**1. Introduction**

Abiotic stresses negatively affect crop production [1]. Water deficit is known to decrease plant growth, significantly reducing yield [2]. Less water is considered a key climatic problem that directly decreases crop production, such as cereals, globally [3]. Drought stress causes severe losses in wheat yield in different growing regions worldwide. As the largest contributor to total consumed calories by humans, wheat represents the principal dietary staple in the world [4]. Yield and its attributes are highly affected by drought in the different stages of the growing cycle of plants [5]. Drought stress decreased wheat yield by as much as 60% [6]. As a strategy for drought tolerance improvement, crops escape water deficit, especially in the climate change conditions currently being experienced [7]. Several physiological and biochemical alterations are induced by drought, causing plants to have many adaptation strategies as defensive survival mechanisms against drought stress. It was reported that different strategies could be followed to reduce food production decreases due to drought in the future [8,9]. The development of drought tolerance mechanisms in food crops is one such strategy. Plants have many options to escape drought stress effects, such as water uptake and flow in plant tissues, production of osmolytes and antioxidant activities, and photosynthesis mechanisms [10]. Moreover, plants were found to produce more osmolyte and soluble sugars and have increased antioxidant defense mechanisms (such as SOD and CAT) to combat the toxic

**Citation:** Abdi, N.; Van Biljon, A.; Steyn, C.; Labuschagne, M. Zn Fertilizer and Mycorrhizal Inoculation Effect on Bread Wheat Cultivar Grown under Water Deficit. *Life* **2023**, *13*, 1078. https://doi.org/ 10.3390/life13051078

Academic Editors: Othmane Merah, Wajid Zaman and Hakim Manghwar

Received: 21 February 2023 Revised: 14 April 2023 Accepted: 20 April 2023 Published: 24 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

effects of the overproduction of reactive oxygen species (ROS) [11]. Due to the water deficit, genes encoding antioxidant enzymes were activated in tolerant genotypes. The wheat genome was known for some modification in terms of genes to control drought-stress conditions [12].

Zn fertilizer and AMF inoculation can contribute to plant survival and tolerance of water deficit conditions in many crops, such as wheat [13]. Zn application maintained nutrient balance and stomata reaction in different crops to reduce the effects of drought stress [14]. As an antioxidant reaction, SOD and CAT enzymes were enhanced due to Zn fertilization in response to water deficit.

Zn is classified as a necessary micronutrient for plant growth due to its involvement in carbon metabolism [15]. Zn plays an important role in plant nucleic acid metabolism. Several biomolecules as lipids and proteins, contain Zn as an essential component; also, it is a cofactor for many enzymes [15,16]. Many studies showed the plant responses to Zn application [17]. Physiological and biochemical processes such as plasma membrane functions and oxidative stress tolerance depend on Zn content [18].

Zn application also reduced the alteration of membrane permeability and the damage caused by oxidative and peroxidative reactions [19,20]. An adequate supply of Zn can reduce the effects of drought on different crops, such as wheat [21].

Many reports confirmed that Zn plays an important role as a strategic component for the root and shoot system and a cofactor of many enzymes [22]. Yield attributes of wheat were increased due to Zn application [23,24]. Soil Zn application increased grain yield by 29%, whole-grain Zn concentration by 95%, and whole-grain estimated Zn bioavailability by 74% [25].

Arbuscular mycorrhizal fungi (AMF) belong to the phylum Glomeromycota, are soil inhabitants, and can colonize 80% of the roots. Mycorrhizal characteristics are mutually beneficial. AMF provide the host plant with essential nutrients (especially P) and water, and photosynthates are transported into endosymbiotic AMF for its development. Mycorrhizal mycelium feeds plants with several secondary metabolites and carbohydrates. It also improves plants to fix nitrogen and increase osmotic adjustments during water deficit.

The effect of AMF colonization depends on the host-plant interaction [26]. To tolerate drought stress, for example, in wheat, symbiosis with AMF can increase plant tolerance against this stress [27,28]. Antioxidant reaction, osmotic adjustments, and root hydraulic conductivity are better regulated in AM-plant association [29]. Zn uptake by the plant increased in the presence of AMF; however, the assimilation depends on the crop–AMF symbiosis. Mycorrhizal association contributed to Zn uptake of up to 24.3% of the total aboveground Zn in wheat and up to 12% in barley. At low Zn application, the highest contribution by the mycorrhizal pathway was observed in barley. Besides this, the grain yield of bread wheat was increased by AMF [30]. The use of Zn and AMF as fertilizer is one of the most effective strategies that can reduce the effect of drought stress and improve yield and plant growth. In addition, the use of biocontrol and chemical fertilizers was increased to reduce the impact of stress factors on crops. Moreover, AMF colonization could improve the nutrient uptake of a crop such as wheat in different types of soil by enhancing the root surface absorption area [31]. In the case of soil containing heavy metals, it was reported that mycorrhizal colonization could reduce the uptake of these metals [32]. Many studies investigated the role of AMF under drought stress to improve plant nutrient uptake. The synergistic interaction of AMF and Zn could improve concentrations of different micronutrients. It was reported that AMF with extraradical mycelium in the soil improved immobile nutrient (such as P and Zn) uptake by the host plant [33], causing an increase in the exchange of photosynthesis products from the plant to the fungus.

Glycine betaine accumulation works as an osmolyte in protecting organisms against abiotic stresses via osmoregulation or osmoprotection. As an osmoregulator, GB enhances root water assimilation, reduces the damage caused by oxidative reactions, and increases drought tolerance [34]. Due to the Zn application, compatible solutes were increased under drought stress [35]. GB maintains water retention in plants owing to Zn application that increases chlorophyll content and plant dry weight [36,37]. GB accumulation helpsplants to overcome drought and saline stress conditions. For example, in transgenic apples expressing the stress regulator gene, *Osmyb4*, accumulation of GB was linked to increased tolerance under drought and cold stress [38]. In chloroplast stroma, GB is produced by betaine aldehyde dehydrogenase (BADH). Under abiotic stress such as salinity, the enzyme choline monooxygenase (CMO) converts choline into betaine aldehyde and then an NAD+-dependent enzyme to improve tolerance against this stress [39].

To determine the effects of Zn application and/or AMF inoculation on bread wheat under drought stress, the regulation of various antioxidants, metabolites, and morphological traits was studied. It was hypothesized that Zn and AMF could improve bread wheat production under water deficit conditions.

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

#### *2.1. Biological Materials and Growth Conditions*

Seeds of one commercial South African wheat cultivar (SST806, official standard for spring wheat quality) were planted in plastic pots containing 2 kg of soil collected from 1.5 m deep subsoil (Table 1). They were grown under glasshouse conditions at the University of Free State, Bloemfontein, South Africa, from May 2019, with day/night temperatures of 18 ◦C at night and 22–24 ◦C during the day. The relative humidity during the day and night was 78%. A soil meter (Efekto Ltd., Caledon, South Africa) was used in this study. A completely randomized block designwas replicated three times for each treatment;control (T0), Zn (T1) = 40 kg ha−1, Arbuscular mycorrhizal fungi = AMF (T2), drought stress (T3), Zn+AM (T4), and Zn+AM+drought (T5).

**Table 1.** Soil, Zn, and AMF characteristics used in the trial.

