**1. Introduction**

Soil and water resource salinization is one of the main abiotic stress factors that reduce plant growth and crop productivity worldwide [1]. It has been estimated that the total land affected by salinization covers approximately 412 million ha and mainly occur in arid and semiarid regions of more than 100 countries in all continents [2]. Generally, a saline environment influences every aspect of crop physiology and growth by causing water deficits due to the low water potential in the root medium, plant toxic ions uptake (e.g., Na+, Cl−, SO4 <sup>2</sup>−), reduction in the uptake/or transport to the shoot of K+, Ca2<sup>+</sup>, and Mg2<sup>+</sup> [1]. Moreover, plants grown in saline environments experience reduction of photosynthetic capacity [3] and growth in terms of number of leaves, shoot length, decrease in photosynthetic pigment content due to the negative effect of Na solutes within plant cells, and a decrease in fresh and dry matter content [4].

Breeding programs to combine salt resistance traits from different germplasm are increasing among seed companies. However, they currently require complicated evaluation and result in delays in the release of new varieties in the market due to the complexity of the salinity traits [5].

Furthermore, plant responses to salinity derive from complex and multifaceted mechanisms [6] and salt tolerance traits involve several physiological and genetic features that overall limit the success of resistance and/or tolerance trait transfer into commercial varieties [1]. To reduce or avoid production losses due to salt stress, a sustainable and fast solution is grafting highly productive genotypes onto potentially salt tolerant rootstocks [1]. Vegetable grafting is a widely used technique in Japan, Korea, the Mediterranean basin, as well as in several European countries to avoid biotic (i.e., soilborne and foliar pathogens, weeds, and arthropods) or abiotic stressors like drought, flooding, heavy metals contamination, sub optimal temperatures, nutritional deficiencies, and salinity. The salt tolerance of grafted plants is influenced by both scion and rootstock [6]. Generally, the use of salt tolerant rootstocks allows the mitigation of the detrimental effects of salinity and guarantees stable yields during the growing cycle through specific morphological, biochemical, metabolic, and physiological mechanisms. Accordingly, under saline conditions, grafted plants tend to accumulate more biomass in the root system, thus allowing for mitigated salinity effects by increasing the root/shoot ratio [1,7–9]. Additionally, to cope with saline stress, grafted plants adopt strategies like salt exclusion in the shoot and retention of salt ions in the root system [1,7]. Besides these strategies, grafting promotes, at the cellular level, a better maintenance of potassium homeostasis, together with accumulation of compatible solutes and osmolytes in the cytosol, along with compartmentation of salt ions in the vacuole through the activation of the antioxidant defense system, and induction of hormones mediated changes in plant growth [1,7,10,11].

*Cucurbitaceae* species like melon (*Cucumis melo* L.), cucumber (*Cucumis sativus* L.), and watermelon (*Citrullus vulgaris* Schrad.) are generally considered salt sensitive or moderately sensitive crops, while being often cultivated in areas undergoing soil and water salinization [12,13]. In cucurbits, grafting commercial varieties into salt tolerant rootstocks was shown to reduce production losses by improving their photosynthetic capacity [1,3]. Watermelon is commonly grafted on bottle gourd (*Lagenaria siceraria* (Molina) Standl.), interspecific hybrids between *C. maxima* and *C. moschata*, and wild watermelon (*Citrullus* spp.) [14]. Cucumber is generally grafted on bottle gourd, luffa (*Lu*ff*a cylindrica* Mill.), *Cucurbita* interspecific hybrids, and *Cucumis* spp [15]. Finally, melon is generally grafted on interspecific hybrids between *C. maxima* Duch. and *C. moschata* Duch. and *Cucumis melo* L. rootstocks and in some cases on luffa [5]. However, their response to salinity, as rootstock vary between the genotypes, and completed screening to identify salt tolerant rootstock varieties to be adopted in commercial grafting program have not yet been carried out.

