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http://journals.tubitak.gov.tr/agriculture/

Turkish Journal of Agriculture and Forestry Turk J Agric For(2019) 43: 593-602© TÜBİTAKdoi:10.3906/tar-1808-1

Change in physiological and biochemical parameters under drought stress in salt-tolerant and salt-susceptible eggplant genotypes

Sevinç KIRAN1,*, Şebnem KUŞVURAN2, Fatma ÖZKAY1

, Ş. Şebnem ELLİALTIOĞLU3

1Soil Fertilizer and Water Resources Central Research Institute, Ankara, Turkey2Kızılırmak Vocational High School, Çankırı Karatekin University, Çankırı, Turkey

3Department of Horticulture, Faculty of Agriculture, Ankara University, Ankara, Turkey

* Correspondence: sevinckiran@tgae.gov.tr

1. Introduction An ever increasing world population, climate change, and resultant variations in precipitation have increased water demands in agriculture. Changing precipitation regimes ultimately result in drought and salinity problems over agricultural lands, and such negative impacts significantly restrict plant production (www.un.org/millennium/summit.htm). Droughts hinder plant growth by reducing cell division and growth and regressing plant nutrient uptake and growth rates.

Drought stress is an environmental stress factor with significant influences on physiologic, biochemical, and molecular processes in plants. Drought tolerance is defined as the ability of plants to sustain vital activities under water deficit conditions (Richardson and McCree, 1985). The changes in physiologic, morphologic, genetic, and biochemical characteristics have significant effects on plant tolerance to droughts. Such changes can be a decrease in leaf areas (Mahajan and Tuteja, 2005), a

thickening of xylem and cell walls (Awasthi and Maurya, 1993), stomal closure (Benesová et al., 2012), a decrease in gas exchange and cell turgor (Garg et al., 2004), and differences in ion accumulation (Hong-Bo et al., 2006), osmotic potential (Stoyanov, 2005), and plant hormone levels (Hosoki et al., 1987). Such changes may also include formation of reactive oxygen species (ROS) in enzymatic and nonenzymatic systems (Keunen et al., 2013) and increases in malondialdehyde production against cell membrane damages (Oliveira et al., 2014).

Genetic differences are quite effective in plant tolerances against drought stress (Kulkarni and Deshpande, 2007). The majority of cultivated plants are sensitive to drought, so stress-tolerant species are usually selected in cultivation practices. Eggplant (Solanum melongena L.) is commonly grown in these arid and semiarid regions. World eggplant production is 48.5 million tons, and Turkey has an annual production of about 850 thousand tons (http://faostat.fao.org). In Turkey, the eggplant has a significant

Abstract: This study was carried out to determine the changes in chlorophyll and ion contents, lipid peroxidation level, and antioxidative enzyme activities of 4 eggplant genotypes (Mardin Kızıltepe (MK), Burdur Merkez (BM), Artvin Hopa (AH), and Kemer (K)) with different salt tolerance levels under drought stress. Experiments were conducted in a greenhouse with controlled climatic conditions. When the plants reached the 3–4 leaves stage, 3 different treatments were applied to create the drought levels. The first condition in the study was obtained through control application treatment (D0), which was depletion of 40% of the available water holding capacity. The second application (D1) was depletion of 90% of the available water holding capacity. The third application (D2) was no applied irrigation water. Plants were analyzed with regards to chlorophyll and ion contents, lipid peroxidation levels, and antioxidative enzyme activities. Significant differences were observed among interactions of genotypes and levels of drought stress in chlorophyll and ion (K+, Ca2+, Zn2+, Mn2+, and Fe2+) contents, lipid peroxidation levels, and activities of antioxidant enzymes (superoxide dismutase-SOD, catalase-CAT, glutathione reductase-GR, and ascorbate peroxidase-APX). The chlorophyll and ion contents of eggplant genotypes under drought stress decreased. However, different levels of lipid peroxidation and antioxidative enzyme activities were observed. The situation changed depending on genotypes. Lipid peroxidation levels increased under drought stress, especially in salt-sensitive genotypes (AH and K). On the other hand, the activities of antioxidative enzymes in the salt tolerant MK and BM genotypes significantly increased. With regard to the investigated traits, salt-tolerant genotypes were also found to be quite tolerant against drought stress.

