Induction of water stress in major Solanum crops: a review on methodologies and their application for identifying drought tolerant materials

Scientia Horticulturae 318 (2023) 112105 Available online 3 May 2023 0304-4238/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Review Induction of water stress in major Solanum crops: A review on methodologies and their application for identifying drought tolerant materials Martín Flores-Saavedra, Mariola Plazas, Santiago Vilanova, Jaime Prohens, Pietro Gramazio * Instituto de Conservaci´ on y Mejora de la Agrodiversidad Valenciana, Universitat Polit` ecnica de Val` encia, Camino de Vera 14, 46022 Valencia, Spain ARTICLE INFO Keywords: Solanum crops Water stress Stress tolerance Stress induction methodologies PEG Wild species ABSTRACT The genus Solanum encompasses several economically important vegetable crops, such as tomato, potato, and eggplant, which are crucial for ensuring food security. Higher temperatures and reduced precipitation are becoming more frequent due to climate change in numerous regions, fostering drought spells and the likelihood of water stress in Solanum crops resulting in decreased yields. Appropriate evaluation techniques are required by researchers and breeders to evaluate the impact of drought on the performance of Solanum crops and identify more tolerant genotypes. This review examines the most important approaches for inducing water stress in Solanum crops, such as withholding irrigation, adjusting field capacity levels, applying evapotranspiration criteria, and utilizing polyethylene glycol as an osmotic agent. We highlight the benefits and drawbacks of each method, enabling researchers and breeders to choose the most suitable conditions for their specific objectives and goals. Additionally, we address the challenges of combining water stress with other types of stress that frequently occur simultaneously in the field and the effects that biostimulants can have in mitigating water stress in Solanum crops. We also provide an in-depth analysis of the impact of water deficit on growth and biomass, as well as on physiological and biochemical traits, and new phenotyping tools that allow the study of stress tolerance in the three major crops belonging to the Solanum genus. Finally, the review discusses the possibility of utilizing wild species to improve water stress tolerance in these Solanum crops. 1. Introduction The Solanaceae family encompasses over 3000 species, with the genus Solanum comprising nearly half of these (Morris and Taylor, 2017). Solanum species are widely distributed worldwide with the greatest diversity found in the tropical regions of America, Australia, Africa and the Indo-Pacific (Echeverría-Londo˜ no et al., 2020). Among them, three crops stand out for their agricultural and scientific importance: tomato (S. lycopersicum L.), potato (S. tuberosum L.), and eggplant (S. melongena L.). Tomato is the most important vegetable crop and a model species for fleshy-fruited plants (Schreinemachers et al., 2018), while potato is the most significant Solanaceae crop in total production and a key staple crop for food security and nutrition (Aksoy et al., 2021). Eggplant is the fifth most produced vegetable crop globally and a basic food product in many Asian countries (Oladosu et al., 2021). Despite the dramatic increase in production and yield of these three Solanum crops in the last 50 years, a deceleration trend has been observed with smaller yearly increases (FAO, 2020). Climate change is a major contributor to this trend, with higher temperatures and changes in precipitation patterns leading to harsher growth conditions and crop losses due to reduced water availability (Arnell et al., 2019; Cook et al., 2018). Higher evapotranspiration caused by higher temperatures, as well as salinity or drought, decreases the water available for plants, resulting in lower yield (Korres et al., 2016). Tomato and potato are highly sensitive to water deficit (Sakya et al., 2018; Hill et al., 2021), while eggplant can withstand low to moderate water deficiency, preserving its physiological processes and production under these conditions, but it is negatively affected by severe water shortage (Díaz-P´ erez and Eaton, 2015). Technological improvements and solutions may mitigate the negative effects of drought in developed countries, but they may not be accessible in developing countries in the short run, where the effects are more severe and accelerated. Therefore, identifying more efficient and resilient genotypes is the most suitable, though challenging, solution (Solh and van Ginkel, 2014). * Corresponding author. E-mail address: [email protected] (P. Gramazio). Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti https://doi.org/10.1016/j.scienta.2023.112105 Received 20 February 2023; Received in revised form 17 April 2023; Accepted 26 April 2023 Scientia Horticulturae 318 (2023) 112105 2 Table 1 Methods of induction of water stress in major Solanum cultivated species and wild relatives. Methods of induction Species Stress level Phenological stage during stress Growing conditions References Irrigation withholding S. lycopersicum Irrigation withholding for 7 days Fruit development Phytotron Goel et al. (2010) Irrigation withholding for 12 days Leaf development Growth chamber Wu et al. (2017) Irrigation withholding for 9 days Leaf development Growth chamber Bian et al. (2019) Irrigation withholding for 6 days – Growth chamber Filiz and Akbudak (2020) Irrigation withholding for 14 days Leaf development Greenhouse Choi et al. (2011) Irrigation withholding for 21 days Leaf development Greenhouse Zhu et al. (2014) Irrigation withholding for 16 days Flowering Greenhouse Landi et al. (2016) Irrigation withholding for 14 days – Greenhouse Mishra et al. (2016) Irrigation withholding for 19 days Leaf development Greenhouse Tamburino et al. (2017) Irrigation withholding for 14 days – Greenhouse Karkute et al. (2018) Irrigation withholding for 30 days Fruit development Greenhouse Waseem et al. (2019) Irrigation withholding for 10 days Leaf development Greenhouse Akbudak et al. (2020) Irrigation withholding for 45 days Fruit development Field Landi et al. (2016) S. pennellii Irrigation withholding for 8 days Leaf development (flowering) Growth chamber Egea et al. (2018) S. tuberosum Irrigation withholding for 14 days Tuber formation Greenhouse with natural light Boguszewska et al. (2010) Irrigation withholding for the remainder of the growing season Leaf development Field Liu et al. (2020) Irrigation withholding for 41 days Tuber formation Field Schafleitner et al. (2007) S. melongena Irrigation withholding for 11 days Leaf development Greenhouse Plazas et al. (2019) Irrigation withholding for 14 days – Field Delfin et al. (2021) S. aethiopicum Until showing symptoms of wilting Leaf development Greenhouse Mibei et al. (2017) Irrigation withholding for 14 days Leaf development Greenhouse Sseremba et al. (2018) Several eggplant CWR Irrigation withholding for 11 days Leaf development Greenhouse Plazas et al. (2022) Field Capacity S. lycopersicum