In the light of these observations, our aim was to evaluate the response to salinity stress in different melons (*Cucumis melo* L.), watermelon (*Citrullus vulgaris* Schrad.), interspecific hybrids of *C. maxima* Duch. × *C. moschata* Duch., bottle gourd (*Lagenaria siceraria* (Molina) Standl.), *Cucurbita moschata* Duch. *cv* Plovdivski 51–17, and luffa *(Lu*ff*a cylindrica* Mill.) *Cucurbitaceae* rootstock and scion genotypes, in terms of plant growth and photosynthetic pigments to identify salt tolerant genotypes to be used in commercial grafting programs.

#### **2. Materials and Methods**

#### *2.1. Growth Conditions, Plant Material, and Salinity Treatments*

The experiment was carried out in a polyethylene covered double span nursery greenhouse at the Department of Horticulture, Faculty of Agriculture, Ege University (38◦27 16.2 N, 27◦13 17.8 E) in Bornova, Izmir Turkey during the spring 2014 growing season.

Plant material included 16 melon varieties (*Cucumis melo* L.), 6 watermelons (*Citrullus vulgaris* Schrad.), 2 interspecific hybrids of *Cucurbita maxima* Duch. × *Cucurbita moschata* Duch., 4 bottle gourd (*Lagenaria siceraria* (Molina) Standl.) varieties, 1 luffa (*Lu*ff*a cylindrica* Mill.) that were obtained

from the Aegean Agricultural Research Institute Department of Biodiversity and Genetic Resources (Menemen, Izmir-Turkey) and 1 *Cucurbita moschata* Duch. *cv* Plovdivski 51–17 obtained from the "Maritsa" Vegetables Crops Research Institute (Plovdiv, Bulgaria) (Table 1). Two salt treatments were considered, by applying 0 (non-saline control) and 150 mM of sodium chloride (NaCl) dissolved in the nutrient solution. Eighty seeds per genotype were sown in polystyrene trays in a mixture of peat and perlite (75:25 *v:v*) on 5 May 2014 and placed in a germination room for three days (temperature of 24 ◦C, 60% RH) and then placed in an unheated nursery greenhouse (mean daily temperature of 28 ◦C) for two weeks. Seedlings were fertigated with commercial fertilizer twice per day until the start of the experiment.

On 22 May 2014, forty seedlings of each species/genotypes were divided per each salinity treatment and placed in separate floating hydroponic systems. The nutrient solution was composed as follows: nitrate 13.14 mM, phosphorus 0.94 mM, potassium 5.83 mM, calcium 3.79 mM, iron 35.8 μM, boron 37 μM, copper 1.6 μM, molybdenum 0.5 μM. The electrical conductivity (EC) of the nutrient solution was 2.0 dS m<sup>−</sup>1. The nutrient solution was replaced every two days and aerated with air pumps. In the saline treatment (150 mM NaCl), salinity stress was induced by progressively dissolving 50 mM of NaCl every two days in the nutrient solution until the final concentration of 150 mM was achieved within one week.


**Table 1.** List of the tested species and the relative genotypes used in this study.

#### *2.2. Plant Growth Measurements*

Plant growth was measured when salinity treatment reached its final concentration of 150 mM (12 days after the start of the NaCl treatment, at 29 days after sowing, DAS) on 3 plants per *Genotype* × *NaCl concentration* combination measuring the number of leaves, the shoot diameter, recorded with an electronic caliper, shoot and root length with a ruler. Shoot, root, and leaf fresh weight (FW) were recorded with an electronic balance and dry weight (DW) was recorded after drying the samples for 48 h at 70 ◦C. Dry matter (DM) was calculated as percentage of DW/FW.

#### *2.3. Photosynthetic Pigments Determination*

Chlorophyll *a*, *b* and carotenoid contents were determined at 29 DAS on 3 plants per each *Genotype* × *NaCl concentration* combination by grinding 250 mg of leaf tissue from fully expanded leaves with quartz crystals in 30 mL of acetone (80% in vol). Leaf extracts were read at 450, 645, and 663 nm with a Varian Carry 100UV-Vis spectrophotometer (Varian Inc, Palo Alto, CA, USA), pigments content was calculated according to Arnon [16] and values were expressed as mg g−<sup>1</sup> FW.