Key words: Antioxidant enzyme, drought stress, physiological traits, salt stress, Solanum melongena L.

Received: 01.08.2018 Accepted/Published Online: 21.06.2019 Final Version: 03.12.2019

Research Article

This work is licensed under a Creative Commons Attribution 4.0 International License.

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production potential and exhibits a great variation in genetic characteristics. However, drought and salinity are common problems in arid and semiarid regions of the country. Although Behboudin (1977) stated that the eggplant is tolerant to drought according to many vegetable species, Fu et al. (2013) emphasized that eggplants (Solanum melongena L.) are very sensitive to water stress during their entire growing period and plants respond to water stress differently depending on the species, duration, and severity of soil water depletion. Considering this view, eggplants can be hypothesized to have novel characteristics, be morohological, physiologically, and biochemically efficient under drought stress. The present study was conducted to investigate some morphological and biochemical characteristics of eggplant genotypes under drought stress. Selected genotypes were previously subjected to experiments for salt tolerance (Yaşar, 2003). Thus, this study also aims to investigate whether there is a mutually supportive relationship between salt tolerance and drought tolerance in eggplants.

2. Materials and methods2.1. MaterialsThe eggplant genotypes used in this study were selected from the genotypes previously tested for salt tolerance (Yaşar, 2003). Among the tested genotypes, 2 salt-tolerant (Mardin Kızıltepe (MK) and Burdur Merkez (BM)) and 2 salt-sensitive (Artvin Hopa (AH) and Kemer (K)) geno-types were used. These genotypes were examined for bio-mass, leaf area, relative water content, stoma conductivity, and leaf water potential under the same drought stress. It was determined that MK and BM had better tolerance to drought than K and AH based on these properties (Kıran et al., 2016). 2.2. MethodsExperiments were conducted in a greenhouse with con-trolled temperature and relative humidity levels (green-house air temperature ranged from 24/20 °C (day/night) with 50%/55% relative humidity). Genotype seeds were sown in 13-L plastic pots containing medium-textured soil (48.9% sand, 17.5% silt, and 33.6% clay) (10 seedlings were grown in each pot). Until 3–4 leaves formed in all pots, which were irrigated daily, the stress of the plant was not allowed to occur. In the experiment, the pot weight was taken into account when determining the amount of irrigation water. For this purpose, pots were weighed daily. All pots were brought to field capacity before the imple-mentation of experimental traits. The first application in the study was the control application (D0), which was depletion of 40% of the available water holding capacity. The second application (D1) was depletion of 90% of the available water holding capacity. The third application (D2) was no applied irrigation water for 33 days after sowing (DAS). It is tolerant to drought with respect to many veg-