Gradually increase of stress (75, 50 and 25% of FC) – Growth chamber Hosseini Tafreshi et al. (2021) 60% of FC Fruit development Greenhouse Rady et al. (2020) 40, 25 and 15% of FC – Greenhouse Krishna et al. (2021) 50% of FC Fruit development Greenhouse Azizi et al. (2021) 75% and 50% of FC Flowering Greenhouse with natural light Chakma et al. (2021) Field Capacity S. tuberosum 40–30% of FC Tuber formation Growth chamber Gong et al. (2015) 25% of FC Tuber initiation Greenhouse and growth chamber Szalonek et al. (2015) 50% of FC Tuber initiation Greenhouse Rolando et al. (2015) S. kurtzianum 50 and 30% of FC Tuber initiation Shade-house Iba˜ nez et al. (2021) S. melongena 30% of FC Leaf development Greenhouse Fu et al. (2013) 25% of FC Leaf development Greenhouse Tani et al. (2018) Gradually increase stress (30 and 10% of FC) – Greenhouse Delfin et al. (2021) 75 and 50% of FC Throughout the crop cycle Field Çolak et al. (2015) Volumetric water content S. chilense 20% of volumetric water content – Growth chamber Blanchard-Gros et al. (2021) Evapotranspiration S. lycopersicum 20% of ET 0 – Field Galm´ es et al. (2013) 75 and 50% of ETc Fruit development Field Tak´ acs et al. (2020) 60% of ETc From flowering Greenhouse Albert et al. (2016) S. pimpinellifolium and S. lycopersicum var. cerasiforme 40% of ETc Throughout the crop cycle Greenhouse Martínez-Cuenca et al. (2020) S. tuberosum 60 and 40% of ET Throughout the crop cycle Field della Costa et al. (1997) S. melongena 67 y 33% de ETc – Field Díaz-P´ erez and Eaton (2015) 70 y 40% de ETc Throughout the crop cycle Field Amiri Rodan et al. (2020) 60% de ETc Throughout the crop cycle Field Semida et al. (2021) Polyethylene glycol S. lycopersicum 10% PEG for 7 days – Growth chamber Karaca and Cekic (2019) 20% PEG for 5 days Leaf development Incubation chamber Meng et al. (2020) 15% PEG for 48 h Leaf development Hydroponic Landi et al. (2016) 14 and 4% PEG Germination Seed germination Esan et al. (2018) 6, 4 and 2% PEG Leaf development In vitro Kulkarni and Deshpande (2010) (continued on next page) M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 3 Fortunately, within the gene pool of these three Solanum crops, genotypes adapted to water deficit have been identified, such as local varieties of tomato (Galm´ es et al., 2013) or wild relatives of potato (Iba˜ nez et al., 2021) and eggplant (Plazas et al., 2022). In this regard, appropriate phenotyping is essential to identify genotypes adapted to drought conditions and high temperatures (Reynolds et al., 2020). However, due to the quantitative nature of the traits related to water deficit and the duration and intensity of water stresses, which are highly variable among the plant phenological stages, environments and crops, no consensus and universal protocols have been developed for comprehensive phenotype candidate genotypes (Passioura, 2012). This paper aims at reviewing the state of the art of procedures and approaches to evaluate water stress in the three most important Solanum crops, comparing the water deficit induction methods according to different criteria and experimental conditions to identify the best stresstolerant genotypes, providing relevant information and guidelines for developing suitable water stress phenotyping toolkits. The effects of drought combined with other stresses and the effects of biostimulants application are also being investigated, as well as the potential of wild relatives for improving the drought tolerance of the target crops is also reviewed. 2. Water deficit induction methods in Solanum crops The methods to induce water deficit and evaluate growth parameters to test water stress tolerance in Solanum materials are very diverse. In greenhouse conditions, water deficit is often induced by withholding irrigation, simulating severe stress (Plazas et al., 2019; Akbudak et al., 2020) or by reducing irrigation to a certain percentage of field capacity (Tani et al., 2018; Azizi et al., 2021). In field conditions, water stress is typically induced by withholding irrigation (Landi et al., 2016) or by reducing evapotranspiration percentage (Galm´ es et al., 2013; Semida et al., 2021). On the other hand, when stress is evaluated in vitro or in climatic chambers, different concentrations of osmotic agents, such as polyethylene glycol, are added (Siaga et al., 2016; Liu et al., 2020). 2.1. Withholding irrigation Withholding the irrigation supply is the simplest and easiest method to evaluate water stress tolerance, even though the developmental stage at which the withholding is applied and its duration are very variable (Table 1). In tomato, withholding of stress can be highly variable, being applied to underdeveloped plants 10 days after transplant (Tamburino et al., 2017) or more developed, after two months of growth (Filiz and Akbudak, 2020; Landi et al., 2016). Thus, the phenological stage where the water stress was applied, in most cases, is in the vegetative phase. However, it has also been evaluated in flowering or during fruit development (Table 1), so the effect of stress could be very different in each trial. In the case of potato and eggplant, less information is available regarding this method of stress induction. In potato, different authors have induced stress at different times of growth, either in leaf development (Liu et al., 2020) or tuber formation (Schafleitner et al., 2007; Boguszewska et al., 2010). In eggplant, stress was applied at the phenological stage of five true leaves in greenhouse trials (Plazas et al., 2019) or five weeks after transplantation in field trials (Delfin et al., 2021), while in the related scarlet eggplant (S. aethiopicum L.), the application of stress was performed five days after transplanting (Mibei et al., 2017) or after eight weeks in cultivation (Sseremba et al., 2018). Withholding the water supply at any phenological stage generates biomass and yield losses; however, the effects may be different at each stage (Yavuz et al., 2021). The duration of stress also varies depending on the growing conditions, the plant material, and the level of stress applied. In tomato trials, for example, stress has been applied for periods ranging from 6 to 45 days (Table 1). In the cases mentioned, the time of exposure to stress did not have a direct relationship with the developmental stage of the crop. Table 1 (continued) Methods of induction Species Stress level Phenological stage during stress Growing conditions References S. tuberosum 15% PEG for 6 h – In vitro Hwang et al. (2011) 25% PEG for 24 h Leaf development In vitro Kappachery et al. (2013) 8 and 4% PEG Leaf development In vitro Pino et al. (2013) 10% PEG for 24 h Leaf development In vitro Liu et al. (2020) S. melongena 10% PEG for 21 days Leaf development In vitro Siaga et al. (2016) 10, 8 and 3% of PEG for 21 days Leaf development In vitro Zayova et al. (2017) M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 4 In the case of S. pennellii, a wild relative of the tomato, stress was generated by withholding the water supply for eight days, being less affected than