Application Methods of Zinc Sulphate Increased Safflower Seed Yield and Quality under End-Season Drought Stress

Abstract

Zinc deficiency is one of the most widespread nutritional problems, affecting nearly one-third of the world population. In addition, it is known that zinc deficiency not only reduces crop yield but also its quality. The effect of different methods of zinc application on the growth, yield, and quality of safflower seeds under regular irrigation and interruption of irrigation from flowering to harvest (82 and 80 DAS in the first and second years, respectively) was evaluated. Zinc sulfate was applied in both soil and foliar methods. The zinc sulfate treatments include no zinc sulfate, soil application of 20, 40, and 60 kg ha⁻¹ at the planting stage; spraying 2.5, 5, and 7.5 g L⁻¹ in the rosette stage; and spraying 2.5, 5, and 7.5 g L⁻¹ in the flowering stage. The end-season drought caused a decrease in the chlorophyll index, leaf area index, relative water content, plant height, yield components, biological yield, seed yield, harvest index, seed oil content, oil harvest index, and seed element content compared to regular irrigation. The decrease in yield occurred with a decrease in the capitol number and diameter, seed number per capitol, and 1000-seed weight. The severity of the damage of the end-season drought stress in the second year was higher than in the first year due to the higher temperatures and the decrease in the rainfall. In both years, the application of zinc sulfate in different ways had an increasing effect on the studied traits in both normal and stress conditions. The application of zinc sulfate reduced the negative effects of unfavorable environmental conditions and improved the yield and nitrogen, phosphorus, potassium, zinc, and iron element content in the seed. In both application methods of zinc sulfate, the increment in the zinc sulfate concentration decreased the seed phosphorus content. However, the phosphorous content was more than that of the treatment of non-zinc application. The application of zinc increased the biological, seed, and oil yield of the treated plants, but the seed and oil yield were more affected. This effect was shown in the seed and oil harvest index increment. Under regular irrigation, higher concentrations of zinc sulfate enhanced plant performance, but under stress conditions, medium and lower concentrations were more effective. The highest 1000-seed weight and potassium and zinc content were obtained by spraying zinc sulfate at 5 g L⁻¹ in the flowering stage under normal irrigation conditions. A comparison of the two methods of applying zinc sulfate showed that foliar spraying was more effective than soil application in improving the seed yield. The soil application is more effective on biological yield than seed yield.

1. Introduction

Climate change and the global increase in population are the two main factors related to the risk of reducing food security and providing enough food for the people of the planet [1]. Improving access to food and achieving food security has become a key issue worldwide. It is predicted that the world’s population will reach 9.2 billion people by 2050, which will increase the food demand by 59–102% [2].

Oilseed crops add important nutritional value to the human diet due to essential fatty acids, high-quality oil, and protein. Therefore, the global production of these crops is continuously increasing, and it is estimated that by 2050, the global need for vegetable oil will double compared to the current production [3]. Safflower (Carthamus tinctorius L.) is an annual plant from the Asteraceae family, which has high-quality oil due to its unsaturated fatty acids, especially linoleic and oleic fatty acids. Safflower is a flexible and suitable crop for cultivation in different environments and has high resistance to various adverse conditions such as drought and salinity. Also, due to its deep roots, it can extract moisture from the depth of the soil [4,5]. Despite the positive and valuable characteristics of safflower, the cultivated area of this plant in the world is limited, and among oilseed crops, it is considered the eighteenth source of edible oil production [6].

Global climate change has caused the weather to become hotter and drier than in previous years. Tabari and colleagues reviewed forty years of meteorological data from different regions of Iran and reported that since the end of the 20th century and the beginning of the 21st century, climate change has significantly increased wind speed, ambient temperature, and ET₀, while the amount of atmospheric precipitation has decreased [7]. According to this report, decreasing rainfall and changing rainfall patterns can cause water shortages and frequent droughts, and especially increase the risk of end-season droughts for crop production. Water deficit is the most important factor limiting the growth and quantitative and qualitative yield of plants in arid and semi-arid regions of the world, such as Iran. Water deficiency has a negative effect on plant growth by affecting cell turgor, enzyme activities, photosynthesis, respiration, and transpiration [8]. In safflower, water deficit decreases the relative water content (RWC), leaf water potential, photosynthetic pigments content, photosynthetic yield, maximum variable quantum efficiency of PSII, and electron transfer rate. In contrast, electrolyte leakage and water use efficiency increased. A reduction in the photochemical efficiency and photosynthetic yield as a function of the water deficit caused a decrease in the capitol number, 100-seed weight, and harvest index [9]. Zafari et al. (2020) also stated that in safflower, drought stress decreased chlorophyll and carotenoid content, RWC, seed yield, and oil percentage [10].

The availability of different nutrients in the soil is under the influence of various environmental factors such as pH, available moisture, and the amount of organic matter in the soil, showing significant variations. The optimal management of plant nutrition with macro and micro elements is considered one of the important issues in plant production, and the optimal supply of elements can increase plant performance in diverse environmental conditions [11]. In addition to increasing the quantity and quality of crops and improving the tolerance of plants to environmental stresses, micro elements are also effective in human and animal health [12,13]. Various methods, such as seed treatment, soil application, foliar spraying, and side dressing, have been proposed to provide nutrients to plants [11].

Zinc is one of the essential micronutrients of plants, which is involved in the synthesis of tryptophan, precursors of auxin, activation of hydrogenase and carbonic anhydrase enzymes, chlorophyll pigment stability, leaf longevity and aging, stability of cell membrane structure, carbohydrate metabolism, protein synthesis, and pollen grain formation and plays a key role in plants’ drought resistance [13,14,15,16]. This element is immobilized in restricted soil moisture, and therefore, a reduction in zinc absorption due to water limitation leads to plants experiencing zinc deficiency stress. A decrease in the amount of available zinc causes disturbances in physiological functions such as photosynthesis, sugar formation, protein synthesis, growth, and fertility, and subsequently decreases the yield quality and quantity [11]. By adding zinc, the production of active oxygen and photooxidation of photosynthetic pigments decreased and the growth and performance of plants improved under stress conditions [15]. It has been reported that the application of zinc improved the yield and yield components of various plants such as cotton [17] and wheat [13]. Also, the application of the zinc element in safflower not only increased seed yield but also increased the seed zinc concentration [18].

Iran is one of the countries with arid and semi-arid regions with an average rainfall of about 250 mm. The low amount of rainfall in the country, as well as global warming and climate change, have caused a significant change in the amount of rainfall and the type of rainfall [7]. Although the effects of Zn fertilization on some crop plants’ growth, performance, and yield have been studied, there are few studies that have investigated the response of safflower to Zn fertilization. On the other hand, the effectiveness of different methods of zinc application at different phenological stages on the performance, yield, and accumulation of mineral elements in safflower has been paid less attention. This study was conducted to evaluate different methods of zinc sulfate application to increase the growth, quantitative, and qualitative seed yield under end-season drought stress and soil with lower zinc availability conditions.

2. Materials and Methods

2.1. Experimental Design and Application of Treatments

A field experiment was conducted as a split-block, complete randomized block design with three replications in two consecutive cropping years (2016 and 2017) in the research farm of the Faculty of Agriculture of the University of Zanjan (1640 m above sea level., latitude: 36°41′ N, longitude: 48°27′ E), Zanjan, Iran.

Irrigation was applied as the main factor in 2 levels (regular irrigation and interruption of irrigation from the flowering stage until harvest). The interruption of irrigation was 82 DAP in the first year and 81 DAP in the second year. The duration of irrigation interruption was 38 days in the first year and 32 days in the second year. The water requirement of the cultivated plants was provided through the drip-strip irrigation system with a 7-day irrigation cycle. The thickness of the irrigation strips was 175 microns, the distance between the holes was 20 cm, the flow rate of the holes was 2 L h⁻¹ at the pressure of 1 bar, and a drip strip was placed next to each row of crops. Each strip was connected to a faucet and the faucet was connected to the water supply pipe. A volumetric water meter was installed at the beginning of the main water supply pipe to accurately control the volume of irrigation water in each round of irrigation. The water requirement of the plant in each round was determined by using the timely data of meteorological parameters of the Zanjan University synoptic station and the FAO–Penman–Monteith relationship (relationships 1 and 2) and the volume of irrigation water from relationship 3.

$$\mathrm{E}{\mathrm{T}}_{\mathrm{C}}={\mathrm{K}}_{\mathrm{C}}\times \mathrm{E}{\mathrm{T}}_0$$

(1)

$$\mathrm{E}{\mathrm{T}}_0={\displaystyle \frac{0.408\Delta ({\mathrm{R}}_{\mathrm{n}}-\mathrm{G})+\mathsf{\gamma }\frac{900}{(\mathrm{T}+273)}{\mathrm{u}}_2({\mathrm{e}}_{\mathrm{a}}-{\mathrm{e}}_{\mathrm{d}})}{\Delta +\mathsf{\gamma }(1+0.34u_2)}}$$

(2)

$$V_{IW}=A_p\times E{T}_c$$

(3)

In this regard, ET₀ is the evapotranspiration–transpiration of the grass reference plant (mm day⁻¹), K_c is the plant coefficient, and ET_C is the evapotranspiration of the safflower (mm day⁻¹). ET₀ values were calculated from Equation (2) [18]. Also, the values of the plant coefficient of safflower in different stages of growth were taken as 0.35 (the initial stage of growth), 1.10 (the middle stage of growth), and 0.25 (the final stage of growth), which were extracted from Allen et al. [18].

