouma 2005). Molecular markers such as RAPD (Sakka et al. 2004), RAMPO, and AFLP (Rhouma Chatti et al. 2011) highlighted the rich polymorphism of this germplasm. The palm sector is also suffering from the problem of monoculture. In fact, the cultivar “Deglet Noor” represented 76% of the total palm cultivation (Hammadi et al. 2015)

2.3 Orange (Citrus)

It is generally assumed that Persia is the first citrus introduction way to the Mediterranean region in the late fourth century BC. Earlier reports assumed that Citrus fruits arrived in Tunisia in the fourth century BC. The weather and soil conditions in Tunisia are suitable for Citrus cultivation. In 2021, Citrus plantations extended more than 50,372 ha in Tunisia, and fruit production in the last 5 years increased to 156,520 Hg/Ha (DGPA 2016).

Intensive works were devoted to characterize the Tunisian Citrus germplasm. The molecular studies conducted based on nuclear markers such as SSR (Mahjbi et al. 2016), SCOT (Mahjbi et al. 2015), SNP, and GBS (Oueslati et al. 2016a) but also on chloroplast markers such as chloroplast trnL-trnF spacer and trnL intron (Oueslati et al. 2016b; Baraket et al. 2019b).

Moreover, the genetic characterization of Tunisian Citrus germplasm by Snoussi et al. (2012) revealed a considerable genetic richness of limes cultivated in Tunisia. In addition, local limes accessions showed distinctive pomological criteria which accentuate the urgency to protect threatened lime species in Tunisia (Saddoud Debbabi et al. 2020).

2.4 Grapevine (Vitis vinifera L.)

In Tunisia, viticulture was introduced by the Phoenicians, who brought with them their taste for wine. Nearly 2500 years ago, the ancient Carthaginians expanded viticulture extensively in Tunisia (Ben Abdallah 1999). Several native grapevine genotypes, highly appreciated for their interesting characteristics such the production quality, lateness and its fitness to take on the tree, and its adaptation to various Tunisian pedoclimatic conditions (Ghaffari and Ferchichi 2011) are still cultivated in Tunisia, from the “Kroumirie-Mogods” mountains (North-West, humid climate) to the desert region of “Rjim Maâtoug” (South-West, arid climate) (Zoghlami et al. 2001).

The exact number of grapevines cultivated in Tunisia is not known. Gribaa (2008) described 60 Tunisian varieties. However, numerous accessions have been subject of neglect, and they were reported as disappeared. Molecular studies were conducted to identify this germplasm, and some varieties such as “Zizet bagra,” “Ressassi,” “Akhal bouslibit,” and “Bazoul adra” have been restored. Nuclear and chloroplast markers allowed identifying a vast molecular polymorphism between wild and cultivated vines (Snoussi et al. 2004). Autochthonous varieties developed adaptative mechanisms to fit the local bioclimatic and edaphic conditions (Ben Abdallah et al. 1998). However, the introduction of newly bred cultivars has caused a rapid reduction in the number of local genotypes. Thus, the conservation of autochthonous varieties is necessary to develop the viticulture.

2.5 Fig (Ficus carica L.)

The cultivation of fig tree in Tunisia since very ancient times has created numerous varieties that adapted well to local agroecological conditions. The first works addressed to describe the Tunisian fig varieties date back to 1931 by Mionangoin (Minangoin 1931), many of them have been disappeared as a consequence of the intensive urbanization and severe genetic erosion (Gaaliche et al. 2012). A surface of 18,500 ha is devoted to the cultivation of fig trees in Tunisia (DGPA 2019). In 2019, Tunisia ranks fourth worldwide in fig production with 27,400 tons per year (approx. 2.2% of the worldwide production) (FAOSTAT 2019; DGPA 2019). The molecular characterization of Tunisian fig germplasm was largely studied using genomic and cytoplasmic markers. Tunisian fig cultivars are characterized by a rich genetic diversity and are well adapted to local agroecological conditions (Gaaliche et al. 2012). They are repartitioned into three types: the common type, the Smyrna type, and “San Pedro” type. ISSR and RAPD markers were used to identify an important number of accessions (Salhi-Hannachi et al. 2004). In addition, other markers were used such as AFLP (Baraket et al. 2009), chloroplast DNA (Baraket et al. 2010), and ITS (Baraket et al. 2015). These studies allowed restoring some accessions as well as the conservation of this germplasm.

