The apple tree is one of the most important economical fruit trees in the world (He et al., 2023) with a global apple production that reaches 93.5 million tons per year, nearly half of which is harvested in China (Food and agriculture organization of the United nation (FAOSTAT), 2020). It is also the most cultivated fruit in Europe with an annual production higher than 12.6 million tons (Food and agriculture organization of the United nation (FAOSTAT), 2020). Apple cultivation occupies an important place in the tree-growing sector in Tunisia (Gara et al., 2019), with a national production that reached 127,000 tons in 2022 (Food and agriculture organization of the United nation (FAOSTAT), 2020). The Kasserine region is the main apple-producing region in Tunisia, with an area of 993 hectares and a production of nearly 60,000 tons in 2023. Sbiba and Foussana are particularly known for their apple trees, and the Sbeitla region has more recently become a new production area with 2,500 tons per year (CRDA Kasserine, 2024). In the Kasserine Governorate, two types of apple cultivars are grown: early cultivars (summer cultivars) such as Lorca, Anna, and Ain Chamir, and late cultivars (autumn cultivars): Golden Delicious, Starkrimson, and Richared (Gara et al., 2019). However, apple production in this region has seen a decline in area and production due to climate change, such as low rainfall, sudden changes in temperatures, and the decline in water resources (CRDA Kasserine, 2024). In fact, production was estimated at 75,000 tons in Kasserine in 2020, while it decreased to 60,000 tons by 2023 (CRDA Kasserine, 2024).

In recent decades, Tunisia has been facing water shortages, highlighting the urgent challenges associated with limited water availability in the country. The effects of climate change exacerbate the situation by increasing water scarcity problems (Gaaloul et al., 2021). This water shortage directly impacts yield and fruit quality, making efficient irrigation essential for maintaining optimal growth conditions. In fact, water requirement for apple trees is approximately 600 mm, and the usual rainfall (P) throughout the summer production season (June to August) is approximately 10 mm, which makes the majority of apple production irrigation-dependent. Regarding the lack of availability of water resources, deficit irrigation strategies are therefore a good choice to improve water use efficiency and fruit quality, including flavor profiles, along with their role in saving water and controlling vegetative growth.

Several water-saving techniques in irrigation were used in recent years to improve water productivity such as supplemental irrigation and deficit irrigation as well as the use of cultivars tolerant to water stress. With regard to this multitude of aforementioned techniques, some researchers have based their knowledge on plant-level physiological knowledge to develop a new irrigation technique by partial root drying [“Partial Root-zone Drying” (PRD)]. This PRD irrigation technique consists of reducing the irrigation water inputs to half compared to conventional irrigation. This reduction is very dependent on the type of crop and is generally accompanied by a non-significant reduction in yield while increasing water productivity (Ben Nouna and Rezig, 2016). This technique, now, is well adopted throughout the world in different fruit crops like citrus, pears, and peaches with promising results (Madhumala et al., 2019).

The efficient use of soil moisture sensors for irrigation requires an understanding of the soil’s water retention capacity (Waldburger et al., 2025). It is also important to consider factors such as root water absorption variability, surface evaporation, and partial soil moistening prior to sensor installation (Shock and Wang, 2011). Soil moisture sensors provide an indirect measure of a plant’s water status. It assesses either the volumetric soil water content or the soil water matric potential that is typically used to display soil moisture levels and provides information on the amount of water required for daily irrigation (Dursun and Ozden, 2019). Recent studies have shown that the use of innovative technologies, including Internet of Things (IoT) and sensor networks, facilitates agricultural land management. In particular, the adoption of IoT-based precision irrigation systems has proven effective in monitoring sensor data in real time, which optimizes water use and improves irrigation productivity (Vinod Kumar et al., 2024; Belkher et al., 2025). A recent study conducted by Seyar and Ahamed (2023) tested a drip irrigation LoRaWAN-based solution in outdoor conditions and showed easy management and robust controlling capability. Moreover, the smart irrigation can be used in an adverse situation through proper integration of the IoT module, sensors, and connection-less environments (Di Gennaro et al., 2024). IoT-based technologies make the irrigation easier and more precise by employing low-cost sensors and controlling modules (Seyar and Ahamed, 2023). Furthermore, it can help to obtain a precise measurement of soil moisture in the plant root zone and apply the required proportion of water at the right time, which helps the farmer to continuously monitor irrigation, save time, and economize water at a low cost.

The amount of water that is administered must be enough to prevent either too much or too little water from compromising the production and quality of the fruit (Thompson et al., 2007). Based on soil moisture data, pre-established thresholds that are based on knowledge of each crop and soil type help avoid both under- and over-irrigation. According to Domínguez-Niño et al. (2020), apples are among the crops for which the use of such irrigation systems can give reliable results.