Zinc sulfate was used as a secondary factor in 10 levels: no application of zinc sulfate, soil application of 20, 40, and 60 kg ha⁻¹ of zinc sulfate at the time of planting, spraying zinc sulfate solution at the rates of 2.5, 5, and 7.5 g L⁻¹ in the rosette stage, and zinc sulfate spraying at the rates of 2.5, 5 and 7.5 g L⁻¹ in the flowering stage. In the soil application method, at planting, furrows were created on both sides of the sowing rows at a distance of 10 cm, zinc sulfate was added to it, and then they were covered with soil. The zinc sulfate was sprayed in the rosette stage and when the plants had 5 to 7 leaves, code 15–17, according to the BBCH scale, 32 DAP in the first year and 34 DAP in the second year [19]. The spraying of zinc sulfate in the flowering stage was performed when the capitol appeared on the main stem, code 50–55, according to the BBCH scale, 71 DAP in the first year and 69 DAP in the second year. Foliar spraying was performed with a knopsack sprayer with a hand-held nozzle and 0.5 MPa pressure. One week after spraying, the irrigation was stopped. The spraying of the zinc sulfate solution was performed in both stages at 6 o’clock in the morning and in windless conditions.

2.2. Field Preparation, Plant Material, and Seed Planting

Field and seedbed preparation was performed with the application of trifluralin herbicide (Terflan, FC48%, Agriplus, Zanjan, Iran) at a rate of 2 mL L⁻¹. Then, disking and ground leveling were performed. After two weeks, furrows were built with furrowers and then the plotting was performed. Each plot consisted of five rows. The length of rows was 4 m and the distance between rows was 50 cm. The area of each plot was equal to 10 m² and the total area of the experiment was equal to 750 m². The distance between the seeds on the planting lines was 20 cm. The distance between plots was 50 cm and the distance between blocks was 1 m. Safflower seeds, cv. Goldasht was obtained from the Seed and Plant Improvement Institute, Oilseeds Department, Karaj, Iran. After disinfection with the fungicide tebuconazole 2% DS (Bahavar Shimi, Zanjan, Iran), the seeds were planted in a line on the furrows and by hand at a depth of 2–3 cm. In the first year, seeding was performed on 1 April 2016, and in the second year, on 5 April 2017. After planting, irrigation was performed immediately using irrigation brigade tape. The irrigation cycle was once every 3 days in the first days until the seedlings appeared, and after the establishment of the seedlings, it was performed once every 7 days. Urea (46.7% N), triple phosphate (23% P), and potassium sulfate (44% K) fertilizers were used at the rates of 200, 100, and 100 kg ha⁻¹, respectively. Potassium sulfate and triple phosphate were mixed with the soil before planting each plot. Urea was applied three times, after 3 to 4 leaves unfolded (code 13–14), in the rosette stage (code 25–27), and at the beginning of stem elongation (code 35–38) on the plots using a drip irrigation system as fertigation. Plants were harvested on 18 August 2016 in the first year and 10 August 2017 in the second year. The physico-chemical characteristics of the farm soil are presented in

Table 1. Physico-chemical characteristics of soil of the Agricultural Research Station of the University of Zanjan.

Year	Soil Texture	Depth	pH	EC (dS m−1)	OM (g kg−1)	Ca (mg kg−1)	K (mg kg−1)	P (mg kg−1)	Zn (mg kg−1)	N (mg kg−1)	S (mg kg−1)
2016	Clay loam	0–30	8.85	1.4	9.45	380	280	9.0	0.54	920	280
2017	Clay loam	0–30	8.90	1.51	10.81	324	210	9.60	0.53	730	-

. Temperature and rainfall parameters in 2016 and 2017 in the Zanjan region are reported in

Table 2. Temperature and precipitation statistics of the Agricultural Research Station of the University of Zanjan in the experiment period in 2016 and 2017.

Year	Month	Average Minimum Temperature (°C)	Average Maximum Temperature (°C)	Average Precipitation (mm)
2016	April	3.7	15.3	62
	May	8.6	24.4	28.1
	June	11.1	28.6	15.9
	July	16.5	32.9	1.6
	August	16.3	34.8	0
	September	13.9	32.3	0
2017	April	4.0	15.8	54
	May	9.3	25.2	22.1
	June	10.9	31.4	0
	July	16.9	34.1	1.3
	August	17.0	35.6	4.8
	September	14.0	33.8	0

.

After the emergence and establishment of seedlings, by thinning the plants, the density was adjusted to about 10 plants per square meter. Weeding was performed manually. At the stem elongation stage, Imidacloprid and Cypermethrin 40% EC (Aria chimistry Co., Teheran, Iran) were used as a foliar spray, and Imidacloprid (Aria chimistry Co., Teheran, Iran) along with irrigation water was used to control turnip moths (Agrotis segetum Denis). Diazinon 60%EC (Moshkfam Co., Zanjan, Iran) was used in intervals of 10 days after the first spraying for the re-attacking of the pest. In the appearance of capitol, deltamethrin (Golsam Co., Zanjan, Iran) was used to control sloe bugs (Dolycoris baccarum) and ground bugs (Oxycarenus pallens H.Sch), and when a high density of the safflower fly (Musca helianthi Rossi) pest was observed, deltamethrin (Golsam Co., Zanjan, Iran) was used. All the spraying was performed in the cool hours of the evening.

2.3. Measured Traits

2.3.1. Chlorophyll Content Index (CCI) and Leaf Area Index (LAI)

At the beginning of flowering (code 61–62), 8 plants were randomly selected in each plot. The CCI was measured non-destructively with a chlorophyll meter (CCM200-OPTI SCIENCE, London, UK) in 3 of the fully opened, young leaves of the upper part of the plants. In the middle of flowering and when the capitols were about a few centimeters (code 65–75), 8 plants in each plot were selected and harvested, and their leaves were separated from the stem. The leaf area was measured with a leaf area meter device (Delta T Device Ltd., London, UK) and reported in cm². Then, the leaf area of the plants was divided by the land area of the plant and the LAI was calculated [20].

2.3.2. Relative Water Content (RWC)

Leaf samples were selected from the third leaf from the end of the stem at the beginning of flowering (code 61–65), and the fresh weight of the leaf was obtained with a scale (accuracy 0.001). Then, the samples were placed in distilled water at room temperature for 16 h and the turgor weight was recorded. Then, they were placed in an oven at a temperature of 70 °C for 48 h and their dry weight was measured. The RWC was calculated with the following formula [20]:

RWC = [(Fresh weight − Dry weight)/(Turgid weight − Dry weight)] × 100

2.3.3. Plant Height, Stem Diameter and Yield Components

At the ripening stage and with the yellowing of the plants (code 97), 8 plants were randomly selected from each plot and the height from the ground level to the highest part of the plant was measured with a ruler and expressed in cm. In the same plants, the diameter of the stem near the soil and all the capitols in the main stem of the plants were measured with a caliper and their average in mm was reported as the diameter of the stem and capitol. The total capitols of the plants were counted, and their average was recorded as the number of capitols per plant. All plant capitols and seeds inside them were manually separated and counted, and after averaging, it was reported as the number of seeds per capitol. Five samples of 1000 seeds were weighed on a precise scale (accuracy 0.01) and, after averaging, they were considered the 1000-seed weight.

2.3.4. Biological Yield (BY), Seed Yield (SY), and Harvest Index (HI)

At the ripening stage, all plants of each experimental plot were harvested, except in the side rows, and the weight of the entire aerial part was reported as BY in kg ha⁻¹. After separating the seeds from the capitols, the seeds’ weight was measured on a scale and reported as the SY per unit area (kg ha⁻¹). The HI for each plot was obtained by dividing the SY by the BY in percentage.

2.3.5. Seed Oil Content (SOC), Oil Yield (OY), and Oil Harvest Index (OHY)

The seed oil content was measured by the Soxhlet method. The seeds were dried in an oven at a temperature of 40 °C until their moisture content reached 5%. Then they were ground in a manual mill, and 1 g of the ground seed was placed on a filter paper and its initial weight was recorded. Then, it was placed in a Soxhlet apparatus (BUCHI extraction system B-811, Berlin, Germany) for 11 h with n-hexane solvent. After that, the filter papers containing the sample were placed in the oven at 50 °C for 2 h to remove the excess solvent. The samples were removed from the oven and transferred to a desiccator. Then their second weight was obtained immediately with a scale (accuracy of 0.001 g). Finally, the oil percent was calculated with the following formula [21,22].

Oil% = [(initial weight − secondary weight)/initial weight] × 100

The OY was calculated by multiplying the oil percentage by the SY and was reported in kg ha⁻¹. The OHI was also calculated by dividing the OY by BY.