2.6 Apple (Malus domestica)

Apple cultivation occupies a prominent place in Tunisia. In Tunisia, apples are cultivated in an area of 27,310 ha, among them 46% in the North, 40% in the Center, and 14% in the South (FAOSTAT 2021), and the country has 3 million apple trees. In 2021, apple production in Tunisia was 155,000 tons compared to 153,000 tons in 2020. Nevertheless, the culture of apples in Tunisia is mainly devoted to foreign varieties. Local accessions are maintained in gardens. The most known local accessions are “Boutabgaya,” “Aigre de Sfax,” “Douce de Sfax,” “Meski,” “Douce de Djerba,” “Chahla,” “Zina,” and “Aziza” (DGPA 2019). Given the importance of these Tunisian apples and to preserve them, characterization studies began in 2018 at the faculty of sciences of Tunis to identify phenotypic and genetic characterization.

2.7 Pear (Pyrus communis L.)

The Flora of Tunisia has described two species of pear Pyrus communis L. and a wild species, P. syriaca Boiss, that occur in the mountainous regions in Northern Tunisia (Carraut 1986). Local Tunisian pear cultivars are numerous and can be encountered in domestic gardens of the north, center, and even south of Tunisia (Brini et al. 2008; Trad and Taoueb 2021). In the last years, the number of local pear cultivars raised the alarm of extinction some of which have disappeared before genetic characterization. In addition, a dramatic destruction of several hundred hectares of pear plantations occurred in Tunisia in the spring of 2012 caused by the propagation of Erwinia amylovora. Therefore, the total pear production has decreased from 60,000 metric tons in 2011 to less than 20,000 metric tons in 2016 (Gaaliche et al. 2018). As a result, an increased introduction of European and American varieties was remarked (Gaaliche et al. 2018).

2.8 Prickly Pear Cactus (Opuntia ficus-indica)

Once upon a time, the prickly pear cactus, was native to Mexico, and then it traveled to Latin America, South Africa, and the Mediterranean area to settle in Tunisia between the seventeenth and nineteenth centuries (Albergamo et al. 2022). The prickly pear culture occupies a surface of 600,000 ha in 2021, making Tunisia among the first countries in the world with regard to the prickly pear culture (Bettaïed 2021; Albergamo et al. 2022). The long-standing crop of prickly pear cactus in Tunisia has created an age-old know-how. The sector owes its success to the studies using morphologic and molecular markers to identify, characterize, and ameliorate this local richness. RAPD (Bendhifi et al. 2013) and RAMPO (Bendhifi et al. 2015) markers were highly investigated to characterize the Tunisian prickly pear. A recent detailed morphologic characterization was conducted by El Hani et al. (2019), highlighting a rich and exceptional phenotypic variability of Tunisian prickly pear cactus.

2.9 Peach (Prunus persica L.) Batsch

Peach is one of the most important fruit tree species in Tunisia. In 2019, peach production amounted to 136,000 tons (Toumi et al. 2022). More than 100 peach cultivars were described in the official Tunisian catalog in 2008. Nevertheless, these accessions represent introduced varieties; most of them were introduced from the USA. The number of local accessions is not known, and molecular studies are scarce. The phenotypes and genotypes of self-incompatibility system were identified for local Tunisian peach by Abdallah et al. (2020).

2.10 Plums (Prunus sp. L.)

In Tunisia, the plum culture dates back to ancient times and occupied an area of 2800 ha compared to 4300 ha in 2010 (FAOSTAT 2020). The Flora of Tunisia (Nabli 2011) described the presence of Japanese plum species (P. salicina and P. cerasifera [2n = 16]) mostly abundant in northern Tunisia and two wild plum species (P. spinosa and P. insititia [2n = 48]) found in northern and north-western Tunisia (Nabli 2011; Baraket et al. 2019a, b). The local plum varieties are mainly planted in small orchards which induced the disappearance of most of them (Baraket et al. 2019a, b).

