The genotypic variation was significant for all leaf morphological traits assessed (Table 1). The Student-Newman-Keuls (SNK) test highlighted four distinct groups of cultivars with regard to mean leaf area (LA). The highest LA values were recorded in “Nabout” (202.11 cm²) and “Aboucharchaou” (334.14 cm²). However, the lowest values were shown by “Brown Turkey” (99.27 cm²), “Jeblia” (110.62 cm²), “Bousbati” (113.01 cm²), “Kadota” (114.89 cm²), and “Chetoui” (115.75 cm²). Several studies affirmed that a small leaf has long been considered a typical mechanism for drought adaptation in many woody species, due to better transpiration regulation [17]. Conversely, a large leaf is generally a sign of sensitivity to water stress. In addition, in previous work on olives, a low LA was singled as a relevant marker to screen cultivars for drought tolerance [18]. According to these findings, low leaf area observed in certain fig cultivars constitutes an asset for them to be drought tolerant.

Leaf blade thickness (LBT) varied from 370.33 to 739.33 µm, with 15 dissimilar groups revealed by the SNK test. Among the fig cultivars studied, “Aicha Moussa”, “Abgaiti”, and “Cuello Dama Blanca” presented the thinnest leaves (less than 470 µm), while the thickest ones were observed in “Hamra” and “Hafer El Brhel” (more than 700 µm). Plants with small and thick leaves are associated with low transpiration, which constitutes an advantage for their adaptation to drought [19]. Within the fig collection studied, the two cultivars, “Chetoui” and “Jeblia” are the closest to meeting this condition. However, some studies highlighted that the thickness of the leaf blade is more related to water loss than its area while segregating between cultivars about drought tolerance. The thicker the blade, the thicker its palisade parenchyma, making dehydration caused by water stress reversible [20,21].

Stomata are the seat of gas exchange, including water vapor and by this role, their characteristics are relevant in discrimination between the genotypes for drought tolerance. Indeed, there is a consensus that a low stomatal density (SD) is associated with a high water-use efficiency under drought conditions [22]. This can be the case of the two fig cultivars, “White Adriatic” and “Abgaiti”, which showed the lowest SD of around 40 stomata mm² in average. On the other hand, the cultivar “Nabout”, “M’tioui”, and “Ounq Hmam” can be less efficient as they showed the highest SD of 99.20, 88.41 and 86.05 stomata mm⁻², respectively. The values for these latter cultivars are in the interval reported by Bercu and Popoviciu [23] for the majority of the fig cultivars of 70–100 stomata mm⁻². However, it was reported that drought tolerance is rather associated with smaller stomata, owing to their rapid closure in response to water stress, compared to large stomata [24]. Within the fig collection studied, stomatal size varied widely, from 376.95 µm² in “Hafer Jmel” to 662.74 µm² in “Aicha Moussa”, thereby making this stomatal trait a relevant descriptor of drought-tolerant cultivars.

Leaf trichomes constitute a physical barrier to water loss through transpiration, and therefore, they are influential in plant performance under drought conditions [25]. In general, leaves containing large and numerous trichomes constitute an advantage for plant drought tolerance [26]. Among the fig genotypes studied, “Brown Turkey” fulfilled this condition well since it showed the highest trichome density of 79.29 trichomes mm⁻², with an average hair length of 309.13 µm. The lowest trichome densities were found in “Jeblia”, “Aicha Moussa”, and “Ournaksi”, containing less than 30 trichomes mm⁻². The hair trichome varied from 195.16 in the genotype “Aicha Moussa” to 402.78 µm in “Fassi”. The value ranges are in agreement with those reported in previous similar studies [27,28].

Relative water content (RWC) is a relevant parameter for assessing plant response to water stress [29]. Within the fig collection studied, the differences were significant in RWC, with range values of 50.23%–76.14% in June and 42.36%–73.97% in July (Table 2). The highest RWC was shown in the genotype “Fassi”, with averages of more than 70% in June and July, while the lowest value was observed in “El Qoti Labied” with an average of 50.23% in June, which decreased to 42.36% in July. Indeed, many genotypes exhibited a decrease in RWC from June to July. Alongside “El Qoti Labied”, a noticeable decrease in RWC between the two measurement dates was shown in the genotypes “Assel”, “Filalia”, “Cuello Dama Blanca”, “Bousbati”, “Kadota”, “Aboucharchaou”, and “Ournaksi”. Contrary, other genotypes such as “White Adriatic”, “Chaari”, and “Hafer El Brhel” maintained a relatively stable RWC. This suggests that there were genotypic differences in leaves’ ability to regulate transpiration and in resistance mechanisms to water stress damages [30,31]. However, in the two genotypes, “Hafer El Brhel” and “Ferqouch Jmel”, an increase in RWC was observed instead. This finding could be explained by leaf blade thickness in this genotype, which was the highest in the collection studied, giving it resistance to water loss [32].