2.3.6. The Seed Nitrogen, Phosphorus, Potassium, Zinc, and Iron Element Measurement

The nitrogen content was measured using the standard Kjeldahl method. Sulfuric acid and a mixture of copper sulfate and potassium sulfate catalysis were used to digest the sample (0.3 g of the dry sample) and it was reported as a percentage [23,24]. The phosphorus, potassium, zinc, and iron content were measured by a wet digestible method. A plant sample (0.3 g of the dry ground sample) with mixed acid (6 g of salicylic acid, 100 mL of 98% sulfuric acid, and 18 mL of distilled water) was used [25]. The phosphorus content was measured by the calorimetric method using ammonium-vanadium molybdate reagent with a spectrophotometer (PerkinElmer model, Lambada 25, Boston, MA, USA) at a wavelength of 470 nm and was reported as a percentage [26]. The amount of potassium was read by the flame photometry method by a film photometer (Jenway PFP7 model, London, UK) and reported as a percentage [27,28]. Also, the iron and zinc content were obtained with an atomic absorption device (Varian 220 AA, Adelaide, Australia) and, finally, was reported as mg g⁻¹ of dry matter.

2.4. Statistical Analysis

The data were analyzed with Bartlett’s test of Homogeneity of Variances for normality before statistical analysis, and they were normal in all studied traits. Then, a combined analysis of variance was carried out for the split-block experiment based on a randomized complete block design over two years in all traits. Data analysis and mean comparisons were performed using the SAS 9.1 [27] program. The means were compared with Duncan’s multi-range test at the 5% probability level.

3. Results

3.1. Chlorophyll Content Index (CCI)

The end-season drought stress decreased the CCI. Also, the CCI showed lower values in the second year compared to the first year (

Table 3. Mean comparison of measured traits of safflower, cv. Goldast as affected by zinc sulfate treatments and irrigation interruption in 2016.

Irrigation	Zinc Sulfate	Chlorophyll Index	Leaf Area Index	RWC (%)	Height (cm)	Stem Diameter (mm)	Capitol Diameter (mm)
Normal	Control	68.3 ± 6.11 d	3.20 ± 0.38 de	83.1 ± 3.20 d	73.9 ± 19.3 d	12.1 ± 0.31 bc	33.0 ± 11.6 cd
	20 kg ha−1 S.A.	72.9 ± 8.19 b	3.81 ± 0.33 ab	87.2 ± 2.06 b	80.8 ± 13.2 a	12.9 ± 0.26 b	33.9 ± 13.8 c
	40 kg ha−1 S.A.	69.8 ± 9.26 cd	3.90 ± 0.37 a	86.7 ± 4.93 bc	79.6 ± 15.8 a	12.8 ± 0.38 b	33.9 ± 11.8 c
	60 kg ha−1 S.A.	75.8 ± 7.13 a	3.91 ± 0.48 a	87.5 ± 2.40 b	75.8 ± 17.4 c	13.9 ± 0.11 a	35.1 ± 13.2 b
	2.5 gL−1 S.R.	72.3 ± 4.65 b	3.23 ± 0.53 de	86.5 ± 3.04 bc	79.2 ± 16.2 a	12.8 ± 0.68 b	36.9 ± 9.128 a
	5 gL−1 S.R.	72.4 ± 3.04 b	3.37 ± 0.64 cd	87.2 ± 5.11 b	75.2 ± 15.7 c	12.9 ± 0.32 ab	34.7 ± 10.8 b
	7.5 gL−1 S.R.	75.4 ± 4.38 a	3.90 ± 0.60 a	89.9 ± 6.11 a	79.5 ± 13.4 a	12.6 ± 0.31 b	34.8 ± 13.8 b
	2.5 gL−1 S.F.	72.8 ± 4.89 b	3.41 ± 0.54 c	86.9 ± 3.27 c	78.0 ± 16.7 ab	12.9 ± 0.22 b	34.4 ± 13.0 b
	5 gL−1 S.F.	76.6 ± 5.37 a	3.41 ± 0.43 c	87.5 ± 4.13 b	77.0 ± 19.9 bc	13.3 ± 0.54 ab	34.1 ± 12.2 bc
	7.5 gL−1 S.F.	69.1 ± 5.37 cd	3.27 ± 0.45 de	88.9 ± 5.87 a	75.8 ± 12.6 c	13.1 ± 0.31 ab	34.3 ± 15.1 bc
Stress	Control	60.7 ± 4.11 f	2.30 ± 0.68 i	70.2 ± 4.31 h	69.7 ± 14.0 f	11.4 ± 0.41 e	30.0 ± 9.48 f
	20 kg ha−1 S.A.	69.6 ± 4.25 cd	2.34 ± 0.63 i	72.0 ± 5.45 gh	73.7 ± 11.3 d	12.3 ± 0.17 bc	30.6 ± 12.7 f
	40 kg ha−1 S.A.	70.0 ± 5.04 c	2.95 ± 0.59 f	72.8 ± 6.00 g	73.2 ± 14.2 d	11.5 ± 0.23 d	32.6 ± 16.4 d
	60 kg ha−1 S.A.	65.8 ± 4.08 eg	2.86 ± 0.47 f	73.8 ± 5.34 g	76.2 ± 13.6 c	11.7 ± 0.39 d	30.1 ± 10.6 f
	2.5 gL−1 S.R.	68.3 ± 4.11 d	2.93 ± 0.58 f	72.6 ± 6.20 gh	71.4 ± 12.4 e	11.5 ± 0.58 d	31.8 ± 14.1 e
	5 gL−1 S.R.	68.0 ± 3.48 de	2.85 ± 0.54 h	79.4 ± 3.88 e	74.3 ± 15.6 cd	11.9 ± 0.24 c	31.8 ± 12.8 e
	7.5 gL−1 S.R.	71.8 ± 6.18 c	2.53 ± 0.67 h	79.8 ± 5.14 e	75.8 ± 14.3 c	11.9 ± 0.51 c	31.1 ± 13.7 e
	2.5 gL−1 S.F.	68.2 ± 5.01 d	2.55 ± 0.63 h	77.1 ± 4.36 f	70.3 ± 11.2 f	12.1 ± 0.32 bc	32.8 ± 12.3 d
	5 gL−1 S.F.	67.1 ± 5.84 de	2.70 ± 0.60 g	79.3 ± 5.49 ef	72.0 ± 14.8 e	11.8 ± 0.45 d	32.5 ± 16.4 d
	7.5 gL−1 S.F.	68.6 ± 5.84 d	2.27 ± 0.66 j	80.2 ± 8.36 e	74.2 ± 15.2 e	11.9 ± 0.52 c	32.7 ± 13.3 d
Irrigation	Zinc sulfate	Number of capitol per plant		Number of seed per capitol	1000-seed weight(g)	Oil content (%)	Harvest index (%)
Normal	Control	24.9 ± 4.01 e		41.0 ± 6.94 i	47.18 ± 4.11 bc	29.4 ± 1.67 d	29.4 ± 1.16 fg
	20 kg ha−1 S.A.	30.8 ± 6.11 c		47.2 ± 7.47 de	47.42 ± 3.05 bc	30.7 ± 2.43 c	33.7 ± 2.18 cd
	40 kg ha−1 S.A.	28.8 ± 5.23 d		47.9 ± 12.23 d	47.31 ± 2.04 bc	31.3 ± 4.25 c	30.9 ± 1.40 f
	60 kg ha−1 S.A.	30.1 ± 4.55 c		55.2 ± 11.50 a	48.75 ± 2.19 ab	30.7 ± 2.13 c	32.2 ± 2.29 de
	2.5 gL−1 S.R.	33.1 ± 6.28 a		51.7 ± 8.66 bc	47.27 ± 2.84 bc	30.1 ± 3.38 c	39.6 ± 2.57 a
	5 gL−1 S.R.	31.5 ± 5.40 b		56.6 ± 12.11 a	48.57 ± 3.77 ab	31.4 ± 4.54 b	38.5 ± 2.67 ab
	7.5 gL−1 S.R.	31.9 ± 6.34 b		46.2 ± 9.29 e	48.7 ± 3.17 ab	31.9 ± 3.29 b	38.0 ± 2.44 b
	2.5 gL−1 S.F.	31.7 ± 4.67 b		51.0 ± 8.42 bc	47.29 ± 2.29 c	33.8 ± 4.19 a	34.5 ± 3.11 c
	5 gL−1 S.F.	31.7 ± 4.83 b		47.3 ± 10.23 de	49.89 ± 2,48 a	31.1 ± 3.13 b	32.8 ± 2.74 d
	7.5 gL−1 S.F.	31.5 ± 4.98 b		52.6 ± 12.2 b	47.3 ± 3.64 bc	31.4 ± 4.11 b	30.3 ± 2.47 f
Stress	Control	20.1 ± 2.85 f		35.1 ± 7.91 j	40.67 ± 1.77 g	28.0 ± 4.44 e	18.4 ± 2.27 j
	20 kg ha−1 S.A.	25.0 ± 4.39 e		49.9 ± 8.32 c	42.6 ± 2.94 ef	29.5 ± 3.61 d	18.3 ± 2.67 j
	40 kg ha−1 S.A.	19.3 ± 3.65 f		47.2 ± 9.12 d	42.93 ± 3.81 e	29.1 ± 4.83 d	30.2 ± 2.46 f
	60 kg ha−1 S.A.	18.1 ± 4.85 h		53.8 ± 12.4 b	43.05 ± 2.39 e	30.7 ± 4.18 c	28.1 ± 2.02 g
	2.5 gL−1 S.R.	18.1 ± 4.54 h		50.0 ± 12.8 cd	40.91 ± 3.28 g	30.7 ± 3.76 c	28.4 ± 2.54 g
	5 gL−1 S.R.	20.8 ± 4.29 f		53.2 ± 13.7 b	42.69 ± 2.55 ef	29.6 ± 2.71 d	29.5 ± 3.11 fg
	7.5 gL−1 S.R.	20.6 ± 4.36 f		47.2 ± 11.2 de	41.95 ± 5.23 f	29.9 ± 3.24 d	26.1 ± 1.39 h
	2.5 gL−1 S.F.	17.9 ± 3.18 h		45.2 ± 12.3 f	45.86 ± 2.13 d	31.3 ± 4.70 c	24.3 ± 2.52 i
	5 gL−1 S.F.	18.1 ± 3.76 h		41.5 ± 12.0 i	44.96 ± 3.06 de	30.8 ± 3.28 c	23.9 ± 3.18 i
	7.5 gL−1 S.F.	17.9 ± 3.77 h		46.3 ± 12.2 e	44.72 ± 2.59 de	29.3 ± 3.43 d	25.9 ± 2.34 h