etable species of eggplant (Behboudin, 1977). Therefore, the rates of water stresses were selected as very high and “depletion of 40% of the available water holding capacity (D0)” treatment was determined as the subject of control in the experiment. The treatment of D1 was chosen in case the seedlings could not tolerate withholding condi-tions (severe drought). Plant samples were taken when the symptomatic effects of drought stress were distinctive on the 12th day of stress treatment (42 DAS). Samples were analyzed with regard to chlorophyll and ion contents, lipid peroxidation levels, and antioxidative enzyme activities. 2.2.1. Chlorophyll content analysesLeaf segments (200 mg) were placed in 5 mL of 80% ethanol and heated in a water bath at 50 °C for 20 min. Total chlorophyll was evaluated in the alcohol extracts from absorbance readings, using the appropriate extinction coefficient. The chlorophyll content (ppm) was calculated as 1000 × A654 / (39.8 × sample FW), according to Luna et al. (2000).2.2.2. Determination of ion content Leaf samples were dried in an oven at 65 °C. The concentrations of K+, Ca2+, Zn2+, Mn2+, and Fe2+ were determined using an Inductively Coupled Plasma Emission Spectrometer (ICP model Liberty 200, Varian Australia Pty. Ltd., Australia). Before the analysis, 50 mg of ground dry material was digested by adding 2 mL concentrated HNO3 (65%) and 1 mL H2O2 (30%) for 30 min at 2600 kPa (80 psi) in a MDS-2100 microwave oven (CEM Corp., USA). After digestion, each sample was brought up to the final volume of 25 mL with deionized water (Kacar and Inal, 2008).2.2.3. Lipid peroxidation Lipid peroxidation was measured as the amount of MDA determined by the thiobarbituric acid (TBA) reaction (Heath and Packer, 1968). Frozen samples (0.2-g leaf sample) were homogenized with a prechilled mortar and pestle with 2 volumes of ice-cold 0.1% (w/v) TCA and centrifuged for 15 min at 15,000 g. Assay mixture containing 1 mL of the supernatant and 2 mL of 0.5% (w/v) TBA in 20% (w/v) TCA was heated at 95 °C for 30 min and then rapidly cooled in an ice bath. After centrifugation (10,000 g for 10 min at 4 °C), the supernatant absorbance was noted as 532 nm, and the values corresponding to nonspecific absorption (600 nm) were subtracted. Lipid peroxidation products were measured as the content of TBA-reactive substances. The MDA content was calculated according to the molar extinction coefficient of 155 m (Mcm)−1.2.2.4. Antioxidative enzyme analyses Approximately 1 g of leaf tissue samples was crushed in liquid nitrogen in a porcelain mortar and homogenized with 50 mM (10 mL) potassium-phosphate buffer solution (pH 7.6) containing 0.1 mM Na-EDTA. Homogenized samples were then centrifuged at 15,000 rpm for 15

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min and kept at +4 °C until the analyses. Measurements were performed in an Analytical Jena 40 model spectrophotometer. During enzyme measurements, final volumes were achieved with buffer solutions. Superoxide dismutase (SOD) activity was determined through nitro blue tetrazolium chloride reduction by O2– under light (Karanlık, 2001). Catalase activity (CAT) was calculated by the increase in absorbance at 240 nm (E = 39.4 mM cm−1) in accordance with Cakmak and Marschner (1992). A unit of CAT activity was defined as µmol H2O2 decomposed min−1mg−1. Glutathione reductase (GR) activity was measured in accordance with Cakmak and Marschner (1992) based on NADPH oxidation at 340 nm (E = 6.2 mM cm−1). 1 µmol mL−1 min−1 was defined as 1 U of GR activity. Ascorbate peroxidase (APX) was measured through the decrease in absorbance at 290 nm (E = 2.8 mM cm−1) as a result of the oxidization of ascorbate according to Cakmak and Marschner (1992). One unit of APX was defined as 1 µmol mL−1 ascorbate oxidized min−1.2.2.5. Data analyses Experiments were conducted in completely randomized 2 factor designs with 3 replications. The first factor was eggplant genotype and the second factor was drought treatment. Experimental results were subject to variance analysis. MSTAT-C software was used in statistical anal-yses (Freed et al., 1989) and means were compared with Duncan’s multiple-range test (Duncan, 1955).