In Tunisia, and especially under semi-arid climates, studies on the use of precise irrigation based on the control of water balance in the soil by intelligent tensiometers are very scarce, especially in the cultivation of fruit trees such as apple. Hence, this study focuses on evaluating the feasibility of using a low-cost automatic smart tensiometers to compare four irrigation strategies and define which is the effective strategy that saves water without affecting the physiology and the apple fruit quality. The effects of the irrigation strategies on plant shoot growth, water status, gas exchange, and fruit quality parameters were evaluated in two young apple cultivars cultivated in the Sbeitla region (west-central Tunisia).

Conventionally, farm managers using traditional irrigation provide a uniform amount of water during the growing season, based on their experiences, without taking into account plant water requirements, phenological stages, and climatic fluctuations. In contrast, appropriate irrigation scheduling promotes benefits such as saving water, decreasing environmental impacts, and generating sustainable agriculture (Domínguez-Niño et al., 2020). The use of smart tensiometers play a crucial role in precision irrigation, ensuring that crops receive the optimal amount of water while minimizing waste. In this study, we examine the effect of deficit irrigation strategies controlled by a precision tensiometer on Galaxy and Richared apple varieties by examining their physiological and pomological parameters.

In fact, we used smart moisture sensors to acquire real-time soil moisture data. These data enabled us to monitor the dynamics of water depletion in the root zone of apple trees and can serve as a reliable reference for determining the appropriate irrigation strategy and monitoring water consumption in the root zone. Our findings revealed that the ETc reached its maximum, 72 and 61 mm decade⁻¹ during the mid and late development stage of the apple tree, respectively. ETc varies with climatic conditions and is also dependent on the tree including vegetative growth, tree height, leaf area, and cover rate. These changes in vegetation result in variations in ETc and, hence, variations in the crop coefficient (Kc). In addition, Ben Aïssa et al. (2000) found that tree transpiration depends positively on the amount of foliage. Indeed, the ratio (Tr/ET₀) increased over the days during the growing season, which was also reported by Zhang et al. (2024). Concerning the dynamics of water depletion, our results showed that under the CI treatment (Figure 3A), the soil water content for the two studied cultivars was in the comfort range in the root zone between 80 and 120 mm, and no water stress was detected during the whole study period. This is consistent with previous findings that control irrigation keeps root zone moisture stable and tree function optimal during critical phenological stages (Hao et al., 2025). After the application of water deficit (123 DABB), we found that, under PRD75% treatment, water was depleted rapidly and soil water content dropped to the permanent wilting point within 10 days of withholding water, in the left side of the root zone; however, on the right side, soil water content was sufficient to support physiological activity until approximately 140 DABB (Figures 3B, C). This is consistent with the mechanism of PRD where alternating wet and dry zones stimulate partial stomatal closure without compromising overall plant water status. Under CDI (50% ETc), there was a decline in soil water content. Indeed, RAM was reached 130 DABB and depletion continued to the WP at 140 DABB (Figure 3A). This gradual and balanced decline suggests that roots were uniformly affected by the reduced water supply and potentially limited the tree’s adaptability (Hao et al., 2025).

Our results showed that shoot growth (Figure 4) was the most sensitive to the irrigation regime and decreased significantly under PRD50% and CDI, with a reduction of shoot growth approximately 31.65% and 38.63% for Galaxy and Richared, respectively. This knowledge aligns with findings from other previous studies in apple (Reid and Kalcsits, 2020). In addition, the vegetative growth of the branches is less affected under the PRD75% treatment compared to other deficit treatments and in both varieties. This was also confirmed by Ben Messaouda et al. (2017) in the “Starkrimson” apple cultivar, cultivated in the Northwest of Tunisia. In fact, according to Lo Bianco (2013), shoot growth reductions are often considered as a result of water shortages, but they can also be seen as an adaptive mechanism and a water-saving strategy through reductions in the transpiration surface. Vegetative growth reductions for apples are also documented under regulated deficit irrigation by Mills et al. (1997), whereas contrasting expectations and results have been observed for trees under PRD.