S.A. Soil application; S.R. Spraying at the rosette stage; S.F. Spraying at the capitula appearance stage. Statistical significance (p < 0.05) is indicated by the different letters.

and

Table 4. Mean comparison of measured traits of safflower, cv. Goldast as affected by zinc sulfate treatments and irrigation interruption in 2017.

Irrigation	Zinc Sulfate	Chlorophyll Index	Leaf Area Index	RWC (%)	Height (cm)	Stem Diameter (mm)	Capitul Diameter (mm)
Normal	Control	51.8 ± 5.04 f	2.75 ± 0.70 b	61.9 ± 5.48 f	51.3 ± 15.8 g	9.5 ± 0.19 c	30.1 ± 15.7 e
	20 kg ha−1 S.A.	52.7 ± 6.43 f	3.15 ± 0.84 a	64.9 ± 6.07 e	57.0 ± 17.1 d	9.2 ± 0.38 c	31.8 ± 9.39 cd
	40 kg ha−1 S.A.	57.5 ± 4.88 d	3.26 ± 0.46 a	67.9 ± 5.86 d	60.5 ± 13.6 c	10.4 ± 0.54 fg	32.9 ± 16.1 bc
	60 kg ha−1 S.A.	72.1 ± 4.64 a	3.15 ± 0.49 a	68.8 ± 5.49 cd	64.6 ± 17.3 a	10.6 ± 0.22 b	34.1 ± 11.4 a
	2.5 gL−1 S.R.	68.1 ± 4.28 b	2.89 ± 0.59 b	67.1 ± 6.39 d	56.6 ± 11.9 de	10.4 ± 0.49 b	33.7 ± 12.1 b
	5 gL−1 S.R.	58.9 ± 9.30 cd	2.97 ± 0.72 ab	81.1 ± 4.39 a	63.7 ± 12.6 ab	11.1 ± 0.32 a	34.8 ± 13.5 a
	7.5 gL−1 S.R.	60.9 ± 4.39 c	2.88 ± 0.57 ab	82.1 ± 5.69 a	60.5 ± 14.1 c	10.6 ± 0.64 b	34.7 ± 11.9 a
	2.5 gL−1 S.F.	56.8 ± 7.46 d	2.77 ± 0.53 b	69.7 ± 7.94 c	60.4 ± 13.4 c	9.5 ± 0.21 c	32.6 ± 16.0 bc
	5 gL−1 S.F.	72.4 ± 5.19 a	2.88 ± 0.50 ab	70.7 ± 4.38 b	59.1 ± 13.1 c	8.8 ± 0.44 cd	32.4 ± 14.1 c
	7.5 gL−1 S.F.	49.9 ± 6.02 f	2.76 ± 0.62 b	71.2 ± 4.17 b	62.2 ± 9.27 b	9.4 ± 0.60 c	33.6 ± 13.2 b
Stress	Control	39.0 ± 4.88 i	1.77 ± 0.63 f	57.9 ± 8.21 h	50.1 ± 14.3 g	8.3 ± 0.41 d	28.9 ± 14.4 f
	20 kg ha−1 S.A.	41.1 ± 6.48 h	2.07 ± 0.57 e	60.3 ± 8.15 g	53.2 ± 11.8 f	9.1 ± 0.52 c	31.8 ± 14.4 cd
	40 kg ha−1 S.A.	50.3 ± 4.65 f	2.62 ± 0.50 bc	61.6 ± 6.56 f	63.2 ± 12.0 ab	10.2 ± 0.17 b	33.5 ± 11.8 b
	60 kg ha−1 S.A.	42.9 ± 7.07 gh	2.77 ± 0.68 b	62.4 ± 10.68 ef	64.1 ± 10.3 a	10.5 ± 0.33 bf	34.1 ± 16.2 a
	2.5 gL−1 S.R.	45.9 ± 4.94 g	2.67 ± 0.48 bc	57.3 ± 6.62 h	61.3 ± 13.5 bc	10.5 ± 0.41 b	33.8 ± 11.8 b
	5 gL−1 S.R.	44.9 ± 5.39 g	2.69 ± 0.41 bc	69.9 ± 7.74 c	56.5 ± 13.0 de	11.4 ± 0.34 a	34.0 ± 11.2 a
	7.5 gL−1 S.R.	56.9 ± 5.29 e	2.55 ± 0.72 c	67.4 ± 6.25 d	59.9 ± 15.3 c	9.6 ± 0.19 c	32.5 ± 16.2 c
	2.5 gL−1 S.F.	42.1 ± 4.46 h	2.49 ± 0.71 cd	61.6 ± 4.94 f	59.2 ± 11.9 c	10.5 ± 0.56 b	30.1 ± 14.1 ef
	5 gL−1 S.F.	44.1 ± 6.84 g	2.29 ± 0.63 de	61.5 ± 9.24 f	52.9 ± 14.1 f	9.3 ± 0.36 c	31.1 ± 13.2 d
	7.5 gL−1 S.F.	48.4 ± 4.44 f	2.09 ± 0.56 e	60.0 ± 6.50 g	52.7 ± 10.8 f	8.8 ± 0.26 cd	30.1 ± 11.7 e
Irrigation	Zinc sulfate	Number of capitol per plant	Number of seed per capitol		1000-seed weight (g)	Oil content (%)	Harvest index (%)
Normal	Control	16.5 ± 3.28 g	37.1 ± 8.13 e		45.57 ± 3.25 bc	28.9 ± 4.23 de	29.8 ± 2.03 e
	20 kg ha−1 S.A.	20.7 ± 4.12 e	43.3 ± 11.4 c		46.18 ± 3.60 b	29.6 ± 4.33 d	30.8 ± 2.87 d
	40 kg ha−1 S.A.	22.1 ± 3.88 c	44.0 ± 11.3 c		46.89 ± 3.18 b	30.8 ± 4.28 c	28.5 ± 2.48 f
	60 kg ha−1 S.A.	23.9 ± 4.03 b	51.3 ± 9.39 a		46.1 ± 3.48 b	31.2 ± 2.14 a	35.0 ± 2.33 c
	2.5 gL−1 S.R.	24.6 ± 5.38 a	47.8 ± 11.9 b		46.73 ± 3.04 b	29.7 ± 3.07 d	37.5 ± 2.98 b
	5 gL−1 S.R.	26.0 ± 4.91 b	52.7 ± 11.2 a		47.4 ± 3.93 ab	29.9 ± 2.97 cd	37.6 ± 3.40 b
	7.5 gL−1 S.R.	24.7 ± 3.37 b	42.3 ± 9.28 c		48.62 ± 3.48 a	30.6 ± 3.04 b	39.1 ± 3.35 a
	2.5 gL−1 S.F.	21.5 ± 3.67 d	47.1 ± 9.18 b		47.13 ± 3.29 ab	31.1 ± 3.33 a	30.2 ± 3.58 e
	5 gL−1 S.F.	19.9 ± 3.41 f	43.4 ± 11.1 c		48.28 ± 3.24 a	31.6 ± 4.04 a	32.1 ± 3.01 d
	7.5 gL−1 S.F.	21.4 ± 3.90 d	48.7 ± 8.35 b		46.41 ± 3.58 b	31.1 ± 2.61 a	29.6 ± 2.47 e
Stress	Control	15.3 ± 2.31 h	32.2 ± 15.2 f		40.77 ± 2.68 e	26.0 ± 3.41 g	18.7 ± 2.98 k
	20 kg ha−1 S.A.	17.1 ± 3.22 fg	46.0 ± 14.8 bc		43.32 ± 2.94 d	26.8 ± 3.32 f	19.3 ± 2.07 jk
	40 kg ha−1 S.A.	21.6 ± 3.59 d	43.3 ± 13.6 c		43.19 ± 3.33 d	27.5 ± 3.36 ef	29.2 ± 3.40 e
	60 kg ha−1 S.A.	23.4 ± 3.79 bc	46.1 ± 11.8 bc		44.33 ± 3.49 c	31.4 ± 3.65 a	27.1 ± 2.46 fg
	2.5 gL−1 S.R.	21.2 ± 3.24 d	49.9 ± 16.4 ab		44.32 ± 3.11 c	28.4 ± 3.24 e	27.6 ± 3.13 f
	5 gL−1 S.R.	24.9 ± 3.49 b	49.3 ± 11.4 ab		43.97 ± 4.32 cd	28.1 ± 3.18 e	22.3 ± 1.43 i
	7.5 gL−1 S.R.	23.1 ± 4.14 bc	43.3 ± 9.47 c		46.11 ± 4.38 b	29.3 ± 3.00 d	21.1 ± 2.50 i
	2.5 gL−1 S.F.	21.8 ± 3.14 d	41.3 ± 13.6 d		46.84 ± 3.37 b	30.4 ± 3.47 b	26.3 ± 2.28 g
	5 gL−1 S.F.	18.8 ± 3.27 f	37.6 ± 5.61 e		42.66 ± 4.56 d	30.7 ± 3.63 b	26.2 ± 2.38 g
	7.5 gL−1 S.F.	19.0 ± 3.84 ef	42.4 ± 11.3 c		43.05 ± 2.28 d	28.7±3.04 e	24.7 ± 2.23 gh