3. Results and discussion3.1. Chlorophyll contentThe ability of plants to tolerate abiotic stresses and a degree of damage to the photosynthetic activity of these stresses is related to chlorophyll (Maxwell and Johnson, 2000). Chlorophyll loss has previously been used in drought studies as an indicator trait (Sayyari et al., 2013). Drought stress had significant effects on chlorophyll content of eggplant plants (P ≤ 0.05), and ‘Genotype × Drought Treatment’ interaction was found to be significant (Table 1). In this study, as expected, the highest chlorophyll values

were found in the control plants. However, these values decreased with drought stress, and this effect changed according to genotypes and drought levels compared to control groups. Under stress conditions, while the greatest chlorophyll contents were observed in ‘MK × D1’ and ‘MK × D2’ combinations (0.39 and 0.38 ppm), the lowest values were seen in ‘Kemer × D1’ and ‘Kemer × D2’ combinations (0.21 and 0.20 ppm). It was observed that MK is the genotype that best protects chlorophyll contents under stress conditions (Figure 1a). Actually, under the same drought stress, MK was able to protect shoot fresh weight and relative water content more than the others (Kıran et al., 2016). However, the photosynthetic apparatus has been reported to tolerate water deficit. A water deficit of 30% is estimated to be the limit that can significantly affect photosynthesis biochemistry (Cornic and Fresneau, 2002). The genotypes previously identified as salt-tolerant were able to preserve chlorophyll contents under drought conditions. The results revealed that the components of the photosynthetic apparatus could be damaged in drought sensitive genotypes, and drought tolerance genotypes could have good adaptability to decrease/evade impairments resulting from drought stress. It can be suggested that genetic differences exist in the reaction of the photosynthetic apparatus to drought. Previous studies have reported a strong correlation between environmental stress tolerance and chlorophyll content, and these may serve as reliable indicators of how tolerant the plant is to the stresses (Chowdhury et al., 2017).3.2. Ion contents With regard to the K+, Ca2+, Zn2+, Mn2+, and Fe2+ contents of eggplant genotypes, the ‘Genotype × Drougth Treatment’ interaction was found to be significant (P ≤ 0.05) (Table 1). Drought stress reduced the K+, Ca2+, Zn2+, Mn2+, and Fe2+ contents of the plants.

The role of K+ in the tolerance of plants to drought stress may be observed in stomatal regulation, energy level, protein synthesis, and internal equilibrium (homeostasis) (Marschner, 1985). At the same time, K+ may prevent

Table 1. ANOVA for chlorophyll and some ion accumulations related to the traits of eggplant genotypes under drought stress.

Source of variance df Chlorophyll K+ Ca2+ Zn2+ Fe2+ Mn2+

G 3 ** ns ** ** ** **DT 2 ** ** ** ** ** **G × DT 6 * ** ** ** ** **Error 24 0.00 2.60 0.07 8.24 40.79 32.61CV (%) 4.28 8.81 9.98 13.49 8.07 8.08

G: genotype; DT: drought treatment; ns: nonsignificant (** and *, significant in a 1% and 5% area, respectively).

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drought damage to the plant by maintaining the turgor pressure and by reducing transpiration (Andersen et al., 1992). Ca2+ plays a significant role in the structure of cell wall and linkage between cells (Maeda et al., 2003). On the other hand, it plays a regulatory role in the balance of cation-anion and acts as an activator for several enzymes (Akıncı and Simsek, 2004). Drought stress caused the decreases of K+ and Ca2+ contents of the plants (Figures 1b and 1c). The highest values were found in the K+ content of ‘BM × D1’ (18.11%), Ca2+ contents of ‘MK × D2’ (2.91%), and ‘MK × D1’ (2.83%) combinations compared to control plants under stress conditions. Considering the genotypes, BM and MK genotypes had higher K+ and Ca2+ contents than AH and Kemer genotypes. MK and BM genotypes lost less K+ than their bodies under drought stress. Thus, they were able to provide more water entry into the cell by protecting the intracellular osmotic potential. It has been reported in the literature that MK has an important place in the preservation of osmotic balance by keeping K+ under plant stress, namely, under salt stress. Yaşar (2003) indicated that differences in the salt tolerance of genotypes were closely related to selective K+ or Na+ uptake and the ratio of these 2 ions and that MK has lower K+ and Ca2+ losses under salt stress. Salt-tolerant plants may hold more K+, Ca2+, Zn2+, Mn2+, and Fe2+ ions that protect the cell membrane and chlorophyll levels. Sivritepe et al. (2008) reported decreasing K+, Ca2+, Mn2+, and Fe2+