Stomatal conductance (gs) is an important indicator of plant water status, which corresponds to the amount of gas that can be exchanged through the total open stomata [33]. High values generally indicate a sensitivity to drought and vice versa [34–36]. Accordingly, the three cultivars “Bioudi”, “Aboucharchaou” and “Zerqa” could present a high plasticity to drought as they displayed the lowest gs value of around 100 mmol m⁻²s⁻¹ in average. However, in similar studies, it was reported that a low stomatal conductance is often associated with a low photosynthesis rate, thereby compromising the water use efficiency of the genotype [37]. Low gs is generally attributed to rapid stomatal closure in response to water deficit, representing a drought tolerance mechanism. On the other hand, the highest gs values were shown by “Kalamata”, “Ournaksi”, and “Brown Turkey” with an average of 503 mmol m⁻² s⁻¹. The maintenance of high levels of stomatal conductance under moderate water stress could confirm that these cultivars are sensitive to drought.

Leaf temperature is a key factor in triggering mechanisms regulating gas exchange at the stomatal level [38]. In well-irrigated plants, leaf temperature may be 1°C to 2°C lower than air temperature. However, in response to drought, it becomes close to the air temperature, or even exceeds it, due to transpiration decrease [39]. Thus, based on leaf temperature values taken under air temperature of 31.4°C, it appears that 70% of the fig genotypes were not able to ensure cooling of their leaf surfaces under water stress applied, displaying a leaf temperature of up to 3.3°C higher than the air temperature. This indicates a sensitivity of the genotypes to water stress, as they are unable to evacuate excess heat via transpiration. Among the other genotypes, only “El Qoti Labied” seems unaffected, with a leaf temperature 4.8°C lower than the air temperature.

Water loss from detached leaves (WLDL) was used in many studies assessing plant drought tolerance as a relevant physiological indicator, which is often linked with leaf cuticular resistance against dehydration and stomata closing speed in response to water stress [40,41]. WLDL was significant for all leaves samples after 1 h under ambient temperature (25°C). It was particularly important in “Brown Turkey” and “Napolitana Blanca”, which reached an average of 50% of leaf moisture. However, the three genotypes “White Adriatic”, “Hafer El Brhel”, and “Zerqa” stand out with a reduced water loss of less than 30%. This indicates that the leaves of these cultivars are better adapted to water retention, which could delay water stress damage on their tissues. Based on the leaf traits assessed herein, this adaptation may be related to their thick blade and high content in trichomes. According to Cominelli et al. [42], these leaf structural traits can confer high cuticular resistance to water loss.

Biochemical compound contents in leaves are among leaf functional traits, which express the magnitude of plant response to water stress. Chlorophyll is among the most vital of these compounds due to its direct involvement in photosynthesis, and its reduction constitutes an apparent sign of a plant’s exposure to water stress [43,44]. Within the fig collection, chlorophyll content index (CCI) varied widely in the range of 29.43%–47.50%, indicating the existence of genotypic differences in chlorophyll accumulation and photosynthetic capacity (Table 3). The highest CCI value was displayed by “Nabout”, while the lowest one was observed in “Aicha Moussa”. A high CCI level suggests that the genotype might have an adaptive behavior to ensure the efficient functioning of its leaves under stressful conditions. Based on the structural traits herein assessed, the thick leaves that characterize the genotype “Nabout” could partly explain this behavior. In fact, the thickest leaves may contain several layers of mesophyll, which increase chlorophyll concentration and improves light absorption efficiency. Note that similar studies reported that high CCI values are linked to full leaf expansion, while low values may be linked to the beginning of leaf senescence [32,45–47], added that variability in chlorophyll content can be generated by differences in nutrient uptake, especially nitrogen, iron, and magnesium.