S.A. Soil application; S.R. Spraying at the rosette stage; S.F. Spraying at the capitula appearance stage. Statistical significance (p < 0.05) is indicated by the different letters.

). In the non-application of zinc sulfate, the interruption of irrigation in both years reduced the CCI by 11% and 24.7%, respectively, compared to regular irrigation in the same treatment (Table 3 and Table 4). Meteorological data show that the second year was a hotter and drier year compared to the first year (Table 2). These data show that in the hotter year, the drought stress could cause more severe damage to the CCI. In both years, zinc sulfate treatments had an increasing effect on the CCI in both regular and end-season drought stress conditions. The highest CCI (76.6) was obtained by spraying zinc sulfate 5 g L⁻¹ in the flowering stage under normal irrigation conditions in the first year, and the lowest value (39.0) was obtained through treatment without zinc sulfate under the interruption of irrigation in the second year (Table 3 and Table 4).

3.2. Leaf Area Index (LAI)

The end-season drought stress reduced safflower’s LAI. Also, LAI decreased in all treatments in the second year compared to the first year (Table 3 and Table 4). On the other hand, in both conditions of regular irrigation and the end-season drought stress, zinc sulfate treatments caused a significant increase in the LAI compared to the non-application of zinc sulfate. In both years, the highest LAI was observed in the soil application treatments of zinc sulfate and then in the foliar spraying of zinc sulfate at the rosette stage under non-stress conditions. Among the soil application treatments, the highest level of zinc sulfate, 60 kg ha⁻¹, caused the greatest increase in LAI. But, in foliar spraying at the rosette stage, the concentration of 5 g L⁻¹ showed a better effect (Table 3 and Table 4). The lowest LAI was related to the non-application of zinc sulfate under the irrigation interruption in the second year (Table 4).

3.3. Plant Height and Stem Diameter

In the second year, the plants had a shorter height than in the first year (Table 3 and Table 4). Also, the lowest plant height was observed in the end-season drought stress conditions and without the application of zinc sulfate (Table 3 and Table 4). In contrast, in both years, zinc sulfate treatments increased the height in both regular irrigation and interruption irrigation conditions. So, the highest height of the plant (80.8 cm) was obtained with the soil application of zinc sulfate at 20 kg ha⁻¹ under regular irrigation conditions in the first year, and the lowest height (50.1 cm) was related to the treatment without zinc sulfate under the interruption of irrigation in the second year (Table 3 and Table 4). Like the height of the plant, the diameter of the stem increased with the application of zinc sulfate, and the end-season drought stress caused a decrease in the diameter of the stem compared to regular irrigation conditions. Also, in the second year, the plants had a smaller stem diameter than in the first year (Table 3 and Table 4). The highest stem diameter was observed in both years in the treatment of soil application of zinc sulfate with 60 kg ha⁻¹ under regular irrigation conditions, and the lowest value was observed in the treatments without zinc application and end-season drought stress, especially in the second year.

3.4. Diameter of the Capitol

The capitol diameter decreased under the influence of irrigation interruption, but the application of zinc sulfate increased it. Also, in all treatments, the capitol diameter decreased in the second year compared to the first year (Table 3 and Table 4). In the first year, the maximum diameter of the capitol was obtained by spraying 2.5 g L⁻¹ zinc sulfate in the rosette stage and in the second year by spraying 5 and 7.5 g L⁻¹ zinc sulfate in the rosette stage under regular irrigation conditions. In both years, the smallest diameter of the capitol was related to the non-application of zinc sulfate under the irrigation interruption condition (Table 3 and Table 4).

3.5. Number of Capitols per Plant

In all treatments, the number of capitols per plant decreased in the second year compared to the first year (Table 3 and Table 4). Also, the number of capitols decreased in both years under the end-season drought stress compared to regular irrigation (Table 3 and Table 4). In contrast, zinc sulfate treatments increased the number of capitols per plant both under regular irrigation and irrigation interruption conditions. So, the highest number of capitols per plant (33.1) was observed in 2.5 g L⁻¹ zinc sulfate spraying in the rosette stage under regular irrigation conditions in the first year; and the lowest number of capitols per plant (15.3) was observed in the non-application of zinc sulfate under the interruption of irrigation conditions in the second year (Table 3 and Table 4). The data show that foliar application in the flowering stage was less effective compared to soil application and foliar application in the rosette stage on the capitol number.

3.6. The Number of Seeds per Capitol

The number of seeds per capitol showed a decrease in all treatments in the second year compared to the first year (Table 3 and Table 4). Also, in both years, applying end-season drought stress reduced the number of seeds per capitol. In the first year, under both conditions of regular irrigation and interruption of irrigation, foliar spraying of 5 g L⁻¹ in the rosette stage and soil application of 60 kg ha⁻¹ zinc sulfate achieved the greatest increase in the number of seeds per capitol without significant differences. In both years, the lowest number of seeds was observed in the non-application of zinc sulfate, especially under stress conditions (Table 3 and Table 4). In non-stressed conditions, soil application of 60 kg ha⁻¹ and foliar application of 5 g L⁻¹ zinc sulfate at the rosette stage had the highest number of seeds per capitol without any significant difference. In stress conditions, soil application and zinc sulfate spray in the rosette stage had a greater effect compared to the spray in the flowering stage (Table 3 and Table 4).

3.7. Seeds Weight

The end-season drought stress caused a decrease in the 1000-seed weight. Also, the 1000-seed weight in the second year was less than the first year (Table 3 and Table 4). The comparison of the methods and application time of zinc sulfate showed that, with some exceptions, the highest effectiveness of zinc sulfate on the 1000 seeds weight was in the form of spray and in the flowering stage. In the interruption of irrigation, the application of zinc sulfate at 2.5 g L⁻¹ in the flowering stage in both years showed a greater effect on increasing the 1000-seed weight (Table 3 and Table 4). On the other hand, in the first year, the highest 1000-seed weight was found with 5 g L⁻¹ of zinc sulfate in the flowering stage, and in the second year, 5 g L⁻¹ in the flowering stage and 7.5 g L⁻¹ of zinc sulfate in the rosette stage under irrigation conditions had the highest 1000-seed weight. The lowest value was related to the non-application of zinc sulfate under the interruption of irrigation conditions in both years (Table 3 and Table 4).

3.8. Biological Yield (BY)

The BY of safflower plants decreased under the end-season drought stress, and on the other hand, the BY increased with the application of zinc sulfate treatments. Also, in all treatments, the BY decreased in the second year compared to the first year (Figure 1a,b). In both years, the lowest BY was obtained with no application of zinc sulfate under the interruption of irrigation conditions, while the application of zinc sulfate as a soil application, especially 60 and 40 kg ha⁻¹, had the greatest effect on increasing biomass production (Figure 1a,b). After the soil application treatments, spraying in the rosette stage showed a higher effect, and the lowest effect was seen for the zinc sulfate spray in the flowering stage.

3.9. Seed Yield (SY)

The SY showed a significant decrease in the second year compared to the first year. Applying the end-season drought stress in both years decisively reduced SY compared to regular irrigation (Figure 2a,b). In both years, zinc sulfate treatments had an increasing effect on SY in both regular irrigation and drought stress conditions. The highest increasing effect of zinc sulfate application in non-stress conditions was observed in high concentrations of soil application and spray in the rosette stage, and after these treatments, zinc sulfate foliar spray in the flowering stage influenced safflower SY. Under stress conditions, low and medium concentrations of zinc sulfate showed a greater increasing effect compared to higher concentrations of zinc sulfate (Figure 2a,b). The lowest SY (1021.73 kg/ha) was observed in the non-application of zinc sulfate under the interruption of irrigation in the second year (Figure 2b). It seems that in the stress conditions, higher concentrations of zinc sulfate, with greater development of the LAI, caused a rapid depletion of soil moisture and had a negative effect on SY production.