concentrations under oxidative stress. Htoon et al. (2014) reported decreasing K+ concentrations under drought stress and indicated that such decreases were more in sensitive cultivars. It was also reported in previous studies that drought significantly decreased Ca2+ contents and degradation in chloroplasts reduced Ca2+ accumulation in plants (Samarah et al., 2004).

Zn2+ also exists in structures of antioxidative enzymes and defense mechanisms (Blasco et al., 2015). As in other ions, drought conditions negatively influenced Zn2+ uptake of plants and different responses were observed in different genotypes. The genotypes with higher drought tolerance were able to better preserve Zn2+ ion contents. Eggplant genotypes showed different responses to water stresses in Zn2+ concentration. The highest Zn2+ content was found in the control plants. On the other hand, with drought stress, Zn2+ decreased to different levels. Accordingly, the greatest Zn2+ concentrations were observed in ‘BM × D1’ and ‘MK × D1’ combinations, respectively (23.43 and 22.96 ppm) (Figure 1d). Nutrient uptake in plants under drought stress plays an important role in the plants’ tolerance to drought (Samarah et al., 2004). The 2 genotypes tolerant to salt in our study also maintained Zn2+ losses at lower levels as they were better adapters in arid conditions. The mechanisms for Zn-mediated drought tolerance are still not fully understood but have been suggested to be involved in the transpiration rate and leaf osmotic potential

rate changes (Sadoogh et al., 2014), increases of water use efficiency (Karim et al., 2012), and decrease of biochemical damage by antioxidant enzymes (Upadhyaya et al., 2013). Sánchez-Rodríguez et al. (2010) indicated differences in Zn2+ uptake of tomato and potato plants under drought conditions while others have reported increased Zn2+ contents in some genotypes and decreased values in others (Nawaz et al., 2015).

Drought affects plant growth by hindering photosynthesis and metabolic activities (Evers et al., 2010). Fe2+ homeostasis is essential for photosynthesis efficiency (Briat et al., 2015). Fe2+ usually decreases under abiotic stress conditions (Hu et. al., 2007; Sánchez-Rodríguez et al., 2010). In this study, the MK genotype, which is tolerant to salt, was able to protect the Fe content better than AH and Kemer. This genotype was able to preserve the Fe2+ concentration of leaves under drought stress (96.80 and 93.53 ppm) (Figure 1e). Under the same stress, BM also kept more Fe2+ in its body and showed more tolerance to stress than Kemer and AH. It has been reported that there are differences between genotypes in Fe2+ and Zn2+ uptake and a loss of plants under drought stress. Compared to the control plants, reductions were noted in the Fe2+ uptake of the plants exposed to drought (Yusuf et al., 2002; Yasar et al., 2014). The results of this study demonstrated that a decrease in Fe2+ contents might affect the related enzymes of chlorophyll biosynthesis. Although Fe2+ is not the main component of the chloroplast, it still plays a vital role in the synthesis of chlorophyll precursors (Yang et al., 2016). Hu et al. (2007) and Nawaz et al. (2015) also reported significant decreases in Fe2+ contents under drought stress.

Mn2+ is essential for photosynthesis, proteins, and enzymes and helps with iron in chlorophyll formation in plants. Manganese accumulation may vary from one plant to another under drought stress. Thus, a great variation was observed in Mn2+ accumulation of the genotypes in the present study. According to our results, eggplant genotypes showed different responses to water stresses in Mn2+ concentration. The highest Mn2+ content was found in the control plants. However, under stress conditions, the greatest Mn2+ concentrations were observed in ‘MK × D2’ and ‘MK × D1’ combinations, respectively (79.82 and 78.29 ppm) (Figure 1f).