Molecular markers were highly investigated to characterize the Tunisian plums, such as chloroplast and mitochondrial DNA CAPS markers (Ben Mustapha et al. 2015). The number of chromosome pairs per accession was identified by flow cytometry (Ben Tamarzizt et al. 2015) and then confirmed by SSR and S-locus markers (Baraket et al. 2019b; Abdallah et al. 2019). A detailed phenotypic characterization of local plum genotypes highlighted the high richness of this germplasm (Baraket et al. 2019b). In addition, the study of the self-incompatibility system in 17 Tunisian plums allowed identifying their S-phenotypes and S-genotypes. Among them, 10 accessions amplified S-genotypes previously defined as rare, and two new genotypes were reported as specific to Tunisian germplasm (Abdallah et al. 2019), highlighting the rich variability of Tunisian plums as an unexplored source of genetic variation (Abdallah et al. 2019).

2.11 Almond (Prunus amygdalus L. = Prunus dulcis)

Almonds have been cultivated in Tunisia since ancient times, most probably since the Carthaginian era. Almond trees occupy an area of approximately 320,000 ha with around 20 million trees extended in the central and northern parts of the country (Maatallah et al. 2022). Molecular studies were largely investigated to identify local almond accessions. These works confirmed the large genetic diversity of local germplasm, but also described new, previously unknown landraces (Gouta et al. 2019). Furthermore, the local Tunisian almond cultivars showed high resistance potential to drought conditions (Gouta et al. 2019).

2.12 Pistachio (Pistacia vera L.)

Pistachio (Pistacia vera L.) is one of the oldest fruit trees growing in Tunisia and its culture increased over the last 20 years. With 30,000 ha of cultivated area, Tunisia is the seventh largest pistachio producer in the world. The local cultivars and ecotypes selection has long been the worries of the country by establishing a national germplasm collection (Chouk et al. 2021).

Several studies have been conducted based on molecular, morphologic, and biochemical markers to explore the genetic polymorphism in Tunisian pistachio germplasm. This germplasm is characterized by an important genetic diversity as revealed by SRAP markers (Guenni et al. 2016), sequence variation of the intron trnL (UAA) (Choulak et al. 2015), and CDDP (Aouadi et al. 2019). These studies aimed to characterize and conserve the Tunisian germplasm of pistachio. In fact, some genotypes such as cv. “Sfax,” which have disappeared and are largely cultivated in California, were reintroduced in Tunisia (Aouadi et al. 2019).

2.13 Grenades (Punica granatum L.)

The pomegranate was introduced to Tunisia several eras back (Evreinoff 1949) and was scattered all over the country except for areas above sea level. The production of grenades reached 75,000 tons in the 2018–2019 seasons (Mekni et al. 2019).

3 Threats and Statement of Problem

Despite the rich genetic variability of the agricultural system in Tunisia, the arboricultural sector faces numerous challenges. First of all, Tunisian fruit tree landraces were the subject of neglect and underutilization. The worldwide spread of improved materials has resulted in a rapid replacement and a dramatic loss of landraces and local accessions (Cirilli et al. 2020). More generally, the problem of native vegetables loss has been observed in many places around the world, making threatened their persistence into the future and the delivery of their benefits to society (Meldrum et al. 2018). Second, monoculture represents also a great threat for arboriculture in Tunisia. For example, many date farms with important nutritional and economic value are disappearing as a result of the monoculture of the cultivar “Deglet Noor” (Hammadi et al. 2015).

The agricultural sector highly depends on climate risks: rising temperatures and varied precipitation levels, coupled with increasing extreme events, such as floods and droughts (Verner et al. 2018). Tunisia is not immune to the degradation and desertification problems (Msadek and Tarhouni 2021). According to the National Environmental Report of Tunisia, 74% of the country is at risk of soil erosion. The last 3 years (2020, 2021, 2022) showed dramatic and continuous drought-threatening agricultural sections. The impact of climatic changes is already observed and noted in almost all fruit trees. To combat desertification, the Tunisian Government has established a guidebook for the 12 local governments (Zaghouan, El Kef, Siliana, Kasserine, Kairouan, Sidi Bouzid, Gafsa, Tozeur, Kebili, Tataouine, Medenine, Gabes) in the south (You et al. 2016). Below, we will present an overview of the solutions and policies adopted by the Tunisian Government, as well as the main activities conducted by the scientific communities to cope with the problem of genetic erosion.