3.10. Harvest Index (HI)

The application of zinc sulfate increased the value of the HI in both years and under non-stressed and stressed conditions (Table 3 and Table 4). The data show that the HI values in the second year and stressful conditions were lower than in the first year and non-stress conditions. Also, in non-stress conditions, the highest HI was observed in the zinc sulfate spray method at the rosette and flowering stages, and the soil application of zinc sulfate had a lower effect on the HI compared to the foliar spray method. It seems that the soil application of zinc sulfate caused more foliage development compared to seed production.

3.11. Seed Oil Content (SOC), Oil Yield (OY), and Oil Harvest Index (OHI)

The SOC was affected by the end-season drought stress, zinc sulfate application, and year. In general, the SOC in the second year was lower than the first year. Also, drought stress had a decreasing effect on SOC compared to regular irrigation. On the other hand, zinc sulfate treatments increased the amount of SOC for treatment without zinc sulfate in both stress and non-stress conditions (Table 3 and Table 4). The comparison of zinc sulfate application times and methods showed that the spray method had the greatest effect on SOC in the flowering stage and caused a greater increase in SOC. The lowest content of SOC (26%) was related to the non-application of zinc sulfate under the interruption of irrigation in the second year (Table 3 and Table 4). Although the highest content of SOC was observed with foliar spraying of zinc sulfate in the flowering stage, foliar spraying in this phenological stage showed a lower OY compared to the other two methods of zinc sulfate application (Figure 3a,b). This difference shows the greater importance of SY compared to SOC for oil production per unit area. Also, in both years, the OHI with foliar spraying showed the highest value in the rosette stage (Figure 4a,b). The lower OHI in the soil application method could be due to the greater effect of zinc sulfate on the development of vegetative parts compared to seed production, and the lower OHI in the foliar treatment in the flowering stage is due to the lower effect of this treatment on seed production. It seems that foliar spraying in the flowering stage would not have enough time for the effect of zinc sulfate on the increase in seed production, like previous treatments.

3.12. The Seed Nitrogen, Phosphorus, Potassium, Zinc, and Iron Element Content

In all treatments, the nitrogen, phosphorus, potassium, zinc, and iron content in the seed decreased in the second year compared to the first year (

Table 5. Mean comparison of the seed N, P, K, Zn, and Fe contents of safflower as affected by zinc sulfate treatment and irrigation interruption in 2016.

Irrigation	Zinc Sulfate	Seed N (mg g−1)	Seed P (mg g−1)	Seed K (mg g−1)	Seed Zn (mg g−1)	Seed Fe (mg g−1)
Normal	Control	48.8 ± 1.18 c	8.6 ± 0.78 d	6.03 ± 0.97 f	0.079 ± 0.011 d	0.070 ± 0.008 f
	20 kg ha−1 S.A.	51.0 ± 1.45 b	9.0 ± 0.88 c	6.35 ± 0.82 de	0.083 ± 0.010 b	0.083 ± 0.011 d
	40 kg ha−1 S.A.	53.8 ± 2.01 a	9.6 ± 0.97 a	6.59 ± 0.62 c	0.084 ± 0.014 b	0.096 ± 0.011 b
	60 kg ha−1 S.A.	49.9 ± 1.16 bc	8.8 ± 0.64 cd	6.33 ± 0.68 e	0.083 ± 0.014 b	0.071 ± 0.009 f
	2.5 gL−1 S.R.	51.5 ± 1.65 b	9.2 ± 0.66 b	6.59 ± 1.01 c	0.080 ± 0.014 c	0.091 ± 0.010 bc
	5 gL−1 S.R.	51.4 ± 1.40 b	9.2 ± 0.80 b	6.33 ± 0.61 e	0.088 ± 0.014 a	0.085 ± 0.011 c
	7.5 gL−1 S.R.	49.4 ± 1.44 c	9.4 ± 0.84 ab	6.06 ± 0.39 f	0.083 ± 0.011 b	0.085 ± 0.011 c
	2.5 gL−1 S.F.	53.5 ± 1.36 a	9.1 ± 0.77 bc	6.33 ± 0.37 e	0.083 ± 0.012 b	0.076 ± 0.014 e
	5 gL−1 S.F.	51.3 ± 1.58 b	9.5 ± 1.01 a	6.87 ± 0.87 a	0.089 ± 0.011 a	0.084 ± 0.013 c
	7.5 gL−1 S.F.	50.6 ± 1.55 b	9.0 ± 0.68 c	6.78 ± 0.64 b	0.086 ± 0.011 ab	0.11 ± 0.019 a
Stress	Control	48.3 ± 1.45 d	7.2 ± 0.67 g	5.54 ± 0.58 h	0.070 ± 0.010 g	0.062 ± 0.013 g
	20 kg ha−1 S.A.	51.2 ± 1.33 b	8.9 ± 0.68 c	6.59 ± 0.43 c	0.074 ± 0.015 e	0.086 ± 0.016 bc
	40 kg ha−1 S.A.	52.2 ± 1.76 a	8.8 ± 0.77 cd	6.60 ± 0.41 c	0.075 ± 0.013 e	0.085 ± 0.011 c
	60 kg ha−1 S.A.	52.5 ± 1.68 a	8.7 ± 0.59 cd	6.43 ± 0.77 d	0.072 ± 0.013 f	0.078 ± 0.016 de
	2.5 gL−1 S.R.	50.4 ± 1.38 b	8.8 ± 0.83 cd	6.60 ± 0.41 c	0.076 ± 0.014 e	0.086 ± 0.008 bc
	5 gL−1 S.R.	50.5 ± 1.54 b	8.1 ± 0.62 e	6.06 ± 0.86 f	0.082 ± 0.011 bc	0.077 ± 0.014 de
	7.5 gL−1 S.R.	49.0 ± 1.69 c	8.1 ± 0.81 e	6.06 ± 0.72 f	0.081 ± 0.011 c	0.088 ± 0.014 bc
	2.5 gL−1 S.F.	50.1 ± 1.33 b	8.2 ± 0.68 e	6.06 ± 0.63 f	0.072 ± 0.008 f	0.079 ± 0.012 de
	5 gL−1 S.F.	51.3 ± 1.70 b	7.6 ± 0.97 f	5.80 ± 0.79 g	0.075 ± 0.013 e	0.076 ± 0.015 e
	7.5 gL−1 S.F.	51.9 ± 1.94 b	8.1 ± 0.62 e	5.80 ± 0.74 g	0.078 ± 0.020 d	0.080 ± 0.017 d

S.A., soil application; S.R., spraying at the rosette stage; S.F., spraying at the capitula appearance stage. Statistical significance (p < 0.05) is indicated by the different letters.

and

Table 6. Mean comparison of the seed N, P, K, Zn, and Fe contents of safflower as affected by zinc sulfate treatment and irrigation interruption in 2017.

Irrigation	Zinc Sulfate	Seed N (mg g−1)	Seed P (mg g−1)	Seed K (mg g−1)	Seed Zn (mg g−1)	Seed Fe (mg g−1)
Normal	Control	34.8 ± 1.60 b	7.7 ± 0.44 b	4.47 ± 0.42 e	0.024 ± 0.018 h	0.053 ± 0.010 c
	20 kg ha−1 S.A.	35.3 ± 2.04 b	8.3 ± 0.83 a	5.61 ± 0.47 a	0.031 ± 0.012 g	0.062 ± 0.011 b
	40 kg ha−1 S.A.	40.1 ± 1.67 b	7.3 ± 0.86 c	5.07 ± 0.49 c	0.057 ± 0.017 c	0.077 ± 0.010 a
	60 kg ha−1 S.A.	47.1 ± 1.85 a	6.9 ± 0.84 cd	4.93 ± 0.62 c	0.079 ± 0.0113 a	0.064 ± 0.017 b
	2.5 gL−1 S.R.	37.8 ± 1.38 b	7.7 ± 0.69 b	5.23 ± 0.90 b	0.038 ± 0.008 f	0.051 ± 0.019 c
	5 gL−1 S.R.	38.2 ± 1.64 b	7.8 ± 0.51 b	4.87 ± 0.86 d	0.046 ± 0.0011 e	0.051 ± 0.016 c
	7.5 gL−1 S.R.	39.4 ± 1.26 b	7.6 ± 0.66 b	4.12 ± 0.77 f	0.060 ± 0.008 c	0.041 ± 0.012 d
	2.5 gL−1 S.F.	32.6 ± 1.27 c	8.1 ± 0.44 a	5.21 ± 0.56 b	0.035 ± 0.012 f	0.050 ± 0.019 c
	5 gL−1 S.F.	36.2 ± 1.69 b	7.5 ± 0.81 bc	5.18 ± 0.81 b	0.051 ± 0.011 d	0.047 ± 0.018 d
	7.5 gL−1 S.F.	35.4 ± 1.44 bc	6.9 ± 0.84 cd	5.25 ± 0.66 b	0.069 ± 0.011 b	0.048 ± 0.011 cd
Stress	Control	22.7 ± 1.94 d	5.9 ± 0.48 f	2.04 ± 0.32 k	0.018 ± 0.007 i	0.031 ± 0.009 e
	20 kg ha−1 S.A.	23.0 ± 1.50 d	6.4 ± 0.66 e	3.76 ± 0.79 h	0.029 ± 0.010 g	0.036 ± 0.014 e
	40 kg ha−1 S.A.	26.2 ± 1.72 d	6.4 ± 0.68 e	3.85 ± 0.83 gh	0.035 ± 0.011 f	0.055 ± 0.013 c
	60 kg ha−1 S.A.	32.2 ± 1.94 c	6.0 ± 0.79 f	3.82 ± 0.64 n	0.051 ± 0.011 d	0.034 ± 0.011 e
	2.5 gL−1 S.R.	24.3 ± 1.36 d	6.1 ± 0.65 f	4.19 ± 0.48 f	0.023 ± 0.011 h	0.036 ± 0.012 e
	5 gL−1 S.R.	25.8 ± 1.31 b	6.4 ± 0.87 e	4.13 ± 0.82 f	0.035 ± 0.011 f	0.036 ± 0.011 e
	7.5 gL−1 S.R.	26.4 ± 1.95 b	6.0 ± 0.69 f	3.94 ± 0.60 g	0.039 ± 0.012 f	0.036 ± 0.011 e
	2.5 gL−1 S.F.	31.4 ± 1.67 c	7.3 ± 0.61 c	3.26 ± 0.65 i	0.024 ± 0.014 h	0.043 ± 0.013 d
	5 gL−1 S.F.	31.7 ± 1.67 c	7.1 ± 097 c	3.12 ± 0.43 j	0.040 ± 0.011 f	0.042 ± 0.013 d
	7.5 gL−1 S.F.	32.3 ± 1.59 c	6.0 ± 0.83 f	2.98 ± 0.76 j	0.053 ± 0.011 d	0.033 ± 0.016 se