cultivated tomato (Egea et al., 2018). In potato, irrigation withholding was evaluated for 14 days (Boguszewska et al., 2010), 41 days (Schafleitner et al., 2007) or until the end of the growing season (Liu et al., 2020). On the other hand, irrigation withholding was assessed in eggplant for 11 days (Plazas et al., 2019) and 14 days (Delfin et al., 2021), while in scarlet eggplant it was applied for 14 days (Sseremba et al. al., 2018). The evapotranspiration demand of a crop can vary spatially and temporally (Christou et al., 2017), so a long time without irrigation does not necessarily imply a higher level of stress for the crop. 2.2. Watering to different field capacity levels Soil field capacity (FC), a concept first introduced by Veihmeyer and Hendrickson (1931), and further developed over time (Zacharias and Bohne, 2008; Robertson et al., 2021), refers to the water content of soil or plant substrate after it has been saturated with water and then drained until the water drainage is no longer significant. Unlike the method of withholding water supply, this approach ensures that a certain level of water is available for the plant. The timing of stress application in tomato was done in plants aged from around 30 d (Azizi et al., 2021; Hosseini Tafreshi et al., 2021) to 50 d (Rady et al., 2020; Krishna et al., 2021). For potato plant, in most cases, induced stress in plants was performed in the phenological stage of the beginning of tuber formation (Rolando et al., 2015; Szalonek et al., 2015; Iba˜ nez et al., 2021), while Gong et al. (2015) did it in the flowering stage (tuber formation). In the case of eggplant, water stress was applied during the leaf development stage, when plants had around five expanded leaves (Fu et al., 2013; Tani et al., 2018) or five weeks after transplanting (Delfin et al., 2021). The percentage of FC used to induce water stress in plants varies depending on the objective of the study. In tomato plants, levels of FC up to 15% (Krishna et al., 2021) and 25% (Hosseini Tafreshi et al., 2021) were used to induce severe stress, while moderate stress was induced with FC levels of 50% (Azizi et al., 2021), 60% (Rady et al., 2020) or 75% (Chakma et al., 2021). In potato plants, severe stress was induced with FC levels of 25% (Szalonek et al., 2015), 30% (Iba˜ nez et al., 2021) and 35% (Gong et al., 2015), while moderate stress was induced with FC levels of 50% (Iba˜ nez et al., 2021; Rolando et al., 2015). For eggplant, more restrictive levels of irrigation were used, with FC levels of 10% (Delfin et al., 2021), 25% (Tani et al., 2018) and 30% (Fu et al., 2013) for severe stress, and less restrictive levels of 50% and 75% under field conditions (Çolak et al., 2015). Generally, to induce severe stress in these major Solanaceae crops, the values of FC range from 10 to 35%, while moderate stress ranges from 40 to 75% (Table 1). However, the level of stress also depends on the duration of exposure. 2.3. Evapotranspiration-based criteria The combination of water loss through soil surface evaporation and crop transpiration is known as evapotranspiration (ET). This concept includes different definitions: reference evapotranspiration (ET 0 ), which is a parameter related to the evaporative power of a specific atmospheric environment, with a reference surface and without water restrictions (well-watered grass), while crop evapotranspiration (ETc) refers to the evapotranspiration of a specific crop, under optimal conditions, in a specific atmospheric environment (Pereira et al., 2015). Evapotranspiration-based criteria have also been used to generate water stress in crops of the Solanum genus by decreasing the replenishment of the crop’s total evapotranspiration. This method is mainly used in field trials. In tomato trials, water stress was induced after a month of well-irrigated cultivation by applying only 20% of the ET0 (Galm´ es et al., 2013). A reduction in irrigation to 40% of the ETc at the beginning of flowering resulted in reductions in tomato vigor and yield (Albert et al., 2016). Similarly, Martínez-Cuenca et al. (2020) evaluated S. pimpinellifolium and S. lycopersicum var. cerasiforme under the same stress level, resulting in negative effects on production. When less restrictive levels were applied, by irrigating tomato plants to 75% and 50% of the ETc until reaching the stage of the beginning of fruit ripening, Tak´ acs et al. (2020) found that plants irrigated to 75% ETc were not greatly affected, although significant yield reductions were observed in plants irrigated with 50% ETc. In potato, moderate stresses were achieved by irrigating to 60% ETc and severe stresses by irrigating to 40% ETc (dalla Costa, 1997). To induce water stress in eggplant, plants were exposed after four weeks of transplanting to 33% and 67% ETc (Díaz-P´ erez and Eaton, 2015), 40% and 70% ETc when the plants had five to six true leaves (Amiri Rodan et al., 2020), and 60% ETc to 37-day-old plants (Semida et al., 2021). Determining the ET requirements of a crop under water stress is complex, and the use of a percentage of the ETc often results in excessive or insufficient irrigation, depending on whether the ET 0 is high or low (Hochberg et al., 2017). Therefore, although it is a practical method under field conditions, the percentage used in one environment can have a different effect on the crop when evaluated in another environment. 2.4. Use of polyethylene glycol solutions The use of polyethylene glycol (PEG) solutions as an osmotic agent results in a decrease in water potential (Al-Taisan et al., 2010), making it a useful tool for simulating water stress in studies of drought tolerance. In the case of Solanum, PEG has been used in hydroponic studies and in vitro culture (Table 1). In tomato cultivation, germination tests were conducted using PEG concentrations of 0%, 4%, and 14% in distilled water, where higher concentrations reduced the percentages of germination and seedling growth (Esan et al., 2018). Under hydroponic conditions, tomato plants were subjected to a 15% PEG solution for 48 h, causing severe chlorosis and leaf loss (Landi et al., 2016). Similarly, tomato plants grown on perlite were watered with Hoagland solution with 10% PEG for four weeks (Karaca and Cekic, 2019). For in vitro evaluation, tomato seedlings were evaluated in Murashige and Skoog (MS) medium with 2%, 4% and 6% PEG concentrations, with growth decreasing as PEG concentration increased (Kulkarni and Deshpande, 2010). Notably, genotypes that were most tolerant in the in vitro assay also tended to be more tolerant under field conditions, suggesting that this method may have potential as a preliminary screening tool. In potato, PEG tests were mainly conducted under in vitro culture conditions, using variable PEG concentrations to replace MS medium. Kappachery et al. (2013), grew plants for 28 days before applying 25% PEG, resulting in clear signs of wilting within 24 h. Pino et al. (2013) evaluated osmotic stress by performing in vitro culture tests with concentrations of 4% and 8% PEG, resulting in water potential values