4 Strategies and Policies Implemented

“The State shall provide the means necessary to guarantee a healthy and balanced environment and contribute to the climate’s integrity.” Article 44 of the Tunisian Constitution (version 2014) engaged the Tunisian Government to develop policies and strategies in favor of environmental protection and climate action. When it comes to biodiversity protection, Tunisia was and is ahead of most Middle East and North Africa (MENA) region countries (Knaepen 2021). In 1976, Tunisia ratified the Algiers Convention on Natural Resource Conservation. Since that, many measures have been adopted focusing on numerous conventions and protocols as highlighted in Table 1. Most interesting perhaps is that, in 1992, Tunisia was engaged to establish a national strategy and conservation action plan in the UN Convention on Biological Diversity (Souissi 2001). Furthermore, Tunisia was the first North African country to formally ratify the United Nations Framework Convention on Climate Change (UNFCCC) in 1993 and the Kyoto Protocol in 2002. It also prepared two National Communications to the UNFCCC (2001, 2014) and ratified the Paris Agreement in 2017 that entered into force during the same year (UNFCCC 2015). In 2012, Tunisia launched its National Climate Change Strategy, developed by the Ministry of Local Affairs and Environment (Knaepen 2021).

Table 1 Major conventions and ratification date of Tunisia
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A National Development Strategy 2016–2020 was prioritized advancing the green economy as the driver of sustainable development. The critical technical and financial actual situation of Tunisia would make it difficult to establish practical solutions despite its engagements of climate policies’ implementation. Thus, a connection between the government, institutions of research, scientific community, and farmers was established for a long time. The role of research centers (institutions, universities, laboratories) is crucial to unlock the potential of agricultural innovation, achieving sustainable development goals, protecting the environment, and preserving landraces. For decades, research activities about crop identification, characterization, and protection kept increasing. To support research activities, scientific relations between Tunisia and foreign parts have intensified in recent years. The publications addressed at studying and evaluating the local genetic resources and developing straightforward strategies are uncountable.

4.1 Inventories

Effective biodiversity conservation and management require a continuous state of knowledge of the existing species as well their states (Stephenson and Stengel 2020). The need for inventories as a basis for planning sound conservation strategies is well recognized (El Mokni et al. 2022) The first inventories of the flora of Tunisia, in general, and the fruit trees, in particular, started since the 30s with Hodgson (1931) and then 1937 by “le laboratoire d’arboriculture fruitière du Service Botanique et Agronomique de Tunisie” (Valdeyron and Crossa-Raynaud 1950), Flora of Tunisia (Cuénod 1954; Pottier-Alapetite 1979, 1981), les cartes phytoécologiques de la Tunisie (Gounot 1958), flore de Tunisie (Nabli 1989). Being aware of their indispensability, the inventories occur typically and permanently by the researchers and students of natural and agricultural sciences.

Most recently, the inventory by El Mokni et al. (2022) showed that 85% of the Tunisian flora is potentially useful as a source of genetic diversity for crop improvement and/or as medicinal and food uses. Indeed, the inventory developed includes 2468 crop wild relatives and/or wild harvested plant taxa, which is about 40% of what is reported for the North African region as a whole (El Mokni et al. 2022).

4.2 Characterization

Numerous accessions remain taxonomically unknown. Studies conducted to characterize these species were mainly based on morphologic and pomological characterization and/or based on molecular genetic markers. In addition, biochemical characterization allowed the highlighting of huge and particular traits in Tunisian fruit tree landraces.

4.2.1 Morphologic and Pomological Characterization

Several studies were conducted using pomological traits to evaluate the genetic diversity of Tunisian fruit trees. The first works were conducted in 1983 with a pomological characterization of Tunisian olive trees. Most recent data collected allowed the establishment of passport data for plums (Baraket et al. 2019b), lime Citrus germplasm (Saddoud Debbabi et al. 2020), grenades (Mekni et al. 2019), and prickly pear cactus (El Hani et al. 2019). Findings obtained based on morphologic characterization allow enhancing both “ex situ” and on-farm genetic conservation programs of Citrus germplasm (Saddoud Debbabi et al. 2020). The main limitation of this technique is that morphological traits are strongly influenced by environmental factors (Baraket et al. 2019b) and are insufficient to identify the classification of species. Therefore, more accurate information can be better obtained with biochemical and molecular data to provide a reasonably detailed and comprehensive account of cultivars’ identification and authentication.