S.A., soil application; S.R., spraying at the rosette stage; S.F., spraying at the capitula appearance stage. Statistical significance (p < 0.05) is indicated by the different letters.

). Also, the application of end-season drought stress reduced the seed element content compared to normal irrigation. On the other hand, zinc sulfate treatments had a positive effect on the seed element contents in seeds compared to the non-applying zinc sulfate. The highest seed nitrogen and phosphorus (38.5% and 0.96%, respectively) content was obtained by the soil application of zinc sulfate at 40 kg ha⁻¹ under normal irrigation conditions in the first year (Table 5 and Table 6). The phosphorus content decreased depending on the increase in the amount of zinc sulfate used in each stage. The highest amount of potassium (0.687%) and zinc (0.089 mg g⁻¹) elements was obtained in the seeds of plants sprayed with 5 g L⁻¹ zinc sulfate in the flowering stage under normal irrigation conditions in the first year. The plants sprayed with 7.5 g L⁻¹ zinc sulfate in the flowering stage under regular irrigation conditions in the first year had the highest amount of iron (0.11 mg/g) in the seeds. The lowest seed element was related to the non-application of zinc sulfate under the interrupted irrigation in the second year (Table 5 and Table 6).

4. Discussion

The growth and performance of crop plants can be affected by environmental factors, such as different environmental stresses, their intensity, and even the time of application. On the other hand, changes in growth conditions and crop management, such as the use of fertilizers, can have an improvement effect on plant growth [9,13,29]. In this regard, the growth of safflower can also be changed under the influence of various environmental factors and nutritional management. The results show a significant reduction in agromorphological traits such as LAI, plant height, capitol diameter, number of capitols per plant, number of seeds per capitol, BY, and SY; physiological traits such as CCI and RWC; and quality such as the content of nitrogen, phosphorus, potassium, zinc, and iron in seeds with the application of drought stress (Table 3, Table 4, Table 5 and Table 6; Figure 1 and Figure 2). The comparison of the two years of the experiment also showed that in the second year, there was a significant decrease in most of the measured traits compared to the first year, especially in the stress conditions. This could be due to the decrease in rainfall and higher temperatures in the second year (2017) compared to the first year (2016) (Table 2). Also, in the second year, the sudden increase in temperature in the July and August months, which coincided with the flowering and seed-filling stages, led to an increase in evapotranspiration demand and a decrease in the period between pollination and seed ripening. Therefore, the seed-filling period was reduced and shortened. This can reduce the transfer of minerals from the soil, as well as the remobilization of elements from the source parts to the sinks, and as a result, reduce the concentration of these elements in the seeds in the second year. Other studies have reported the negative effect of drought and heat stress on the uptake and availability of mineral elements and the reduction in quantitative and qualitative seed yield [13,30]. Drought and heat stress in each stage of plant life can affect growth and yield, but the seed-filling stage is very important for determining the final seed weight and seed quality, and therefore, the final quantitative and qualitative yield [31,32]. The negative effect of unfavorable environmental conditions, such as soil moisture deficiency and high temperatures, on physiological parameters and seed yield has been reported [10,13,29,30]. Accelerated leaf aging in stressful conditions due to dehydration and desiccation leads to incomplete remobilization and causes an insufficient supply of elements to seeds during seed filling. Also, the late initiation of nutrient mobilization may prevent rapid seed growth [32].

The decrease in the chlorophyll content observed in the end-season drought stress conditions could be due to the decrease in the availability and absorption of elements such as iron and nitrogen. Also, it has been reported that in drought stress conditions, the oxidation of photosynthetic pigments due to the production of reactive oxygen species and the activation of chlorophyllase and peroxidase enzymes decreases the chlorophyll concentrations [10,33]. On the other hand, the application of zinc sulfate increased the CCI. This increase may be due to the positive role of zinc in the absorption of nitrogen, iron, and other elements that are either directly involved in the structure of chlorophyll or are effective in its biosynthesis [12,34].

The decrease in RWC under water stress conditions could be due to a decrease in water absorption from the soil or due to more transpiration from the leaves [8]. This disturbance in the water condition reduces the turgor pressure, which reduces cell division and development and the plant’s leaf area. Among other factors, the reduction in the total leaf area due to the end-season drought can be mentioned due to the premature aging of the leaves and the falling of the leaves. Like these results, it has been reported in safflower [9] and soybean [35] that drought had a negative effect on the CCI, leaf area, and RWC of plants. Decreasing the RWC, by increasing the stomatal and non-stomatal resistances, causes a decrease in the rate of photosynthesis, growth, biomass production, and crop yield [9]. In this study, the decrease in plant height and BY of safflower under stress conditions can be attributed to the decrease in CCI and RWC. Similarly, reduced plant height and BY under drought conditions have been reported in other studies [17,29].

The end-season drought stress had a significant and decreasing effect on yield and yield components, HI, and seed mineral element content. The decrease in safflower yield and yield components may be due to the decrease in leaf area, CCI, and RWC under drought stress conditions, which causes a decrease in the light interception area and chlorophyll pigments, and finally, an increase in stomatal resistance. The water supply for safflower in the post-flowering stage can increase the capitol diameter, yield components, and finally, seed yield. On the other hand, the occurrence of drought stress in this stage can reduce the growth of inflorescences, the number of flowers, capitol size, and other yield components, causing a decrease in seed yield [36]. Decreasing the photosynthesis rate in drought stress conditions reduces the release of assimilates toward the reproductive organs and reduces the number of fertile flowers. On the other hand, the decrease in seed number can be attributed to the increase in the proportion of sterile florets before seed filling. In addition, water deficiency can cause the abortion and death of fertilized flowers, disturbances in pollination and relative sterility of pollen, and disturbances in seed filling [37]. The decrease in the seeds’ weight under stress conditions could be due to the decrease in leaf area, chlorophyll content, and, eventually, the photosynthesis rate. Under stress conditions, the direct transfer and remobilization of photoassimilates is limited, and if this happens between flowering and ripening, it causes a decrease in the seeds’ weight [38]. In this study, the reduction in yield components under drought stress conditions led to decreased safflower seed yield. It has been reported that drought stress decreases plant height, seed weight, SY, BY, CCI, RWC, and leaf area in safflower [15] and soybean [35]. Also, decreases in the number of pods per plant, the number of seeds per pod, seed weight, and the SY of canola exposed to drought have been reported [39].

Drought stress reduced the HI of plants. Also, among the zinc sulfate application methods, soil-treated plants had a lower HI compared to sprayed plants. This decrease observed in plants under drought stress was due to a sharper decrease in seed production compared to biomass production, while for the plants treated by the soil method, it was due to a greater increase in biomass compared to seed production. It has been reported that the HI of plants decreased under water shortage conditions [29]. It has been found that stresses such as water shortage [38] and heat [13] affect the SY more than the BY and therefore reduce the HI, which was also observed in our study.