Regarding relative leaf water contents, our results showed a slight decrease of this parameter in both cultivars even in trees treated with 100% ETc (CI). The increase of evaporative demand, under high temperature and lack of precipitation, may explain this reduction. Furthermore, the reduction of RWC in apple leaves under deficit irrigation has been proven in many other species such as pomegranate (Pourghayoumi et al., 2017), peach (Guizani et al., 2019), and plum (Hajlaoui et al., 2022). The effect of PRD75% treatment was moderate, and the trees were able to maintain high leaf water content (approximately 49% during 190 DABB) in both cultivars. This can be explained by the fact that in the PRD system, the roots on the irrigated side absorb enough water to maintain a high water potential while those on the non-irrigated side produce abscisic acid (ABA), which, in turn, regulates stomatal conductance and optimizes water use (Kang et al., 2000; Ahmadi et al., 2010). In addition, in the present study, the results of gas exchange showed that no significant difference was recorded during the fruit growth period under all irrigation treatments. This may be explained by the fact that since the water deficit was not very severe, the trees are able to absorb their needs from the water stock in the soil. However, during fruit ripening, there was a significant decrease of these parameters (transpiration and stomatal conductance) mainly in the Richared cultivar under 50% PRD (from 2.97 to 1.62 mmol m^–2 s^–1 for Tr and from 140.18 to 124.98 mmol H₂O m⁻² s⁻¹ for gs). Similar findings have been reported by Azzeddine et al. (2019). In fact, the reduction of net CO₂ assimilation (Pn) was probably elucidated by the closure of stomata when the leaf water potential fell below a given threshold (Centritto et al., 2002). However, the absence of a significant reduction of Pn and gs for trees exposed to PRD75% ETc may be explained by the fact that during a light PRD deficit irrigation, the wet part captures water to maintain the water balance of plants, while the dry part causes a partial closure of the stomata. This reduces their transpiration losses due to the production of ABA. However, a small amount of water is evacuated via the stomata, which causes the absorption of CO₂ and thus the production of glucose during the photosynthesis process. The fluctuations in stomatal opening and closing influence the photosynthetic rate and stomatal conductance (Iqbal et al., 2020). According to Raza et al. (2017), mild water stress does not influence the photosynthetic rate as much as leaf enlargement.

Moreover, the reduction of gs under water deficit conditions is related to the plant’s ability to withstand drought conditions. The linear correlation of gs and Pn that is documented in previous studies (Šircelj et al., 2007; Guizani et al., 2019) confirms that gs controls Pn, and the limitation of gs induces a decrease in photosynthetic assimilation. On the other hand, the variation of photosynthesis rate from the fruit development, fruit ripening, and post-harvest stage was also reported by Laužikė et al. (2020), who mentioned that photosynthetic indices of apples are most affected by development stages during the season. This variation is related to physiological changes of metabolite transport and distribution during fruit ripening and leaf senescence. For both varieties, a significant reduction of gs was also noted under continuous and PRD (50%) deficit irrigation compared to control irrigation. Previous studies confirmed these findings under different environmental conditions (Lo Bianco, 2013). In fact, this decrease reflects the crop’s sensitivity to water deficit. Similarly, other studies have shown that photosynthesis is significantly affected by PRD irrigation. According to Zegbe et al. (2004), cultivar type and environmental conditions significantly affect the results of PRD irrigation.

Our results displayed that during the first days of the application of deficit irrigation, shoot growth and photosynthetic parameters were not significantly affected by water shortage. However, a few days later and especially under deficit water regimes (CDI and PRD50%), with soil water content below the readily available water until the wilting point (Figure 3), physiological traits were gradually affected. Similar results were mentioned by Lawlor and Cornic (2002) and Bhusal et al. (2019) in apple trees. In contrast, our work also revealed that there was an increase in instantaneous water use efficiency (Pn/Tr) in trees subjected to PRD50% (5.56 and 6.06 μmol mmol^–1, for Galaxy and Richared, respectively, during fruit ripening). In fact, according to Kang et al. (2000), WUE increases under PRD irrigation compared to CI treatment. This increase can be explained by the fact that under PRD irrigation, less water is required to maintain yield, which leads to a reduction in overall water consumption although inducing some degree of water stress in the plant, which can improve WUE.

Fruit quality analysis showed that Richared exhibited the highest yield and the best fruit (in terms of weight and size) and even TSS, despite both cultivars undergoing the same irrigation, fertigation, and climatic conditions. This may be related to the varietal effect. It is important to mention that even for control irrigation conditions, the yield is considered low, because the trees are still young (3 years) and are not in full production. The application of deficit irrigation strategies (CDI and PRD50%) during the last phase of fruit development significantly reduced yield. This was not the case for trees exposed to PRD75%; this treatment does not have a depressive effect on physiological parameters and especially gas exchange parameters. According to Wen et al. (2024), apple is sensitive to severe water deficit during this period because it can lead to the loss of elasticity in plant cell walls, damage to leaves, and reduced photosynthetic capacity, causing a reduction in yield.