of −0.362 and −0.478 MPa, respectively. Liu et al. (2020) subjected potato seedlings to 10% PEG for 24 h to analyze gene expression under water stress. In eggplant, Siaga et al. (2016), evaluated the growth of seedlings cultivated in vitro with 10% PEG for 21 days, which negatively impacted survival and growth, while Zayova et al. (2017) cultivated eggplant seedlings on 3%, 8% and 10% PEG for 21 days for stress induction, observing that higher PEG concentration resulted in lower survival rates. This method allows for greater control over the level of stress generated compared to others, but it is not a practical method for evaluating crop biomass and yield, so results may not always apply to field conditions. While relationships have been reported between in vitro culture with PEG and open field culture for drought tolerance (Kulkarni and Despande, 2010; Gopal and Iwama, 2007), further trials are needed to validate the method. M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 5 2.5. Advantages and disadvantages of stress induction methods The different stress induction methods are very diverse, each of them presenting advantages and disadvantages for evaluation (Table 2). The space required for each method can vary greatly. For instance, using polyethylene glycol (PEG) in an in vitro culture setting requires less space, while inducing stress through withholding irrigation or reducing the percentage of evapotranspiration (ET) is typically done under field conditions (Amiri Rodan et al., 2020 and Semida et al., 2021) and requires more time, space, and resources (Table 1). Meanwhile, when using the method of reducing field capacity (FC) and withholding irrigation in a greenhouse setting, the area needed is typically smaller than in field conditions, but still larger than in vitro cultivation. Another important aspect to consider is the control of environmental parameters and the level of stress induced. When simulating water stress through PEG, the environmental conditions can be controlled and replicated, as well as the level of stress generated. However, in experimental designs involving reduced replenishment of field capacity or evapotranspiration, even though the water level remains constant and the substrate is controlled, the environmental conditions can be highly variable (Hochberg et al., 2017). In this sense, withholding irrigation is considered the most unstable and least replicable method, as the water level of the soil and environmental conditions can vary greatly. Drought tolerance screening is for simulating real conditions of the crop at a commercial level. In this sense, the methods based on withholding irrigation or at certain levels of the FC or the ET can be performed at the field level or in greenhouses simulating a lower availability of daily irrigation (% of FC or ET), by a reduced frequency of irrigation or by irrigation withholding. Conversely, for water stress induction with PEG, although it decreases water availability for plants, the environmental conditions of in vitro culture and the use of germination chambers can be very different from real field conditions. Additionally, yield is generally not evaluable with this method, which is an important consideration for Solanum crops (Pino et al., 2013; Zayova et al., 2017; Meng et al., 2020). Another important aspect of the methodology used is the easiness and convenience of assessment. Withholding irrigation is the simplest method of inducing stress, followed by the application of PEG (Table 1). Lastly, the method of reduced replacement of FC or ET requires greater irrigation control, often on a daily basis during the experimental period (Szalonek et al., 2015; Martínez-Cuenca et al., 2020). 3. Water stress combined with other factors 3.1. Water stress combined with other stresses The increasing prevalence of multiple stressors on plants, particularly in the context of climate change, makes studying the combined effects of these stresses increasingly important (Mahalingam, 2015). Research has shown that in Solanum crops, the presence of multiple stressors, including water stress, does not always lead to an exacerbation of negative effects and largely depends on the specific genotype under study (Fig. 1). For abiotic stress, drought and salinity conditions generate a similar effect of stress and tolerance mechanisms in plants (Uddin et al., 2016). Due to this, the effects of water and saline stress are often studied together. Studies observed that within the genus Solanum, plants are negatively affected by saline conditions (Abdel-Farid et al., 2020; Jaarsma et al., 2013) and water limitations (Díaz-P´ erez and Eaton, 2015; Rady et al., 2020). However, important advances were made in genetic regulation that allows greater tolerance to both salt and water stress, through the generation of transgenic plants (Choi et al., 2011; Goel et al., 2010; Waseem et al., 2019; Zhu et al., 2014). On the other hand, when combining hydric and high temperature stresses in tomato, it was observed a positive effect on plant height and the number of leaves; however, the biomass can be higher or lower when compared to heat stress alone, depending on the genotype (Blanchard-Gros et al., 2021). Adverse weather conditions, such as drought, can make plants more Table 2 Comparison of stress induction methods in major Solanum crops. The number of asterisks indicates a higher relationship with the corresponding characteristic. Characteristic Irrigation withholding Field capacity level Evapotranspiration level Polyethylene glycol Area used ***/** ***/** ***/** * Control and replicability * ** ** *** Duration of the trial ***/** ***/** *** * Relationship with real conditions *** ***/** *** * Easy implementation *** * * ** Fig. 1. Effects of water stress combined with biotic and abiotic stresses in Solanum crops. The symbols (↑) and (↓) mean a positive or negative effect on the susceptibility of the plant to water stress. M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 6 vulnerable to biotic stressors, such as pests and diseases (Teshome et al., 2020). Studies have also investigated the combined effects of drought and damage caused by insects and pathogens. For example, research has shown that tomato plants under water stress conditions are more susceptible to infestations by Bactericera cockerelli, with greater numbers of nymphs observed than in well-irrigated plants (Huot and Tamborindeguy, 2017). However, in some cases, such as in S. dulcamara, the combined stress of drought and the insect Spodoptera exigua, resulted in improved resistance to herbivores (Nguyen et al., 2016). In eggplant, the combined stress of drought and Verticillium dahliae can have positive or negative effects depending on the genotype under study (Tani et al., 2018). 