4.2.2 Biochemical Characterization

Studies were conducted to explore and compare the biochemical properties of trees in Tunisia. These findings highlighted the rich biochemical compounds of Tunisian trees and enhanced their qualities from a biochemical point of view. For instance, local cultivars of almonds showed high tolerance to drought conditions. As for the results of flavonoids, local cultivars showed the highest values in flavonols, which revealed the highest content of O-diphenol (Maatallah et al. 2022). Another importance of biochemical characterization is to study the response to biotic and abiotic stresses. These works were numerous, and many cultivars were revealed as tolerant or resistant to stress such as the case of plum accession “Chaaraouia” (Prunus domestica) identified as resistant to water deficiency (Baraket et al. 2021). On the other side, some local cultivars showed low qualities limiting their competitiveness toward exportation. Thus, these cultivars gain special attention for the enhancement of quality. For example, “Chemlali,” a well-appreciated olive variety, contains however high levels of palmitic and linoleic acids and low levels of monounsaturated fatty acids (especially oleic acid) and phenolic derivatives, which could influence the Tunisian olive oil quality. For this reason, Tunisia has encouraged the plantation of several Mediterranean varieties (Tekaya et al. 2022).

4.2.3 Molecular Characterization

Molecular markers have been considered for a long time as the best tool for crop characterization, barcoding, and population structure. Among the key success factors of molecular markers is their polymorphic character as well their independence to developmental stages and environmental conditions (Saddoud et al. 2022). There has been lot of interest in using molecular markers, to evaluate the level and structure of genetic variation within fruit tree species. One of the pioneer markers for plant molecular characterization studies is restriction fragment length polymorphism (RFLP), extensively used in molecular works (Baraket et al. 2017).

Advanced more reliable and polymorphic markers were highly used by researchers like AFLP (amplified fragment length polymorphism), chloroplast and mitochondrial DNA CAPS markers (Ben Mustapha et al. 2015 for plums), SSR (simple sequences repeat) (Ouni et al. 2020 for pear; Ben Abdallah et al. 2020 for date palm), SNP (single nucleotide polymorphism) (Ben Ayed and Rebai 2019 for olives), SRAP (sequence-related amplified polymorphism) (Guenni et al. 2016 for pistachio), SCoT (start codon targeted) (Mahjbi et al. 2015 for citrus), and CDDP (conserved DNA derived polymorphism) (Aouadi et al. 2019 for pistachio). With the advent of next-generation sequencing (NGS) technology, the genotyping-by-sequencing (GBS) approach was used in many important crop genomes and populations such as Citrus (Oueslati et al. 2017) for discovering and genotyping SNPs.

4.3 Conservation

As a principle of the conservation strategy, there have been concerted efforts on the part of the government, scientific community, and farmers to cope with the problem. Two methods have been established for the conservation of Tunisian arboriculture genetic resources: “in situ” and “ex situ” (Saddoud Debbabi et al. 2020).

4.3.1 In Situ Conservation

In situ conservation may be defined as the maintenance of viable populations of species in their natural habitats (Marone et al. 2021). The establishment of national collections is a methodology adopted by Tunisia as a genetic reserve of the fruit trees’ germplasm. As a sort of sustainable management of genetic diversity, existing parks have been maintained and others natural reserves have been build up (DGF, Ministry of Agriculture, and Ministry of the Environment), all containing flora on the way to extinction (Souissi 2001). Today, Tunisia has 17 national parks (Table 2), 27 natural reserves, 20 protected sites, and 40 RamsarFootnote 1 sites, all aimed to human welfare, poverty alleviation, and sustainable development (UNESCO 2023).

Table 2 National parks established in Tunisia
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According to Ferchichi (2013), among 237,000 ha of the protected zones, 250,000 ha are national parks. One of the oldest botanical gardens in Tunisia is the National Arboretum of Tunis, which age dates back to 1891. “Bou-Hedma” National Park, created in 1980 to preserve arid plant communities and fauna, contains the unique Acacia tortilis (Forssk.) in Tunisia (Msadek and Tarhouni 2021). The protection of Tunisian oases represents also a prerequisite for the scientific community. One of the important factors of these oases resides in the old permaculture emergence by our ancestors, associating many fruit trees and vegetable plants in the same oasis (Ferchichi 2013). Permaculture, one of the newest concepts of sustainable agriculture, was already observed 2000 years ago when Tunisians combined various crops in the same space, on several levels, under the protective shade of all trees.

