**1. Introduction**

Grapes are one of the most important fruit crops on the planet. Grape is a member of the Vitis genus, which is part of the Vitaceae family, which contains more than 60 genera. Grapes (*Vitis vinifera* L.) are cultivated in more than 100 countries throughout the world, with an estimated area of 7.8 million hectares in 2016. Wine, jam, juice, grape seed extract, dried grapes, vinegar, and grape seed oil are among the many goods made from grapes. In 2016, the world produced 75.8 million tons of grapes, with 39% produced in Europe, 34% produced in Asia, 18% produced in the Americas, and 9% produced in Africa [1]. Grapes are Egypt's second most important fruit crop, after citrus. Egypt's agriculture has succeeded in increasing vineyard area by 220,665 hectares over the past decade, yielding 1,586,342 tons of grapes [2]. The grapevine is one of the most important horticultural crops in the world. The high value of table grapes is primarily attributed to bio-compounds

**Citation:** El-Ezz, S.F.A.; A., L.A.; Al-Harbi, N.A.; Al-Qahtani, S.M.; Allam, H.M.; Abdein, M.A.; Abdelgawad, Z.A. A Comparison of the Effects of Several Foliar Forms of Magnesium Fertilization on 'Superior Seedless' (*Vitis vinifera* L.) in Saline Soils. *Coatings* **2022**, *12*, 201. https:// doi.org/10.3390/coatings12020201

Academic Editor: Yu Shen

Received: 6 December 2021 Accepted: 25 January 2022 Published: 3 February 2022

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required for human health, such as antioxidants, anthocyanins, and phenolics, which include gallic acid, catechin, anthocyanins, and resveratrol [3].

The fundamental issue with newly reclaimed and cultivated fields was that they were often sandy and calcareous soils with poor nutrient concentration, especially magnesium. Recently, research on magnesium nutrition has begun, with the goal of determining the Mg requirements of Egypt's most important crops. Magnesium deficiency has been discovered in some Egyptian soils such as clay or newly reclaimed soils [4]. Therefore, magnesium (Mg) is the most essential element constituent in chlorophyll molecules that regulates the photosynthesis processes [5,6]. The deficiency of Mg during growth seasons limits photosynthesis performance [7]. The physiological functions of Mg in plants have also been characterized for flowering induction [8]. Mg is required for the growth and development of plants [9]. It is also a cofactor in the biosynthesis of various enzymes, including those involved in respiration and photosynthesis. It is a phloem-mobile nutrient that migrates between older and younger leaves [10]. Mg is also a significant component of the chlorophyll molecule's ring structure [11]. Additionally, it alleviates abiotic stress conditions, such as dryness and heat, which can significantly enhance Mg deficit by inhibiting its absorption due to its mass flow transit [9]. Additionally, it mitigates aluminum toxicity in acid soils at micromolar concentrations, as opposed to calcium, which is required at millimolar concentrations [12]. A Mg shortage has been shown to adversely influence ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is involved in CO<sup>2</sup> fixation [13], resulting in a decrease in photosynthetic performance [14], which is correlated to a decrease in photosynthesis performance and stomatal mechanism [15]. Furthermore, it plays a role of metabolism nitrogen in plant [16]. The inhibitory influence of Mg loss on photosynthetic capacity and net CO<sup>2</sup> absorption was marked in several plant species [5,17,18]. As a result, in certain species, magnesium deprivation affects the structure and function of the PSI and PSII systems [19]. As a result, a decrease in the Fv/Fm ratio (maximum quantum efficiency of PSII) was observed in citrus seedlings [20]. Despite this, the Mg shortage had no effect on Fy/Fm and other fluorescence metrics in *Helianthus annuus* plants under Mg deficiency conditions. A rise in the chlorophyll a/chlorophyll b ratio is typically reported [21]. The decrease in light-harvesting complex II ( LHC-II) abundance in Mg the absence of *Arabidopsis thaliana* leaves is caused by thylakoid membrane dysfunction [22].

Many researchers have begun to investigate magnesium nutrition and the determination of magnesium requirements for economically important crops [23] such as 'Washington navel' orange trees [24] and banana plants, and they have reported on the influence of magnesium on yield and fruit quality, stating that magnesium fertilization increased the yield and fruit quality of the aforementioned fruit species [25]. In addition, using the magnesium application can induce a state of magnesium deficiency during growing [26]. Furthermore, fertilizing "Grand Nain" bananas with 100 g/plant magnesium chelate plus a foliar spray of 2% magnesium sulfate increased growth metrics, yield, and fruit quality [27]. In addition, treating Le Conte pear plants with compost 45 kg/tree + biofertilizers 20 g/tree plus 1.5% magnesium sulfate produced the best production and fruit quality [28]. The foliar Mg (137.5 ppm) application boosted the growth characteristics and yield of Washington Navel orange trees [29]. Moreover, some studies were conducted to improve bunches of color quality of Crimson seedless by using foliar application of Mg [30].

'Superior seedless' is one of the first seedless table grapes to be produced in the Mediterranean region, and it adapts well to and performs well in Egyptian circumstances as well. It was harvested when the meat was yellow-white and the skin was green, as requested by the European market [31]. 'Superior Seedless' is also considered as one of the most important international grape variety with a good economic return [32]. Consumers value this grape selection for its excellent nutritional value, great taste, versatile application, and higher economic returns [33]. The world's vineyard area is growing as a result of a continual and unrelenting shift [34]. The purpose of this study is to determine the difference among foliar magnesium forms on 'Superior Seedless' vines grown in salinity sandy soil. Furthermore, this study also aims to determine the optimal magnesium form for vine nutrition under soil salinity conditions.

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

#### *2.1. Vine and Experimental Setup*

A commercial vineyard in the Nobaria area of Egypt (31.23◦ N, 29.96◦ E) was studied for two growth seasons (2020 and 2021). The soil was sandy in texture (*Entisol-Typic Torripsamments*), and the soil composition is described in Table 1. The farm consists of 6-year-old vines of the 'Superior Seedless' cv. grafted on 1103 Paulsen rootstock. Three-bythree-meter vines were planted in sandy soil using a drip watering system. The pruning level was done on all vines at 70 bud vines−<sup>1</sup> (7 cans <sup>×</sup> 10 buds can−<sup>1</sup> each on four cardons), and all vines were trained by the Y system. Table 1 summarizes the physical and chemical examination of the field experiment with 'Superior Seedless' vines [35,36]. All vines were pruned to a height of 60 buds' vine−<sup>1</sup> , with the length of the cans ranging from 6 to 8 buds per can, and each can contain 12–14 buds and were produced until mid-July in European countries. Additionally, according to the Egyptian Agriculture Ministry, all vines received the same management program as for NPK fertilizer (300, 200, and 250 Kg were afforded on three portions from growth starting until harvesting (one portion was added at the vine dormancy stage) in sandy soil. Uniform vines (48) were chosen and treated with four different types of magnesium; each treatment consisted of three duplicates with four vines per replication. All treatments receive 750 g of magnesium sulfate per 600 L of irrigation water, which was employed to avoid magnesium shortages. It is distinguished by the yellowing of older leaves and a yellow tint between the veins of the leaves.

**Table 1.** Soil and irrigation-water traits analysis.