3.2. Water stress combined with biostimulants Biostimulants can alleviate abiotic stress and, in consequence, are of interest for a more sustainable agriculture (Matthews et al., 2022), In this way, methods for inducing water stress in Solanum crops combined with the use of biostimulants have been investigated. In this way, the application of biostimulants in tomato, such as 4-Vita (Campobenedetto et al., 2021) and Eranthis (Sudiro et al., 2022), under limited irrigation conditions was reported to increase photosynthetic capacity, chlorophyll content and response to oxidative stress. However, their positive effect on yield remains unproven (Top et al., 2023) and fruit quality responds differently to each biostimulant (Fernandes et al., 2022). Promising results were also observed in potato, where the application of the biostimulant clove fruit extract (CFE) improved the antioxidant response, growth and yield of tubers under water stress (Desoky et al., 2021). Despite the lack of research on the effects of biostimulants under water stress in eggplant, it was observed that the application of Ascophyllum nodosum standardised extract (G¨ oemer BM-86) increased yield and fruit quality under field conditions (Pohl et al., 2019), making it a promising tool to evaluate the stress mitigation under drought conditions. 4. Traits evaluated under water stress conditions 4.1. Growth and biomass Water stress in plants can greatly reduce their growth rate due to a decrease in water potential and transpiration rate, which leads to a reduction in cell turgor. As a result, important plant characteristics such as height, leaf area index, biomass, and yield are negatively impacted (Imadi et al., 2016). 4.1.1. Germination and survival of seedlings The availability of water greatly affects the germination and survival of seedlings. However, there are significant variations depending on the genotype studied. For example, Esan et al. (2018) found that tomato germination was slightly reduced when exposed to a solution with 4% PEG, and in more severe stress conditions with 14% PEG, germination percentages were severely reduced and some seeds did not germinate at all. Meng et al. (2020) evaluated transgenic and wild-type tomato seedlings under water stress conditions simulated by 100 mM and 200 mM mannitol. Germination percentages decreased in both cases, but more significantly in the transgenic lines with silenced SlWHY2 gene (Fig. 2). In potato, it was observed that transgenic seedlings expressing the ScCBFI gene were less affected by severe water stress and had a higher percentage of survival (around 100%) in vitro conditions than wild-type plants (which had a 40% reduction in survival) (Pino et al., 2013) (Fig. 2). Similarly, transgenic plants expressing the STANN1 gene under water stress had a survival rate of 82% compared to 12% for wild-type plants (Szalonek et al., 2015) (Fig. 2). When evaluating the effect of water stress on eggplant in vitro, the percentage of live explants was not significantly affected when cultured with 10% PEG (Siaga et al., 2016). However, Zayova et al. (2017) found that plants were severely affected when exposed to the same level of stress (10% PEG for 21 days), with survival percentages ranging from 10% to 0% depending on the variety, while with 8% PEG, the effect was less severe with survival percentages ranging from 60% to 40%. 4.1.2. Plant height and foliar and root development In tomato, a lack of water can significantly affect the plant’s height (Bian et al., 2019; Hosseini Tafreshi et al., 2021). Additionally, stem diameter is one of the growth parameters that is commonly affected by water stress (Albert et al., 2016). However, other studies have not found significant differences for this trait (Landi et al., 2016; Tamburino et al., 2017), or in the number of nodes affected (Tamburino et al., 2017). In contrast, leaf development is clearly impacted by water deficit in tomato plants. The number of leaves decreases by approximately 28% in plants at the end of the cultivation period (Rady et al., 2020), and the leaf area is reduced following a decrease in irrigation (Bian et al., 2019; Chakma et al., 2021; Albert et al., 2016). The height of potato plants decreases when irrigation is reduced, as observed by della Costa et al. (1997) and confirmed by Pino et al. (2013), even in genotypes with higher stress tolerance. Rolando et al. (2015) also found that length and vegetation cover are reduced with a decrease in water supply, with reductions of 25% and 43%, respectively. Similarly, when evaluating the effects of water stress on eggplant in vitro culture, the height decreases significantly when PEG is added at 8% (Zayova et al., 2017). However, other studies under in vitro conditions Fig. 2. Genes involved in drought tolerance in tomato or potato plants and their effects. M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 7 did not find an effect on the number of leaves or shoot height when PEG is added at 10% (Siaga et al., 2016). Under greenhouse conditions, eggplant is affected by decreased irrigation, with stem growth rates in stressed plants being significantly lower than in well-watered plants (Tani et al., 2018). Severe water stress conditions can result in the growth of the plant in height being up to seven-fold lower than in non-stressed conditions (Plazas et al., 2019). In trials evaluating plants at the end of the crop cycle, height was found to be reduced by 16% (Semida et al., 2021), although Díaz-Perez and Eaton (2015) did not find an effect of irrigation level on plant growth. The leaf area of eggplant is also affected by water stress, with reductions of 37% (Çolak et al., 2015), 27% (Semida et al., 2021), and 28% (Delfin et al., 2021) under water stress conditions. Regarding root length, it decreases under water stress conditions in tomato plants (Esan et al., 2018). However, genotypes with greater tolerance have a greater root length compared to susceptible genotypes (Kulkarni and Deshpande, 2010). Advances in root development have also been made through transgenesis, where tomato plants expressing the AtGRXS17 gene had a longer tap root than WT plants under water stress conditions (Wu et al., 2017) (Fig. 2). In tolerant potato genotypes expressing the ScCBFI gene, root length and area increase under moderate stress conditions (Fig. 2), but decrease under severe stress conditions for both tolerant and susceptible genotypes (Pino et al., 2013). Root growth in eggplant was found to be susceptible to water stress, as two genotypes evaluated under water stress showed reductions in length, biomass, and water content (Plazas et al., 2022). 