4.3.2 Ex Situ Conservation

The oldest and ideal form of Ex situ conservation is the creation of core collection which is a subset of species with important genetic diversity and limited redundancy (Cirilli et al. 2020). In Tunisia, the first core collections were established for olive in 1940 and peach in 1954. During the last decade, the number of private collections increased remarkably; most of them are, however, devoted to commercial varieties. The local accessions preserved in core collections are only those of great economic importance. For instance, in “Nefzaoua,” many modern oases have been established showing a trend toward the monoculture of the “Deglet Noor” cultivar (Hammadi et al. 2015). Four stations, located in “Degache,” “Ibn Chabbat I,” “Ibn Chabbat II,” and “Atilet,” are maintaining live specimens of 100 endangered cultivars (Hammadi et al. 2015). Another example is the “Boughrara” collection which contains the most important Tunisian olive varieties, such as “Chemlali,” “Chetoui,” and “Chemchali.” The core collections maintaining the local grapevine were established in four areas: Cape Bon (Grombalia, Takelsa, Kelibia, Baddar), Tunis (Sidi Thabet, Mornag, Tebourba, Ariana), Bizerte (El Alia, Ras Jebel, Metline, Raf Raf), and Béja (Jendouba Tibar, Sedjnane) (Ben and Ghorbel 2000).

Table 3 describes the most important core collections in Tunisia. For some species such as plum, peach, apple, and pear, there are no core collections, to the best of our knowledge, devoted to local fruit tree accessions. The conservation of these landraces is maintained thanks to the concerted efforts of farmers (Fig. 2).

Table 3 List of core collections of fruit trees established in Tunisia for ex situ conservation
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Fig. 2
2 photos. A depicts a farm of plum trees with small trunks and clustered leaves. B depicts a farmer working on a cactus farm.

Example of farms planting local varieties of fruit trees. (a) A farm of plum trees in “Raf Raf” in Northern Tunisia (photo by D.A.). (b) A farm of prickly pear cactus in “Kasserine” in central-western Tunisia (source: Bettaïed 2021)

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Therefore, these collections provide, on the one side, a natural laboratory allowing studying the evolution of many morphological, biochemical, and genetic traits as response to climate changes, and on the other side, they ensure the economic incomes for the farmers (Marone et al. 2021). The good management of the ex situ and in situ conservation policies takes advantage of the research laboratories, such as those of the Institute for Arid Regions at Medenine; National Research Institute for Rural Engineering, Water and Forests (INRGREF); National Institute for Scientific and Technical Research (INRST); National Agronomic Education and Research Institution (IRESA); National Research Agronomic Institute in Tunisia (INRAT); National Agronomy Institute in Tunisia (INAT); and the regional offices of the Agriculture Development of the Ministry of Agriculture (CRDAs) for citrus and dates (as well as cereals, potatoes, and organic agriculture), and the universities such as Faculty of Sciences of Tunis.

Numerous inventories and research studies, conducted by the team of “Fruit Trees Valorization” at the Faculty of Science, allowed the establishment of a DNA bank of many crop species. These institutions provide the necessary data related to the status of biodiversity and develop new technologies, research findings, and innovations. In 2003, Tunisia created the gene bank of Tunis, which conserves most of the genetic resources.

4.4 Improvement and Amelioration

The Tunisian Government has set up a series of strategies and programs to improve the agricultural sector, in particular in the field of fruit tree cropping. Numerous breeding programs were conducted to assure a better quality of the Tunisian species such as looking for adequate pollinizers or increasing the resistance. The first amelioration programs date back to the 1930s. For example, the first amelioration program started in 1930 (INRAT) to obtain apricot with staggered fruiting. Some intercrosses between local and introduced cultivars started in 1954 resulting in the emergence (1970–1974) of “Jazil,” “Ouardi,” “Sayeb,” “Amal,” and “Ezzine.” The first apple breeding program in Tunisia was conducted in 1960 at the INRAT institute, which aimed to create varieties adapted to local conditions (Trad et al. 2015).