A salt-tolerant plant is a better protective of Mn2+ content and is able to better tolerate drought stress. Mn2+ plays a vital role as a cofactor, including Mn-SOD and Mn-CAT, which participate in plant defense against oxidative stress. Although not clear, it is also assumed that Mn2+ acts as a scavenger of O2− and H2O2 (Ducic and Polle, 2005). Several studies also revealed that supplemental Mn2+ plays an important role in the adaptive responses of plants under various environmental stresses (Rahman et al., 2016). In a study by Yusuf et al. (2002) carried out on a chickpea plant by the application of drought stress, the

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results were similar to the results of this study. There were significant differences among the genotypes in terms of the Mn2+ decrease in that study. Samarah et al. (2004) reported that there was a significant relationship between mineral element uptake and tolerance to drought in soybeans and potatoes grown under drought stress conditions. The current findings comply with the results of Sánchez-Rodríguez et al. (2010) and Peuke et al. (2013).3.3. Lipid peroxidation levels Drought significantly increased the malondialdehyde (MDA) contents of eggplant plants (P ≤ 0.05) (Table 2).

MDA is a lipid peroxidation product and an indicator of drought stress-induced oxidative damage. Under stress treatments, the MDA content increased in leaf tissues of all genotypes. While the lowest MDA contents were observed in ‘BM × D2’ and ‘BM × D1’ combinations (6.19 and 5.93 µmol g-1 FW), the greatest values were observed in ‘Kemer × D2’, ‘Kemer × D1’, ‘AH × D2’, and ‘AH × D1’ combinations (9.48, 9.13, 8.77, and 8.67 µmol g−1 FW, respectively). However, the differences among these combinations were not significant (Figure 2a). Generally, salt-tolerant genotypes accumulated less MDA levels

Figure 1. Changes in chlorophyll (a); potassium (K+) (b); calcium (Ca2+) (c); zinc (Zn2+) (d); iron (Fe2+) (e); and manganese (Mn2+) (f) contents under drought stress conditions. The different superscript letters indicate statistically significant differences (P ≤ 0.05). Error bars represent the standard error (±SE) of the means. (MK: Mardin Kızıltepe; BM: Burdur Merkez; AH: Artvin Hopa; K: Kemer; D0: control [depletion of 40% of the available water holding capacity]; D1: depletion of 90% the available water holding capacity; D2: no applied irrigation water).

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than sensitive genotypes and thus the genotypes MK and BM had the lowest cell damage. However, Yaşar (2003) reported that MK had lower MDA contents than Kemer and AH genotypes. Studies conducted by Sairam and Saxena (2000) on wheat and Sanchez-Rodriguez et al. (2010) found that oxidative damage was lower in cultivars tolerant to drought stress and thus less lipid peroxidation occurred than in sensitive cultivars. It was reported in several previous studies that drought stress increased the MDA contents of the plants (Sayyari et al., 2013; Chen et al., 2016).3.4. Antioxidative enzyme contentsDrought stress influences SOD, POD, CAT, and APX enzymatic antioxidant defense mechanisms of various plants (Sharma and Dubey, 2004). Drought stress treatments in the present study also significantly increased antioxidative enzyme activities of eggplant genotypes (P ≤ 0.05) (Table 2). SOD catalyzes the dismutation of O2 to H2O2, and O2. It was reported in earlier studies of some plant species (Baroowa et al., 2016) that SOD activities increased at different rates by drought stress depending on the structure of plant genetics. Acar et al. (2001) suggested that a correlation between plant resistance to drought stress and SOD activities existed. SOD enzyme activity, responsible for the elimination of oxidative stress-induced superoxide radicals, increased under stress conditions, and the greatest values were observed in ‘AH × D2’ and ‘AH × D1’ combinations (510.01 and 505.00 U min-1 mg-1 FW) (Figure 2b). SOD enzyme activity increased more at the end of stress in tolerant genotypes (MK and BM). Increased SOD activity in MK and BM also correlates with improved protection against oxidative damage. Tolerant genotypes were able to protect themselves more effectively against superoxide radicals. Yaşar (2003) reported that tolerant eggplant genotypes use SOD enzymes more actively under salt stress. Karanlık (2001) states that plants increasingly produce O2 during stress and that they are trying to destroy these radicals by increasing SOD activity