4.1.3. Biomass production The effects of water stress on growth and biomass in tomato have been widely studied, with a focus on characterizing gene function or gene expression (Wu et al., 2017; Akbudak et al., 2020). Typically, these studies have been conducted using visual inspections of wilting symptoms (Table 3). Although Tamburino et al. (2017) did not observe significant differences between stressed and control tomato plants for biomass fresh weight, many other studies reported opposite evidence ((Azizi et al., 2021; Rady et al., 2020; Tack´ as et al., 2020; Meng et al., 2020) as well as for biomass dry weight (Bian et al., 2019; Rady et al., 2020; Azizi et al., 2021; Chakma et al., 2021 For this parameter, it was reported that plants in which the gene SlCBL3–1 is silenced have an even greater loss of growth under water stress conditions (Hosseini Tafreshi et al., 2021) (Fig. 2). Potato biomass is also affected by water stress, with reductions of up to 92% under severe conditions (Pino et al., 2013). Tolerant transgenic plants with the ScCBFI gene were found to experience a lesser reduction (52%) in fresh weight. Similarly, Rolando et al. (2015) observed a decrease in aerial fresh and dry weight in stressed plants (36% compared to control plants). However, leaf dry weight was not affected to the same extent (della Costa et al., 1997). In eggplant, water stress has a significant impact on fresh weight, with the most affected organs being leaves and roots (Plazas et al., 2019). Studies have shown reductions in both fresh (26% reduction, Semida et al., 2021) and dry (24% reduction, Semida et al., 2021; 52% reduction, Çolak et al., 2015) weight. Additionally, water-stressed plants have been found to increase their root-to-shoot ratio (Delfin et al., 2021; Tani et al., 2018). Overall, it is clear that the biomass of Solanum crops is affected by Table 3 Biomass and growth variables evaluated under water stress conditions. Species Growth and biomass measurements References S. lycopersicum Root and shoot length, root and shoot weight Kulkarni and Desphande (2010) Visual symptoms Goel et al. (2010) Visual symptoms Choi et al. (2011) Visual symptoms Zhu et al. (2014) Height Landi et al. (2016) Visual symptoms Mishra et al. (2016) Height, number of knots, dry and fresh weight Tamburino et al. (2017) Visual symptoms and root length Wu et al. (2017) Germination percentage, root and shoot length, fresh weight. Esan et al. (2018) Visual symptoms Karkute et al. (2018) Height, leaf area and dry weight Bian et al. (2019) Visual symptoms Waseem et al. (2019) Visual symptoms Akbudak et al. (2020) Visual symptoms Filiz and Akbudak (2020) Visual signs, germination percentage and fresh weight Meng et al. (2020) Number of leaves, fresh and dry weight, fruit weight, number of fruits and yield Rady et al. (2020) Dry weight, yield Tak´ acs et al. (2020) Fresh and dry weight Azizi et al. (2021) Height, leaf area, dry weight, number of fruits, length of the fruit and yield. Chakma et al. (2021) Leaf mass area Galm´ es et al. (2013) Height, fresh weight, root length Hosseini Tafreshi et al. (2021) Fresh weight, root length, fruit weight, number of fruits and yield. Krishna et al. (2021) S. tuberosum Visual symptoms Kappachery et al. (2013) Height, dry weight, tuber weight, tuber number and yield della Costa et al. (1997) Tuber number, size and yield Schafleitner et al. (2007) Yield Boguszewska et al. (2010) Plant survival, height, foliage weight, length, diameter and root area Pino et al. (2013) Leaf length, plant cover, aboveground dry weight, tuber dry weight Rolando et al. (2015) Visual symptoms, plant survivals and yield Szalonek et al. (2015) S. kurtzianum Height, total biomass, leaf biomass, leaflet thickness, tuber number and yield Iba˜ nez et al. (2021) S. melongena Leaf area, dry weight, fruit weight, length and width of fruit, yield Çolak et al. (2015) Height, stem diameter, dry weight, number of fruits, yield Díaz-P´ erez y Eaton (2015) Percentage explants live, height, number of leaves, percentage explants callus Siaga et al. (2016) Survived plants, rooted plants, height, root length Zayova et al. (2017) Stem and root length, fresh and dry shoot/root weight ratio Tani et al. (2018) Height, fresh weight Plazas et al. (2019) Yield Amiri Rodan et al. (2020) Height, number of leaves, stem diameter, dry and fresh weight, leaf area, fruit length, number of fruits, fruit weight, yield Semida et al. (2021) Leaf area, dry weight, surface area, density and root length Delfin et al. (2021) M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 8 water stress. The extent of the reduction varies depending on the species and genotype, with some genotypes displaying a higher level of tolerance. These differences in biomass loss could be attributed to variations in environmental conditions or inherent genetic differences. 4.1.4. Yield and yield components Drought has a negative impact on food production worldwide and is widely recognized as the most influential abiotic stress in terms of yield loss (Begna, 2020). The effect of water deficit on tomato yield varies, with greater or lesser losses depending on the level of stress applied. For example, Rady et al. (2020) found decreases of between 62% and 69% for water stress conditions, depending on the season. Similarly, Tack´ as et al. (2020) found slight decreases of 16% and 25% when irrigating at 75% FC, and from 24% to 42% at 50% FC, depending on the year evaluated. Chakma et al. (2021) evaluated the performance at 100%, 75% and 50% FC, observing reductions in yield of 19% and 94% for 75% and 50% FC, respectively. In terms of transgenic plants, Krishna et al. (2021), evaluated double-transgenic plants for AtDREB1A and BcZAT12, finding that the double-transgenic plants presented a higher yield compared to individual ones and the control under water stress (Fig. 2). The decrease in yield is usually due to a decrease in the number of fruits per plant and the weight of the fruit (Rady et al., 2020; Chakma et al., 2021; Krishna et al., 2021). However, it is worth noting that the water deficit often leads to an increase in fruit quality (Albert et al., 2016). Potato tuber yield is also affected by water scarcity, with a decrease of 84%, 72% and 58% when cultivated at 80%, 60% and 40% of their irrigation needs (della Costa et al., 1997). Boguszewska et al. (2010) found percentages of decrease in yield ranging from approximately 25% to 58% when evaluating 10 genotypes under water stress conditions, while Rolando et al. (2015) observed a 59% decrease when irrigating at 50% FC. On the other hand, Schafleitner et al. (2007) found yield losses that averaged around 50% when withholding the water supply, observing genotypes that combined moderate decreases with high yield potential. In the case of transgenic plants (STANN1 gene), Szalonek et al. (2015) found improvements in yield quality under well-irrigated conditions and a slight decrease under water stress compared with WT plants, which had a decreased yield by half. The effects of drought stress on yield components vary depending on the genotype studied, so in some cases, the yield loss is due to the tuber weight, number of tubers or both (Schafleitner et al., 2007). It was also found that the weight of the tubers is promoted when plants are grown at 80% of the ET and not at 100% ET, but in more stressful conditions considerably decrease the weight. In relation to the number of tubers, these are affected by stress to a greater extent than those of larger size (della Costa et al., 1997). Like tomato and potato, eggplant yield is negatively affected by water scarcity (Díaz-P´ erez and Eaton, 2015), although the losses are not as severe. In a study by Semida et al. (2021), a 17% decrease in yield was found when irrigation was at 60% ETc. Amiri Rodan et al. (2020) found an even greater decrease in yield of 32% under more restrictive irrigation at 40% ETc, while Çolak et al. (2015) reported losses of up to 49%. The weight of the fruits, as well as their length and width, were all affected by water stress, with decreases of 13%, 17%, and 23%, respectively (Çolak et al., 2015). The number of fruits was also impacted by water stress and had a stronger correlation with yield than fruit weight (Díaz-P´ erez and Eaton, 2015). However, Semida et al. (2021) found that the majority of the yield loss was due to a reduction in the weight of the fruits, rather than the number of fruits. 