Cuticular wax accumulation in leaves constitutes a biochemical adaptation mechanism to drought. It is a complex hydrophobic substance that covers the leaf surface and forms a protective barrier between the plant and its environment, insulating plant tissues from harsh external conditions [48,49]. The waxy layer of the leaf cuticle is made up of very long-chain aliphatic lipids (alkanes, primary and secondary alcohols, esters, sterols, and flavonoids) [50]. This leaf waxy coating plays an essential role in plant tolerance to abiotic stress, including drought, through reduced transpiration and also in reducing attacks of pests and diseases [51]. Cuticular wax content (CWC) in fig leaves varied widely, from 38.60 mg g⁻¹ in the genotype “Kalamata” to 202.37 mg g⁻¹ in “Cuello Dama Blanca”. Cuticular wax biosynthesis occurs at the cuticle, which is generally not related to the leaf structural traits. It is regulated by complex genetic expressions linked to various waxy aliphatic compounds [52], which seem to be differently increased by water stress depending on fig genotypes. In general, high leaf CWC constitutes an advantage for plant drought tolerance by reducing water loss through transpiration, particularly in conditions of low atmospheric humidity or soil water deficit [50,53]. In a similar study on olive, it was singled as a relevant biochemical marker for drought tolerance alongside soluble sugar content [18]. The latter trait is often included in similar studies as a mechanism indicator of plant response to water stress. Soluble sugar content (SSC) in leaves generally increases in response to water stress, as an osmotic adjustment strategy to maintain cell turgor, which is induced by the expression of various specific genes [54,55]. Hence, an increased leaf SSC strengthens the plant’s plasticity to drought [56]. Within the fig collection studied, variation in SSC was highly significant, displaying 21 levels according to the SNK test. Maximum values were found in “Nabout” and “Aboucharchaou”, with 92.40 and 93.45 mg g⁻¹, respectively, while the lowest ones were exhibited by “White Adriatic” (28.35 mg g⁻¹) and “Ferzaoui” (28.50 mg g⁻¹). Genotypes that tend to accumulate higher levels of SSC may have improved drought tolerance through more active osmotic adjustment mechanisms or more efficient sugar metabolism. SSC content is a reliable marker for assessing the adaptability of genotypes to water stress, and its variability among fig cultivars could be used in breeding programs for drought-tolerant varieties. Phenolic compounds are secondary metabolites whose biosynthesis becomes more activated in response to water stress as a defense mechanism. They are antioxidants that protect plant cells against oxidative damage caused by reactive oxygen species (ROS) [57]. Contrary to SSC, there was non-significant difference in total phenolic compounds (TPC) content between the fig genotypes, with an overall average of 7.57 mg g⁻¹. This leaf biochemical trait, therefore, has no impact on discrimination between the fig genotypes herein studied with regard to drought tolerance. Note that similar studies reported significant differences in phenolic compounds, as found by Petruccelli et al. [58] on a collection of Italian fig varieties. The contradictory results concerning this trait could be explained by differences in the genetic pool studied, but also in the environmental conditions.

Proline accumulation in the leaf is considered among the adaptive strategies observed in fig trees to cope with water stress [59]. Proline is an amino acid that contributes in leaf structure stabilization under drought conditions [60]. In addition, it enables pH regulation and can serve as a carbon and nitrogen reserve for plants during a stress period [61]. There was a notable difference in leaf proline concentration among the fig genotypes studied. Some of them showed high proline levels, such as “Zerqa” and “Kalamata”, with respective average values of 23.52 and 21.37 µmol g⁻¹. The lowest values were found in “Bousbati” and “Bioudi”, of 5.07 and 5.56 µmol g⁻¹, respectively. The genotypes exhibiting high leaf proline accumulation may have a greater ability to withstand water stress than those with low proline levels [62]. Several previous studies have underlined that the increased biosynthesis of proline in plant cells enables the recovery of reactive oxygen species (ROS), which play an important role in the plant’s defense system against drought-induced oxidative damage [63].