in their bodies. In plants that are tolerant to increased SOD enzyme activity, studied under drought stress conducted by Liu et al. (2009) and Çelik et al. (2017), this increase was more pronounced. It was observed in the present study that relevant enzymes had positive impacts on tolerance to stress conditions, but significant increases in SOD activity were not found to be sufficient by themselves to provide tolerance to drought stress. As a result, coordination with other enzyme activities should be taken into consideration to provide sufficient tolerance to drought stress.

The CAT enzyme is responsible for the elimination of reactive oxygen species created under oxidative stress by converting them into water and molecular oxygen (Dionisio-Sese and Tobita, 1998; Kuşvuran, 2010). CAT enzyme activity of genotypes generally increased with drought treatments, but genotypes exhibited different responses with regard to CAT activity. The greatest values under drought conditions were observed in ‘BM × D2’, ‘BM × D1’, ‘MK × D2’, and ‘MK × D1’ combinations, respectively (Figure 2c). On the other hand, Kemer and AH genotypes had lower CAT enzyme activity levels. In general, salt-tolerant genotypes had higher CAT enzyme activities under drought stress. CAT enzyme activity plays a protective role for drought tolerance in MK and BM. The antioxidant activity of CAT, in particular, plays a protective role for drought stress tolerance in MK and BM. In addition, drought increases the amount of MDA in the MK and BM genotypes with the increase of the sensitive genotypes, indicating lower oxidative damage in the membranes. CAT content may inhibit the production of free radicals at a given level in both genotypes and thus may result in less lipid peroxidation. The results are consistent with the findings of Yaşar (2003) that the same genotypes have undergone CAT activity under salt stress. It has been reported that the CAT activity of varieties tolerant to drought stress in tomatoes, canola, wheat, and potatoes is higher than that of susceptible varieties (Sanchez-Rodriguez et al., 2010; Hosseini et al., 2015; Nawaz et al., 2015; Li et al., 2017).

The results on GR enzyme activities were similar to ones on catalase enzyme activities. GR is one of the antioxidant defense mechanisms and plays a significant role in the elimination of H2O2 in chloroplasts and mitochondria (Kuşvuran, 2010). The greatest GR activities were observed in ‘MK × D2’, ‘MK × D1’, ‘BM × D2’, and ‘BM × D1’ combinations (482.59, 470.0, 456.64 and 468.57 μmol min−1 mg−1 FW, respectively) (Figure 2d). With regard to GR enzyme activity, the AH genotype was found to be insufficient and exhibited a close value to control plants under drought stress. ‘AH × D2’ and ‘AH × D1’ combinations had GR activities, respectively, of 214.33 and 212.35 μmol min−1 mg−1 FW. There was an increase in GR activities in all genotypes against drought stress. The most significant increases compared to the control

Table 2. ANOVA for malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase (APX) enzymes related to the traits of eggplant genotypes under drought stress.

Source of variance df MDA SOD CAT GR APXG 3 ** ** ** ** **DT 2 ** ** ** ** **G × DT 6 ** ** ** ** **Error 24 0.36 138.61 86.17 413.15 1152.68CV (%) 9.39 3.69 4.80 6.17 2.46

G: genotype; DT: drought treatment; ns: nonsignificant (** and *, significant in a 1% and 5% area respectively).