4.2. Physiological and biochemical traits One of the first responses of plants to water stress is to close their stomata. This triggers several effects on physiological and biochemical processes, such as adjustments to the photosynthetic system, accumulation of osmolytes, and a number of mechanisms aimed at overcoming the period of water limitation (Kapoor et al., 2020). In addition to this, water stress also causes oxidative stress in plants, leading to an increase in the production of reactive oxygen species, which ultimately leads to cell death, as well as an increase in the production of antioxidant enzymes as a response to water stress (Mattos and Moretti, 2016). Among Solanum species, tomato has the most documented information on the biochemical and physiological processes that occur during water stress. 4.2.1. Water content Drought causes a significant reduction in the relative water content (RWC) of tomato plants (Bian et al., 2019). Reductions of 22% were observed when irrigated at 60% FC (Rady et al., 2020) and 13% when irrigated at 50% FC (Azizi et al., 2021). Chakma et al. (2021) also found decreases in RWC when applying lower amounts of water, although the decreases were not as significant. The decrease in RWC is related to a decrease in water potential and lower plant productivity, as reported by Nunes et al. (2022). To address this issue, scientists have developed transgenic plants that can increase RWC under conditions of limited water (Fig. 2). For example, Goel et al. (2010) reported higher RWC values in some modified lines overexpressing the osmotin gene. In addition, Krishna et al. (2021) evaluated double transgenic plants of AtDREB1A and BcZAT12 exposed to 21 days of water deficit and observed that RWC was less affected than in WT plants. On the other hand, the functionality of genes involved in drought tolerance was assessed by silencing them. In this way, Zhu et al. (2014), when evaluating plants in which the SlNAC4 gene had been silenced, found that the RWC decreased to a greater extent after 15 and 21 days of treatment than in the WT lines. On the other hand, Hosseini Tafreshi et al. (2021) observed that tomato plants with transient silencing of the SlCBL3–1 gene had a higher RWC than non-silenced plants when grown under water stress conditions. Another way to assess the hydration status of leaves is to determine the loss of water content in the leaf by subtracting the weight of the leaf material at the time of detachment from the plant from the initial fresh weight of the leaf. Using this method, Choi et al. (2011) saw a 13% weight loss after 10 min, while resistant transgenic plants (CaXTH3) only lost between 3.4% and 4.7% of their weight. In addition, transgenic lines expressing the AtGRXS17 gene were found to have higher water content than WT plants (Wu et al., 2017). In potato, when evaluating ten genotypes under water stress, it was observed that RWC decreased between 45% and 65% with respect to well-watered plants, but this decrease did not correlate with the loss of yield under stress (Boguszewska et al., 2010). Similarly, in the wild relative S. kurtzianum, the RWC decreased when water availability was lower (Iba˜ nez et al., 2021). Through biotechnological approaches, advances were made in this aspect. In this way, when evaluating transformed potato plants with overexpression of the STANN1 gene, after 12 days without irrigation, the RWC was higher than that of WT plants (Szalonek et al., 2015) (Fig. 2). Similarly, in eggplant, water stress also affects the water status of the plants, causing a decrease in RWC by 7% when subjected to a stress of 60% ETc (Semida et al., 2021). This reduction is less than that observed in tomato plants, indicating a higher tolerance to water stress in eggplant (Rady et al., 2020; Azizi et al., 2021). Additionally, the effect of water stress on the water potential of eggplant can also be seen through a decrease of −1.5 MPa when cultivated with up to 10% of the field capacity replacement (Delfin et al., 2021). 4.2.2. Gas exchange Gas exchange parameters in tomato are affected by hydric stress conditions. Drought causes a decrease in photosynthesis and stomatal conductance a few days after the stress is imposed, with mesophyll conductance being subsequently affected (Bian et al., 2019). Photosynthesis in well-irrigated tomato plants ranges between 15.6 and 20.0 µmol m −2 s −1 , which is significantly reduced under stress conditions, with values ranging between 1.5 and 10.9 µmol m −2 s −1 . Similarly, mesophyll conductance varies between 0.17 and 0.29 mol m −2 s −1 under well-irrigated conditions, while under stress, values range from 0.02 to 0.13 mol m −2 s −1 (Galm´ es et al., 2013). Landi et al. (2016) M. Flores-Saavedra et al. Scientia Horticulturae 318 (2023) 112105 9 evaluated stomatal conductance of tomato under hydroponic, greenhouse, and field conditions and found a strong effect of water stress in hydroponically grown plants exposed to PEG, with stomatal conductance values decreasing by 72%. In the case of greenhouse plants, with the suspension of irrigation, stomatal conductance was reduced by 92%, and in field conditions, the difference was less marked, with a decrease of 43% after 45 days of withholding irrigation. Tamburino et al. (2017) also found an important effect of water stress in tomato, with values reduced by up to 98% compared to control plants, after 19 days of withholding irrigation. The intrinsic and instantaneous water use efficiency (WUE), calculated as the ratio between photosynthesis and stomatal conductance and the ratio between photosynthesis and transpiration rate, respectively, shows a significant increase in tomato plants under water stress, with a trend of increasing values as days of withholding irrigation pass (Bian et al., 2019). Like tomato, potato also experiences a decrease in photosynthesis and stomatal conductance under water stress conditions, while the instantaneous WUE increases (Rolando et al., 2015). Similarly, in the wild parent, S. kurtzianum water stress decreases stomatal conductance, although genotypes were found to be less affected than others (Iba˜ nez et al., 2021). It is important to note that stomatal conductance cannot be used as a reliable indicator of yield under water-limited conditions, as stomatal resistance does not necessarily correlate with higher yield under stress (Schafleitner et al., 2007). Studies have also shown that in water-stress tolerant potato plants