Correlations between variables allows to predict the relationship between them and to identify among them the traits that can be potentially used as drought tolerance markers. The bilateral correlations within leaf morphological traits and their connections with physio-biochemical traits are summarized in Table 4. It emerges that leaf area is negatively correlated with blade thickness, thereby indicating that smaller fig leaves are thicker and vice versa. The significant correlation between these two traits seems obvious in most plants. A reduced leaf area combined with increased leaf thickness constitutes a structural strategy for adapting plants to stressful situations [19]. This can be proven by the positive and significant correlation herein observed between leaf thickness and chlorophyll concentration (r = 0.387). However, fig genotypes with thicker leaves exhibited a significant decrease in leaf water status, more pronounced than genotypes with thinner leaves, as can be deduced from the negative correlation between leaf thickness and RWC (r = −0.415). It appears that a high blade thickness constitutes a favorable structural trait to maintain chlorophyll concentration relatively stable, even at low water potential levels. In this sense, Taratima et al. [64] reported that in thick leaves, chlorophyll concentration is high, the reduction of which under water stress does not severely affect the chloroplast activity. Based on these results, leaf thickness seems to be associated with plasticity to drought in fig trees.

As for trichomes, the correlation matrix shows that their hair length is more related to drought tolerance degree than their density, as can be presumed by its positive and significant correlation with chlorophyll concentration (r = 0.369). It is known that a high coverage of leaf surface by trichomes is associated with a high drought tolerance level by regulating transpiration [65,66]. According to the result herein found on the fig tree, the leaf surface coverage rate by trichomes seems to be more determined by their hairs, especially when they are longer. Up to then, the trichome hair length, alongside the blade thickness appears to be leaf structural traits that are significantly involved in fig drought tolerance owing to their correlations with chlorophyll content. The involvement of the other leaf structural traits in drought tolerance could not be highlighted based on the results obtained. Among these, leaf area and stomatal density allow to predict richness of leaves in soluble sugars, with highly significant positive correlation coefficients. This indicates that soluble sugar accumulation is higher in leaves that are large or contain a high stomatal density. High leaf SSC may be linked to the requirements in metabolite translocation generated by fruit, as it may be an adaptive response of the genotype to stabilize leaf water potential under increased transpiration [67].

On the other hand, two important observations emerge from the correlation matrix between the physio-biochemical traits in Table 5. The first one concerns the non-significance of correlations between chlorophyll content and all of them. Therefore, water stress’s effect on leaf water status was not intense enough to induce a decrease in chlorophyll concentration. This functional behavior in fig leaf has been reported on other known drought-tolerant species, which are able to maintain their photosynthetic activity under severe water stress, such as olive [68], carob, eucalyptus, Moringa [69], and jujube [70]. The second observation is related to the significant negative correlation between relative water content and leaf temperature. This result suggests that leaf temperature can be used as a reliable indicator for easy and quick assessment of fig tree water status. It is, therefore, a typical functional relationship in fig leaf since similar studies on other species have not reported this relationship to be significant, as observed in peach, almond and plum [13].

Among the measured leaf traits, chlorophyll concentration emerges as a pivotal factor in clustering fig cultivars according to their drought tolerance level, given that its decline signifies an advanced stage of water stress effects on both physiological and biochemical characteristics [68,71]. Furthermore, these traits exhibited a comparable impact on discrimination among the cultivars, as shown by principal component analysis (PCA), 77.84% of the total variance is explained in seven principal components (Table 6). The first two principal components represent 33.31% (PC₁ = 17.74% and PC₂ = 15.57%). These results highlight the complex interaction of structural and functional traits in drought resilience. Generally, the obtained results reveal that stomatal traits, water status, trichome characteristics, and biochemical properties are the primary drivers of variation in the dataset. Thus, to effectively assess the drought tolerance of the 30 fig cultivars, they were initially grouped into four primary clusters according to the chlorophyll concentration index (CCI), which proved to be a discriminating primary criterion for the selection of water-stress tolerant genotypes. Subsequently, within each primary cluster, the cultivars underwent classification using the unweighted pair group method with arithmetic mean (UPGMA), relying on the average values of RWC, gs, SSC, and proline content (Fig. 1). These four traits were chosen given their significant correlations with the other physiological and biochemical parameters, as outlined in Table 7. These traits were distinguished as secondary discriminating indicators for genotype classification.

