1. Introduction
Wheat is one of the most important cereal products, which has a significant effect on human nutrition. Wheat has been cultivated for more than 10,000 years as a primary human food and it has had a significant impact on human social, demographic, and cultural development [
1]. In addition to rice, bread made from wheat grains is a leading human food. As with other grains, wheat grains are a major source of energy, carbohydrates (such as starch), protein, minerals, vitamins, and fiber. The protein content of wheat grains varies between 8 and 20%, depending on the type and class [
1]. Wheat grain quality parameters heavily depend on growth conditions, soil fertility, fertilizer application, water ability, genotype, grain transport, and storage situations [
2,
3,
4,
5]. Some of the essential quality factors of grains such as wheat include the grain structure, grain cover, resistance to breakage (mechanical damage), and grain composition, which are affected by the rates of inorganic fertilizers [
6,
7]. Additionally, the yield and quality parameters of grains are affected by agronomic practices, such as tillage systems [
8,
9,
10,
11,
12,
13,
14]. Grain size is one of the most important yield components for cereals such as wheat. The grain protein content is the main factor in terms of wheat quality [
15]. Some studies have shown little correlation between wheat grain protein and soil tillage systems [
16,
17,
18], while others have found positive effects and reported that CT systems tend to produce higher protein contents than other systems [
19,
20,
21,
22].
Color is an important factor in the food industry. The surface color of wheat grain is an important attribute in the sorting and marketing of the grains [
23,
24]. The color of wheat grains is the key to the milling and baking industry, which determines the color of the whole wheat products [
24]. In the literature, several studies on the color measurement of wheat grains are available, in which the researchers reported that the color and appearance of the final product made from wheat grains, such as bread and tortillas, are influenced by the color of the wheat grains [
25,
26,
27].
Soil tillage is one of the most important processes in the production of agricultural products. More than half of the total energy required for cereal production is consumed during soil tillage [
15]. On the other hand, soil tillage is also an important factor affecting soil structure properties and crop yield [
28,
29,
30]. Wheat production in Iran is based on the CT system, which includes the use of a moldboard plow and seed bed peroration using a disk harrow. The CT system is characterized by high energy consumption and has the most destructive effects on soil properties [
31]; therefore, there is an ongoing effort to replace the CT system with conservation systems such as RT or NT systems. However, this effort may be limited by certain problems such as environmental conditions and economic factors [
32,
33]. As stated by many researchers, in the conventional tillage and conservation tillage systems, the yield and quality parameters of crops are functions of the interactions of many parameters, which are difficult to predict [
34,
35,
36]. Generally, from the review of the results of research studies conducted in this paper, it can be stated that when reducing the levels of tillage from the conventional system to conservation tillage systems (RT or NT systems) in cereals, there is a decrease in crop yield [
37]. The soil tillage system also affects the quality parameters of grains, such as the physical dimensions and chemical composition [
34,
38,
39]. Lopez-Bellodo et al. [
19] stated that higher protein content for wheat grains was obtained for the CT system than in the NT system. Wozniak et al. [
40] reported that the herbicide tillage system (use of Roundup 360 SL herbicide to control weeds) reduced the volume weight and increased ash content in the wheat grains as compared to CT and RT systems. In addition to the cases mentioned above, the tillage system influences weed infestations. Many investigations show that the NT system increased the weed infestation of crops [
41].
The effects of several tillage systems on the yield, yield components, and physicochemical attributes of wheat grains are required to achieve more stable wheat production. According to the needs mentioned above, the purpose of this study was to investigate the effects of tillage systems, namely conventional tillage (CT), reduced tillage (RT), and no tillage (NT) systems, on wheat grain yield, yield components, physical properties (grains geometrical dimensions, arithmetic mean diameter, geometric mean diameter, equivalent mean diameter, sphericity, 1000-grain mass, volume, water absorption capacity, and density), chemical composition (protein content, fiber content, ash content, and water content), and surface color parameters (whiteness, redness, and yellowness) of wheat grains over 2 experiment years.
2. Materials and Methods
2.1. Site Description
Field experiments were conducted during 2017–2018 and 2018–2019 growing seasons in the Kohdasht region in the southwest of Iran (33°34′, 47°36′ E), at an elevation of 1195 m above sea level, with a mean annual precipitin of 450 mm, average yearly temperature of 16 °C, and with a semi-arid climate. The monthly rainfall during the experimental years is shown in
Figure 1. The sum of annual rainfall during the growing season in 2017–2018 was 835 mm, which was much higher (about 2 times) than the long-term rainfall (450 mm). Additionally, the sum of annual rainfall during the 2018–2019 growing season was 526 mm, which was more than the long-term rainfall data (450 mm).
2.2. Experimental Procedure
The cropping system used in this experiment was dry land continuous cultivation in three tillage systems. Trials were established using a randomized complete block design over 2 years, with three tillage treatments and three replications. The tillage treatments were a combination of three systems: (1) conventional tillage (CT), (2) reduced tillage (RT), and (3) no-tillage (NT) in the autumn. Conventional tillage (CT) included using a moldboard plow (depth close to 25 cm) + disk harrow (depth close to 10 cm) + seed drilling using a conventional seed driller. Reduced tillage (RT) included using field cultivation (depth close to 10 cm) after harvest of the previous crop (chickpea) + seed drilling using a direct drilling machine. No-tillage (NT) included using a direct drilling machine after harvest of the previous crop (chickpea), without any prior tillage.