In addition, no significant impact was recorded on fruit diameter under diverse water deficit treatment during the beginning of application. However, we noted a remarkable decrease of fruit growth under PRD and continuous 50% treatments compared to control treatment. It is important to indicate that the fruit diameter was greater under 75% PRD treatment, and even fruit size was less affected. These results are in agreement with Goldhamer et al. (2006) and Badiee et al. (2023) who found that continued daily application of deficit irrigation during the rapid fruit growth phase would most likely decrease apple and almond fruit growth. During the experiment, the irrigation treatments had significantly affected fruit quality in the two varieties studied. In fact, there was a significant decrease in the yield, size, and weight among the fruits exposed to deficit irrigation; however, those exposed to PRD75% were the least affected. These results are in accordance with previous studies in different crops, including apple (Badiee et al., 2023), peach (Guizani et al., 2019), and plum (Maatallah et al., 2015).

According to the results of this study, a significant improvement in fruit quality was reported by the PRD75% water regime, despite the slight decrease in size and weight, which does not generally affect commercial quality. According to Hao et al. (2025), PRD irrigation improved leaf photosynthetic efficiency, which increased the transport of photosynthetic products to the fruit and enhanced nutritional content and market quality. In fact, fruit flesh firmness was shown to be significantly affected by the deficit irrigation treatments. The greatest values of firmness were recorded under PRD50% for Richared (7.70 kg cm⁻²) and under CDI (50% ETc) for Galaxy (8.50 kg cm⁻²). These findings are in agreement with previous results in the apple fruit (Mpelasoka et al., 2001; Küçükyumuk et al., 2013). Moreover, there was a significant increase of TSS under deficit irrigation strategies. The high values of TSS may be explained by the conversion of starch into sugars (Wan Zaliha and Singh, 2010). Indeed, fruit exposed to PRD75% recorded the highest level of TSS (17.50 and 19.50°Brix in Richared and Galaxy, respectively). Similar results were reported by Leib et al. (2006) and Mpelasoka et al. (2001). On the other hand, Ben Messaouda et al. (2017) found opposite results, and they showed that PRD50% treatment did not alter soluble solids and firmness. Generally, controlled irrigation PRD75% had no negative impact on apple quality, but instead increased water use efficiency. In this context, Zegbe and Behboudian (2008) reported that an increase of WUE by 120% allowed a reduction of 0.14 megaliters of water per hectare, which increased the farmer’s economic profits.

In addition, our results showed that fruit skin color varied significantly among irrigation strategies. In fact, red skin color (a*) increased significantly with deficit irrigation. The study’s findings, regarding skin color, were consistent with those of previous studies (Kilili et al., 1996; Mills et al., 1997; Wan Zaliha and Singh, 2010), which found that redness increased as a result of insufficient watering. In fact, according to Wan Zaliha and Singh (2010), the improvement in fruit color can be attributed to the higher concentration of total anthocyanins on the apple skin. This could be associated with better sunlight penetration into the canopy and onto the fruits, due to sparse leaf abscission induced by water deficit. Furthermore, Küçükyumuk et al. (2013) explained the impact of deficit irrigation on skin color by the shoot length reduction, where the lowest shoot length values were correlated with the highest red color values.

Besides the important role of deficit irrigation strategies (PRD) in saving water and improving fruit quality, recent studies on the chemical and biological fertility of orchard soils have demonstrated that sustainable and innovative soil management systems are based on water management (Futa et al, 2024; Sofo et al., 2012). According to our results, PRD75% can establish an ideal nutrient balance for the plant and avoid nutrient accumulation, soil salinization, and the risk of leaching. In addition, optimized irrigation efficiency can prevent soil erosion and root asphyxiation. The use of smart irrigation based on smart tensiometers, which have a low environmental impact, can restore or increase soil fertility levels in fruit agro-ecosystems.

Accurate smart irrigation scheduling is essential for estimating the real need of the plants without excess or deficit. In this study, we investigated the irrigation scheduling techniques under four different irrigation treatments. The results illustrated that deficit irrigation strategies save water, but at higher levels of stress, the plant’s physiology and fruit quality were affected. However, under PRD75%, the plants reached the RAM after irrigation for the two cultivars Galaxy and Richared. This allowed us to conclude that the PRD75% strategy controlled by smart tensiometers seems to be the most recommended in the region of the present study, given its moderate effect on the shoot growth as well as on the parameters of gas exchanges. Similarly, this irrigation strategy increased the sugar level and firmness and did not alter the weight and size of the fruits. The irrigation strategy PRD75% saves water, promotes physiological stability, improves fruit quality, and preserves soils for better sustainability of fruit production. Therefore, it could be a good choice for orchards of apple trees in areas with frequent rainfall. Furthermore, Richared seems to be the most suitable choice for this climate, given its fruit quality and the best physiological behavior under deficit irrigation treatments.