The first main group (G1), classified as the most sensitive to drought due to its low chlorophyll concentration, is subdivided into two subgroups. The first subgroup (G1.1) contains 7 cultivars, including “White Adriatic” and “Chetoui”, widely cultivated in the Mediterranean. Meanwhile, “Nabout” and “Brown Turkey”, also common cultivars, are classified in a distinct subgroup (G1.2), relatively less sensitive to water stress, compared to G1.1. The differentiation of these two cultivars is linked to their stomatal conductance, which was twice as high as that of subgroup G1.1, as well as their soluble sugar content, which was 38% higher. The second main group (G2) comprises 8 locally less widespread cultivars, classified as moderately sensitive to water stress. Within this group, the cultivar “Kalamata” is notably distinguished due to its superiority in terms of relative water content, stomatal conductance and proline accumulation. The third main group (G3), categorized as moderately tolerant to water deficit, comprises 6 cultivars divided into two subgroups. The first subgroup (G3.1) encompasses the two familiar cultivars, “Cuello Dama Blanca” and “Ghoudane”, along with 3 local cultivars. However, the second subgroup (G3.2) consists solely of the cultivar “Ournaksi”, distinguished notably by its significantly higher stomatal conductance, approximately three times greater than that of subgroup G3.1. Finally, the fourth main group (G4), the most drought-tolerant within the studied fig collection due to its chlorophyll concentration index being less affected under water stress, contains 7 cultivars. Among them, “Kadota” and “Royal Blanck” are two exotic varieties, ranked alongside five local cultivars, among which “El Qoti Labied”, “Chaari”, and “Fassi” are the most locally known. Within this group, the exotic variety “Kadota” and the local cultivar “Ferqouch Jmel” showed high stomatal conductance values, thereby classifying them into a distinct subgroup (G4.2) as the most drought-tolerant genotypes within the studied collection.

This ranking of cultivars is in line with that found by Oukabli et al. [10] for 7 cultivars herein studied, including “El Qoti Labied” and “Ferqouch Jmel”, classified as drought tolerant based on shoot growth decrease and leaf fall rates during the summer period. The results also align with those of Tapita et al., ranking “Kadota” variety as more drought-tolerant than “Brown Turkey”. The phenotypic clustering of the 30 fig tree cultivars is highly useful for deepening the analysis of functional and structural characteristics related to fig tree adaptation. It is of great interest in guiding fig variety selection and diversification in arid areas. In this regard, the cultivars of the group G4 could be suggested as a first step to enhance the resilience of the sector to climate change.

The plant material consisted of 30 fig cultivars (Table 7), 12 years old, gathered in an ex-situ collection at the experimental station of the National Institute of Agricultural Research (INRA) in Ain Taoujdate, northern Morocco (33°56^′ E, 5°13^′ N, 499 m a.s.l.). Three individual trees represented each cultivar, which were planted in parallel lines, with a spacing of 5 × 3 m. The climate in the experimental station is of semiarid type, with hot and dry summer. The soil is of calcimagnesic type, sandy clay textured, with a useful water reserve of 1.7 mm cm⁻¹, quite rich in organic matter (2.8% in the top layer). The experimental year, 2022, was particularly very dry, with annual rainfall of 300 mm and reference evapotranspiration (ET₀) of 1128 mm, which were recorded from the weather station near the trial. The monthly distributions of these two climatic parameters are presented in Fig. 2, which clearly shows the occurrence of a pronounced rainfall deficit from April to November.

All trees were drip irrigated daily from early April (vegetative departure) to end October (3 months after fruit harvest), except during the rainy days. The daily amount of irrigation was determined to ensure a deficit water regime, corresponding to 60% of crop evapotranspiration (ETc). The daily ETc values were calculated using the formula [ETc = Kr x Kc x ET₀−R_eff], where Kr is the reduction coefficient, which was equal to 1 as the tree canopy cover was high, Kc is the crop coefficient of 0.6 as recommended by Andrade et al. (2014), and R_eff is the effective rainfall, equivalent to 80% of the recorded rainfall. The monthly amounts of irrigation applied to produce a water deficit of 60% are presented in Table 8.

Measurements on the leaf were taken in mid-June when the rainfall deficit was severe, and the fig trees were in the fruit-set stage. During this period, the activity of the fig trees was intense to ensure their needs for rapid growth of fruits and shoots, while being under a marked water deficit. The samples consisted of 20 fully developed leaves, taken from the four sides of each fig tree (60 leaves per genotype).

Statistical analysis was carried out using SPSS v22 software. Analysis of variance (Two-Way ANOVA) followed by a post-hoc test using Student-Newman-Keuls (SNK) test was employed to ascertain significant differences among leaf samples for each trait. The difference between LT and AT was determined by Student test (t-test). To discern the leaf structural traits most associated with fig drought tolerance, a correlation test was performed using the Pearson model, examining relationships among all measured traits. Additionally, principal component analysis (PCA) and a two-level clustering approach based on relevant traits, were employed to rank cultivars with regard to drought tolerance.