Wheat seeds of Aftab, a popular winter bread wheat variety in Iran, which is suitable for dry land cultivation, were chosen for use in this research and planted at a rate of 160 kg ha−1 (the rate for rain-fed conditions). The row spacing (11 rows in each plot) was 0.2 m. Individual tillage plots were 15 m in length and 30 m in width (450 m2 plot size), which were divided into three sub-blocks. The wheat cultivar and planting dates were chosen based on the best recommendations for farmers in the area of the study. Crop management exercises (except tillage methods) were diverse among locations depending on the requirements based on soil test results. Fertilizers were applied at the planting date (100 kg ha−1 P2O5 and 75 kg ha−1 N). Herbicides were used for weed control by application of 20 g ha−1 of glyphosate and 0.8 L ha−1 of clodinafop–propargyl in spring.
2.3. Soil Sampling
Soil samples from four locations were collected from each main plot at 0–30 cm soil depth after harvest of the previous crop (chickpea) to determine the properties of the studied field soil [
42]. Soil bulk density was determined by taking soil samples at 0–10 and 11–20 cm soil depths using a standard core sampler (4 cm long and 8 cm in diameter) and oven-dried at 105 °C for 48 h, then the bulk density was calculated by dividing the sample dry weight to the sample’s total volume [
43]. Soil-available P was determined by extracting samples with 0.5 mol L
−1 NaHCO
3 and by determining P calorimetrically [
44]. Soil-available K was measured using the 1 mol L
−1 ammonium acetate extraction–flame photometry method [
45].
Table 1 shows the properties of the studied field soil.
2.4. Crop Sampling
After physiological maturity, all plants in the central area of plots were cut above soil using a hand sickle. The edges (0.5 m) of the plots were cut and discarded before harvest. The plant samples were air-dried, threshed, and cleaned in an air screen cleaner to obtain grain yield and yield component values. The grain yield per hectare, number of plants per unit area (m2), plant height, spike length, number of grains per spike, spike mass, and thousand-grain mass were measured and examined. The number of plants per unit area of plots was determined using a wooden frame with 50 × 350 cm over three replications. Plant height and spike length values were measured using a ruler. The spike mass was measured using a digital scale with an accuracy of 0.01 g. The thousand-grain mass (TGM) of each grain sample treatment was measured by counting 1000 grains and weighing them in an electronic balance with a precision of 0.001 g.
The quality properties of the grains are affected by the moisture content [
46]. Harvested grain samples were stored for six months (at 4 °C and a humidity of about 60%) to standard and equalize their moisture content and determine the effects of the tillage system on the water content (WC) (stored moisture for six months) of grains [
47,
48], then their quality properties were measured. To investigate the effects of the year of study and tillage systems on the physical properties of wheat grains, including grain size, 100 grains randomly selected from each sample and their three linear dimensions (namely length (L), width (W), and thickness (T)) were measured using a digital micrometer over three replications, with an accuracy of 0.01 mm. The arithmetic mean diameter (D
a), geometric mean diameter (D
g), equivalent mean diameter (D
P), and sphericity (φ), respectively, of the grain samples were computed using the following relationships [
49]:
To measure the density of tested samples, firstly 100 grains of each treatment were weighed, then their volume (V) was measured using the liquid displacement method (toluene). The density of the grains was computed by dividing the mass of the sample by the actual volume. To measure the water absorption (WA) capacity of grain samples, 50 whole grains of each sample were weighed and soaked in 100 mL of distilled water in a cylindrical container covered with foil, which were left at room temperature (22 °C) for 24 h. The grains were then dried using absorbent paper and weighed. The water absorption capacity of each sample was computed as (weight after soaking − weight before soaking)/50.
The chemical compositions of whole wheat grains, including the protein content (PC), fiber content (FC) (natural detergent fiber (NDF)), ash content (AC) and water content (WC) (stored moisture for six months), were computed using near-infrared reflectance (NIR) technique using a Perten DA 7250 NIR analyzer instrument (Perten-Instruments, Ab, Huddinge, Sweden) [
5].
For surface color parameter measurements, glass dishes containing wheat grain samples were placed under 4 LED lamps (12 W, 230 V, AC, and 50 Hz). The images of whole grain surfaces were captured using a 24.5 megapixel digital color camera (Nikon digital SLR camera D53000). The camera was positioned 20 cm vertically above the grain sample surface before capturing pictures with no flash or zoom. The color properties of captured images were analyzed using Image J software version 1.32 g. Finally, for quantization of the grain surface color, L*, a*, and b* coordinates of the CIE color scale were calculated from RGB values from digital images of the grains using a ColorMine color converter (
http://colormine.org, (accessed on 20 December 2020)
In the color space of the CIE L*a*b* scale, L* values indicate lightness, with a score of 100 being white and 0 being black (dark). The values of a* indicate red to green colors, for which positive values of a* demonstrate redness and negative values demonstrate greenness. The b* values illustrate yellow-blue colors, with positive values representing yellowness and negative values representing blueness [
50,
51,
52,
53,
54].
2.5. Data Analysis
Analysis of variance (ANOVA) was performed to determine the effects of different treatments (year of study and tillage system on wheat grain yield and properties. All statistical analyses were carried out using SPSS software version 19 (SPSS Institute Inc., Chicago, IL, USA). The levels of significance were shown as * p < 0.05 and ** p < 0.01 using Duncan’s multiple range tests.