In this study, the end-season drought stress caused a decrease in oil percent and oil yield per unit area, which was consistent with the results of studies on sunflower [40] and safflower [15]. The decrease in oil percent due to drought stress could be due to the decrease in the ability to convert carbohydrates into oil and the oxidation of some unsaturated fatty acids in drought conditions. Also, under stress conditions, the inhibition of fatty acid synthesis and damage to synthesized fatty acids can reduce oil content [40]. Under stress conditions, not only the quantity of yield but also its quality decreases. For example, in safflower [41] and wheat [13], the seed mineral element content decreased under drought stress. This may be due to the decrease in the availability of elements in the soil and the decrease in the development of the root system in water deficit conditions, which causes a decrease in the concentration of elements in plant tissues [12]. Also, water deficiency in the soil causes a decrease in mass flow and the diffusion of elements in the soil, which reduces the transfer of elements to the root surface and leads to the insufficient absorption of elements by the plant [41]. On the other hand, with the reduction in transpiration flow, the transfer of mineral elements to the aerial part and seeds also decreases. Another source of accumulation of mineral elements in the seed is remobilization from the vegetative parts. In stress conditions, due to the premature aging of tissues, the chance of remobilization decreases and causes the reduction in accumulated elements in the seeds [42]. It has been stated that it is possible to improve this reduction in physiological parameters, growth, yield, and seed quality under stress conditions using nutritional management, such as zinc micronutrient application [13,17].

The data show that the application of zinc improved parameters such as RWC, especially under dry conditions. One of the reasons for increasing RWC with zinc application is the positive effect of this element on potassium absorption and the improvement of turgor pressure [11]. Improving and increasing the RWC, along with the positive effect of zinc on auxin hormone biosynthesis, can increase cell division and elongation [14,34], which finally increases the leaf area in safflower plants treated with zinc sulfate.

Plants treated with zinc sulfate had greater height compared to untreated plants under both stress and non-stress conditions. It has been stated that zinc can increase stem height and other growth parameters [13] by increasing the biosynthesis of tryptophan (auxin precursor) [11]. Also, improving the CCI, leaf area, and RWC by applying zinc sulfate can improve photosynthesis and increase dry matter accumulation, which will result in increasing the vegetative parameters and BY of plants. Other researchers also reported that the application of zinc sulfate (spraying and soil application) increased agronomic and physiological parameters in plants such as cotton, safflower, and wheat and reduced the negative effect of drought on these traits [13,17,29,43].

The application of zinc sulfate in soil and spray forms increased yield and yield components under both stress and non-stress conditions. Applying zinc sulfate increased the number and diameter of capitols, which increased the number of seeds per capitol and plant, and finally, SY. It has been reported that the foliar spraying and soil application of zinc sulfate increased the capitol diameter of the sunflower, which itself increased the number of seeds in the capitol and ultimately increased the SY. This increment was attributed to the increase in the number of fertile flowers in the capitol, which directly affects SY [44]. Zinc plays a positive role in increasing pollination, fertilization, and pollen survival, and by increasing the number of florets, it can increase seed formation and the number of seeds [11]. Similarly, spraying zinc sulfate on safflower plants under drought conditions increased the number of seeds and the 1000-seed weight [29]. It seems that supplying any factor such as water and mineral elements that give the plant more opportunity to grow will lead to the formation of more potential places for the production of capitol and will also increase the yield. As a result, the application of zinc sulfate significantly increased the yield of safflower through the improvement of yield components.

The data show that the application of zinc sulfate, although increasing biomass production, had a greater effect on seed production because the HI value showed a significant increase in zinc application treatments. It has been reported that the application of zinc sulfate in safflower and wheat led to an increase in the HI by increasing SY [13,29]. Also, the application of zinc sulfate increased the oil content of safflower seeds, which was consistent with the results of other studies on safflower [15,45]. It seems that the application of zinc increased the metabolism of fatty acids, and the amount of oil produced and stored in the seeds was increased. It has been reported that the increase in seed oil with zinc treatment was due to the increase in the fatty acid content, which could be due to the prevention of the oxidation of these fatty acids in the presence of zinc in the seed [45].

The improvement of nitrogen, phosphorus, potassium, zinc, and iron content in safflower seeds was observed with the application of zinc sulfate under stress and non-stress conditions. Similarly, an increase in the accumulation of nitrogen, potassium, zinc, and iron in seeds in different crops has been reported with the application of zinc sulfate [13,34,46]. The application of high levels of zinc sulfate, using both soil and spray methods, caused a significant decrease in the phosphorus content in safflower seeds compared to lower levels; however, they were higher than the phosphorus content in the seeds of untreated plants with zinc (Table 5 and Table 6). It has been reported that there is an antagonistic relationship between soil zinc and phosphorus for plant uptake [11]. It seems that the application of high concentrations of zinc sulfate has increased the zinc absorption or transfer in the plant and reduced the transfer of phosphorus from the roots to other parts. The antagonistic effect of zinc on the amount of phosphorus in seeds has also been reported in other studies [13]. Weisany et al. also reported that spraying maize plants under drought conditions improved the nitrogen, potassium, phosphorus, zinc, and iron content of seeds [41]. The increase in seed potassium with the application of zinc may be due to the synergistic effect between zinc and potassium, where the presence of zinc increases the flow of potassium from the roots to the shoots and seeds [11,46]. Like the present report, the application of zinc sulfate led to an increase in the concentration of zinc and manganese in safflower seeds [47]. It has been found that there is a positive relationship between seed zinc and the iron concentration. It is reported that zinc and iron elements can accumulate in the seed due to pleiotropic effects or linkage between the responsible genes [13,34]. The positive role of zinc in increasing macro and micro elements’ use efficiencies has been reported [13]. The application of zinc improves root growth, facilitates the absorption of water and nutrient elements, increases the quantitative and qualitative yield of seeds, and improves drought tolerance [12].

In calcareous soil conditions and increased soil alkalinity, the absorption of elements, especially micro nutrients, is disturbed [11]. The decrease in soil water content led to a reduction in the zinc element’s activity in the soil solution and, due to the limitation of root growth, the plant faced a double deficiency of this element [11,41]. The strategy of fertilizer application as an agricultural method is one of the most promising sustainable solutions to overcome the problem of zinc deficiency in humans by improving zinc accumulation in seeds. Therefore, due to the slow absorption of zinc and other similar elements by the roots, it is better to provide these elements to the plant through foliar spraying. In the present study, zinc foliar application had a greater effect on the growth, yield, and quality of safflower seeds compared to the soil application method. In this context, it has been reported that the foliar application of zinc is preferable to other methods of its application [48] because it can improve the yield and zinc content of seeds, even to 80% [49]. It has been reported that different methods of zinc application (spraying, soil application, and spraying + soil application) had an increasing effect on the zinc content of wheat cultivars [13,43], which was consistent with the results of our study. Chattha et al. announced that the soil application and spraying of zinc on wheat increased the length of the stem and spike, biomass, SY, HI, and carbohydrate, protein, and zinc content in the seeds [50]. It seems that foliar spraying of zinc sulfate in each growth stage of safflower had the ability to increase the content of zinc in the seed. This issue can be considered related to the relationship between the vessel and the phloem in the inflorescence of the plant and the proper exchange of elements between them. Ishimaru and colleagues stated that after being absorbed through the stomata and transferred to the leaf cells, zinc is stored there, and during the growth stages, it is moved inside the plant by specific protein transporters that also play a role in transporting iron [51]. This causes zinc to move from the aged leaves at the end of growth through the phloem to the seeds and, as a result, increases the amount of this element in the seeds. It has been reported that the transfer of zinc from lower leaves to young leaves and seeds through the phloem at the end of the plant growth stage led to an increase in zinc content in seeds [52].

5. Conclusions

The crop plant’s yield is a result of the interaction of three components: genotype, environment, and agronomic management. The application of end-season drought stress, as well as the decrease in rainfall and the increase in temperature in the second year (2017), caused a significant reduction in safflower yield. This reduction in yield, through the reduction in plant height, leaf area, relative water content, and chlorophyll content, led to a decrease in the capitol diameter, number of capitols per plant, and number of seeds per capitol, which led to a decrease in seed yield. The data from the harvest index and the oil harvest index showed that adverse environmental factors had more severe effects on seed yield and seed oil content than plant biomass production. In contrast, the application of zinc sulfate in different ways reduced the harmful effect of stress on these traits. The comparison of zinc sulfate application methods showed that foliar application affected the quality of safflower seeds by increasing the content of nitrogen, phosphorus, potassium, zinc, and iron. The foliar spraying of zinc in both stages is a promising method to improve the concentration of zinc in the seed, which can quickly compensate for the zinc deficiency in the seed and improve the quantitative and qualitative yield. The comparison of the two methods of soil application and spraying of zinc sulfate showed that soil application, in addition to the lower absorption efficiency by the plant, is expensive, and therefore foliar application is a more useful alternative solution. Our results showed that the application of 7.5 g L⁻¹ of zinc sulfate in non-stress conditions and 5 g L⁻¹ in stress conditions had the most effect on safflower yield. Foliar spraying provides the possibility of the rapid use of micronutrients by the plant and causes it to overcome the element’s deficiency in a short duration compared to soil application. On the other hand, soil application can store zinc in the soil for the following years. The data show that the application of zinc can increase the yield in non-stress conditions, but it can also improve plant performance against drought and heat in stressful conditions.