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plants were determined in the salt-tolerant BM and MK genotypes. Both genotypes can protect the membranes of GR enzyme under dry conditions. Additionally, the MDA levels of these 2 genotypes are found to be lower. The same genotypes showed similar responses under salt stress. The increases in GR enzyme activities were found to be higher than those of the aged genotypes (Yaşar, 2003). In addition to Sánchez-Rodríguez et al. (2010), Chakraborty et al. (2015) indicated that GR enzyme activities increased during drought treatment by comparison with their controls.

APX has been found to be an efficient regulator of ROS, as it helps maximize H2O2 (Pandey et al., 2017). In

the study, under drought stress, MDA, SOD, CAT, and GR, as well as APX activities, have common characteristics in genotyping. The salt stress tolerant genotype was less affected than the BM plants. With regard to APX activity, the BM genotype exhibited the greatest response against drought stress. ‘BM × D2’ and ‘BM × D1’ combinations were in first place with a quite higher difference than the others (respectively, 2310.00 and 2295.67 μmol min−1mg−1 FW). In terms of GR activity, the AH genotype had the lowest APX activities (1083.33 and 1095.00 μmol min−1mg−1 FW) (Figure 2e). Thus, our results suggest that CAT, APX, and GPX activities coordinated with SOD activity and played a central protective role in the O2

− and H2O2  scavenging

Figure 2. Changes in MDA (a) and superoxide dismutase (SOD) (b); catalase CAT (c); glutathione reductase GR (d); and ascorbate peroxidase APX (e) activities under drought stress conditions. The different superscript letters indicate statistically significant differences (P ≤ 0.05). Error bars represent the standard error (±SE) of the means. (MK: Mardin Kızıltepe; BM: Burdur Merkez; AH: Artvin Hopa; K: Kemer; D0: control [depletion of 40% of the available water holding capacity]; D1: depletion of 90% the available water holding capacity; D2: no applied irrigation water).

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process (de Azevedo Neto et al., 2006). In addition, the active involvement of these enzymes is related, at least in part, to drought-induced oxidative stress tolerance in eggplant plants. Plants under slight water stress treatment generally showed smaller increases in  enzyme activity when compared to those submitted to more intense stress. It has been shown that tolerant genotypes (BM and MK) often have increased  antioxidant  enzymes activity and a lesser degree of membrane damage (as indicated by the low MDA content) when compared to sensitive ones (AH and K), suggesting that these enzymes play an important role in tolerance against drought stress, with the balance between SOD, CAT, GR, or APX being crucial for determining the steady-state level of H2O2 and superoxide radicals. As mentioned by Celikkol Akcay et al. (2010), increased production of free radicals under severe drought stress treatment might cause an interaction with the antioxidant enzyme primary with its potential oxidation and inactivation. Accordingly, Sánchez-Rodríguez et al. (2012) and Nawaz et al. (2015) reported increased APX enzyme activities under drought stress. Sánchez-Rodríguez et al. (2010) and Awasthi et al. (2017) reported that drought stress leads to an increase in APX enzyme

activity. They found that this increase was greater in susceptible genotypes than in tolerant genotypes. On the other hand, APX enzymes were not activated as much as SOD, CAT, and GR in MK. Lotfi et al. (2015) indicated that decreases might be seen in APX enzyme activities with increasing drought severity and durations.

In conclusion, chlorophyll, K, Ca, Zn, Mn and Fe ion contents, lipid peroxidation levels, and the antioxidative enzyme activities of 4 different eggplant genotypes were analyzed under drought stress conditions in this study. It was observed under drought stress that BM and MK genotypes had lower losses in analyzed traits. On the other hand, salt-sensitive AH and Kemer genotypes presented greater losses and exhibited more sensitive responses to drought stress. Antioxidative enzyme activities played a significant preventive role against stress conditions and such defense mechanisms were found to be quite effective in providing tolerance to drought stress. MK and BM genotypes may be utilized in eggplant breeding aimed to develop varieties for areas with limited water. Thorough knowledge and an understanding of drought tolerance mechanisms need careful and continued research.

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