overexpressing the STANN1 gene, their photosynthetic rate and stomatal conductance decrease when exposed to water stress, just like WT plants (Szalonek et al., 2015). In eggplant, gas exchange shows a drastic decrease in photosynthetic rate, stomatal conductance and transpiration under water stress, with decreases of 73%, 88% and 82%, respectively. However, the intrinsic WUE, increases by 48% under stress conditions (Delfin et al., 2021). Tani et al. (2018) found that when evaluating two different genotypes, the levels of gas exchange (photosynthesis, stomatal conductance, transpiration, and CO 2 concentration) decreased significantly under water stress conditions (25% field capacity), while the instantaneous and intrinsic WUE increased. On the other hand, Díaz-P´ erez and Eaton (2015), saw that when the temperature is lower, there were no differences in photosynthesis, stomatal conductance, and WUE resulting in a low evapotranspiration demand. However, under conditions of higher temperature, photosynthesis and stomatal conductance decrease due to less water availability. In Solanum crops, a clear trend is observed where the gas exchange parameters decrease in a water stress condition, while the WUE increases. Although an increase in WUE might suggest greater tolerance to stress, it is not always related to reaching higher yields when the plants are grown in limited water conditions. This is because if the plant cannot maintain adequate levels of carbon gain, biomass production will be low, regardless of whether WUE is increased through stomatal closure (Leakey et al., 2019). 4.2.3. Photosynthetic pigments Chlorophyll is the primary pigment responsible for photosynthesis in plants, and its levels have been found to decrease under water stress conditions in tomato plants (Hosseini Tafreshi et al., 2021). However, some studies have not found significant differences in chlorophyll levels after withholding irrigation for 19 days (Tamburino et al., 2017) or only slight decreases when using PEG application (Karaca and Cekic, 2019). When evaluating the chlorophyll content using a portable chlorophyll meter (SPAD), it was found that it was reduced between 25% and 23% when tomato was irrigated to 60% of the FC (Rady et al., 2020), while Chakma et al. (2021), when evaluating the chlorophyll content in the same way, it was reduced by 8% when it was irrigated to 50% FC. By evaluating transgenic tomato plants, significant effects on chlorophyll content were observed (Fig. 2). In this way, when evaluating plants tolerant to water stress, which overexpress the osmotin gene, it was seen that chlorophyll levels increase by approximately 50% under limited water conditions (Goel et al., 2010). Additionally, transgenic plants with overexpression of the SlbHLH22 gene have been found to maintain chlorophyll levels four times higher than WT plants when subjected to severe stress for 30 days without irrigation (Waseem et al., 2019). On the other hand, transgenic plants with silencing of the SlNAC4 gene have been found to have a greater reduction in chlorophyll content under stress conditions than wild-type plants (Zhu et al., 2014). In the same way, when the SlCBL3–1 gene is silenced, the plant exhibited a greater reduction in chlorophyll and carotenoid content under water stress conditions (Hosseini Tafreshi et al., 2021). Similarly, CRISPR-edited plants with the SLWHY2 gene silenced have also been found to be more affected than non-transgenic plants, with chlorophyll levels decreasing by around 30% (Meng et al., 2020). In potato plants, water stress has been found to have varying effects on photosynthetic pigments. Some studies have found that SPAD values, which measure chlorophyll content, are higher under water stress conditions than in control conditions (Rolando et al., 2015). However, other studies have not found a significant increase in chlorophylls a and b when plants are subjected to 14 days without irrigation (Szalonek et al., 2015). In water-stressed S. kurtzianum plants, it has been observed that the content of chlorophyll and carotenoids decreases significantly under severe stress conditions (Iba˜ nez et al., 2021). In eggplant, research has found that chlorophyll a, chlorophyll b, and carotenoids decrease in content when the plants are exposed to severe water stress. These photosynthetic pigments also display a positive correlation with biomass levels (Plazas et al., 2019). Under moderate water stress conditions, a trend towards a reduction in carotenoids has also been reported (Amiri Rodan et al., 2020), while chlorophylls, as measured by a SPAD meter, tend to increase (Díaz-P´ erez and Eaton, 2015). When comparing results from different studies, it is clear that the effects of water stress on photosynthetic pigments can vary greatly. The way in which the results are expressed, such as in mg of chlorophyll per g of fresh weight or dry weight, or as a chlorophyll index when using a SPAD meter, can also play a decisive role. Additionally, the results, when expressed in fresh weight or using a SPAD meter, do not take into account the effect of nutrient dilution that occurs when comparing irrigated plants to those under water stress. Therefore, to accurately evaluate the effects of stress on photosynthetic pigments, it is best to express the values in dry weight, although this method is not commonly used in research articles (Plazas et al., 2019; Iba˜ nez et al., 2021). 4.2.4. Oxidative stress To assess the effect of drought on oxidative stress in tomato, measurements of the content of malondialdehyde (MDA), hydrogen peroxide (H 2 O 2 ) and superoxide (O 2 − ) are used as biomarkers, where they generally increase their concentration under water stress conditions (Rady et al., 2020). Several studies have reported oxidative stress in tomato plants exposed to different levels of water stress and genotypes. For instance, Akbudak et al. (2020) observed that the contents of H 2 O 2 and MDA increased by more than 100% in plants that were stressed for 10 days without irrigation. Similarly, Krishna et al. (2021) found that the concentration of H 2 O 2 increased as the stress level increased, with values more than two-fold higher in plants that were stressed for 21 days compared to control plants. When comparing tolerant and susceptible genotypes, it was found that the MDA and H 2 O 2 content increased in both, but the increase was lower in the tolerant genotypes (Hosseini Tafreshi et al., 2021). Additionally, Meng et al. (2020) reported an increase in H 2 O 2 and O 2 − levels in tomato plants stressed with PEG. However, when evaluating the oxidative stress levels in leaves and roots separately, Filiz and Akbudak (2020) found that the H 2 O 2 and MDA changes were only significant in the leaves and not in the roots. An increase in MDA content indicates a higher level of oxidative stress that can be induced by lack of water, which correlates with a lower level of growth; however, there are eggplant genotypes that are able to maintain their MDA levels under water stress conditions, which is M. Flores-Saavedra et al.