In 2013, an international project “Promotion of sustainable agriculture and rural development in Tunisia” (“PAD”) was implemented by GIZ and the Ministry of Agriculture, Water Resources and Fisheries to improve apricot production of the rural population in the center-west of Tunisia. As a result, new markets of sun-dried apricots were created and oriented toward the exportation of dried apricots (Lachkar et al. 2021).

More than 1600 plantlets were produced by plant tissue culture methods such as micropropagation and somatic embryogenesis. These methodologies represent an ideal way for rapid propagation and genetic resources conservation. “Deglet Bey,” “Boufeggous,” “Gondi,” and “Cheddakh” are examples of Tunisian palm cultivars preserved by plant tissue culture methods (Hammadi et al. 2015).

5 Achievements and Perspectives

Adaption to climate change, protection of natural resources, and conservation of the landscape are the most important challenges faced by Tunisia’s arboriculture. In response, several planned actions and programs have been drawn up in Tunisia. The Tunisian achievements regarding the fruit tree resource conservation strategies are significant. “Ichkeul” National Park was included in the UNESCO’s list of World Heritage Sites in 1980, and other national parks are in the UNESCO tentative list of World Heritage Sites like “El Feidja National Park,” “Bou-Hedma National Park,” “Chott el Djerid,” and “Oasis de Gabes.” “Chambi National Park” was declared a UNESCO biosphere reserve in 1981.

Two Tunisian regions have been recognized as Globally Important Agricultural Heritage Systems (GIAHS), which aim to identify and preserve agricultural systems of global importance with their landscapes, agrobiodiversity, traditional knowledge, and associated culture. The first was Gafsa Oases in 2011 and then, in 2020, Tunisia’s traditional “Ramli” agricultural systems in the lagoons of Ghar El Melh and its hanging gardens from “Djebba El Olia,” situated in the hills of northwestern Tunisia (Fig. 3). “Through the use of natural geological formations and the use of stones, local communities have been able to transform the landscape into fertile and productive lands,” the UN agency said. The FAO praised the diversity of local crop varieties grown by the area’s farmers, as well as their use of wild plants to repel potential pests and livestock to “plow” and fertilize the soil (FAOSTAT 2020).

Fig. 3
A photo of a hanging garden. The trees are cluttered with stones intersecting in the middle.

The hanging gardens of “Djebba El Olia” in northwestern Tunisia. (Source: https://www.fao.org/giahs/giahsarounstheworld/designated-sites/near-east-and-north-africa/hanging-gardens-from-djebba-el-olia/fr/)

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Tunisia has recently obtained a “Protected Designation of Origin” (PDO) quality label for “Téboursouk” olive oil (March 2020, records of the World Intellectual Property Organization (WIPO)).

6 Conclusion and Prospects

One of the most important keys to preserve plant biodiversity is the sustainable management of genetic resources. Tunisia, called “the Green,” owes this qualification to the abundance of greenery and the extensive forest areas. The Tunisian local fruit trees represent a cultural authenticity and a genetic heritage. Intensive studies showed a wide genetic diversity for several Tunisian fruit trees. The importance of this local germplasm resides in their adaptation to severe conditions such as water scarcity as well as the presence of unique and rare genotypes. For that, many efforts have been concerted to better conserve this natural richness for future generations. Thus, a vibrant research ecosystem and scientific programs were conducted to investigate and identify these genetic resources. Many DNA banks were created in different research units at the universities and research institutes. In addition, national programs were traced to preserve fruit tree landraces by implementing many collections and natural parks that hold a wide genetic diversity for several fruit trees.

Further policies and efforts must be drawn to identify the rich genetic heritage of the fruit trees’ germplasm. The conservation and valorization of the local species need concerted efforts of research institutes, farmers, students, industries, and also regional and community-based organizations.

In conclusion, this report describes, to the best of actual knowledge, the status of local tree fruit biodiversity and genetic resources in Tunisia. We presented an insight into the main threats, limitations, and policies toward conservation. Being conscious of the importance of this heritage, scientists, farmers, and social organizations need to collaborate to train, enhance, and boost the awareness of Tunisian tree patrimony.