Introduction
Wheat has the highest role in supplying human food in terms of high adaptability to a wide range of environmental conditions and the highest global production among other crops (Grote et al., Reference Grote, Fasse, Nguyen and Erenstein2021). This crop is economically the most important cereal in the world (Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). The economic importance of wheat and the increase in its consumption due to the growth of the world's population require strategies such as the introduction of high-yielding cultivars (Lima et al., Reference Lima, Gracia-Romero, Rezzouk, Diez-Fraile, Araus-Gonzalez, Kamphorst, Amaral Júnior, Kefauver, Aparicio and Araus2021) and the improvement of seed quality through better plant nutrition management or plant breeding (Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). Soil nutrient deficiency is a limiting factor for cereal production and quality in many countries, especially in developing countries (Joy et al., Reference Joy, Ahmad, Zia, Kumssa, Young, Ander, Watts, Stein and Broadley2017; Grote et al., Reference Grote, Fasse, Nguyen and Erenstein2021).
Although zinc is an important component of many enzymes responsible for metabolic reactions in plants, zinc deficiency is one of the most important and widespread micronutrient deficiencies in the world, and its deficiency is a public problem among people in developing countries (Joy et al., Reference Joy, Ahmad, Zia, Kumssa, Young, Ander, Watts, Stein and Broadley2017). It has been reported that more than two billion people in developing countries suffer from zinc deficiency (Grote et al., Reference Grote, Fasse, Nguyen and Erenstein2021). Zinc deficiency causes human immunodeficiency, stunted growth, hair loss, infertility and cognitive disorders (Ramazan et al., Reference Ramazan, Hafeez, Khan, Nadeem, Rahman, Batool and Ahmad2020; Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). Calcareous soils, high bicarbonate of irrigation water, low soil organic matter and excessive use of phosphate fertilizers are among the reasons for zinc deficiency. The result of these factors causes zinc deficiency in the plant, decrease in yield, and decrease in nutritional value due to increase in the amount of phytic acid. These eventually cause zinc deficiency in the human society that uses these products (Sharma et al., Reference Sharma, Patni, Shankhdhar and Shankhdhar2013). Wheat is a poor source of zinc for human nutrition, and the zinc content in most cultivars is less than 20 mg/kg, while its amount in the dry weight of wheat grains should be more than 50 mg/kg (Ghasal et al., Reference Ghasal, Shivay, Pooniya, Choudhary and Verma2017). Zinc application increases seed yield and its quality, and also increases the shelf life of bread.
In plants, zinc plays a role in processes such as chlorophyll synthesis, indole-3-acetic acid biosynthetic pathway, chloroplast development, nitrogen metabolism, protein quality, and activation of enzymes such as carbonic anhydrase (Taiz and Zeiger, Reference Taiz and Zeiger2003; Sharma et al., Reference Sharma, Patni, Shankhdhar and Shankhdhar2013; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). On the other hand, the application of micronutrients such as zinc increases the utilization efficiency of macronutrients and reduces their utilization costs (Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). In wheat, the application of zinc in the form of soil or foliar application improved the seed yield and its components, biological yield, gluten and zinc content (Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021; Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). It has been found that zinc application can cause accumulation of zinc in wheat seeds in high amounts (Ram et al., Reference Ram, Rashid, Zhang, Duarte, Phattarakul, Simunji, Kalayci, Freitas, Rerkasem, Bal, Mahmood, Savasli, Lungu, Wang, de Barros, Malik, Arisoy, Guo, Sohu, Zou and Cakmak2016; Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). Different methods of zinc application show a positive effect on the zinc content of seeds (Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017; Ma et al., Reference Ma, Sun, Wang, Ding, Qin, Hou, Huang, Xie and Guo2017). Produced seeds show higher germination potential and growth rate, and reduce the concentration of pollutants such as cadmium in the edible parts of agricultural crops (Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017; Safari et al., Reference Safari, Nahari Alishah, Kari Dolatabad, Ndu, Schulthess and Sorooshzadeh2019; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021).
Among cereals, wheat and rice are more susceptible to zinc deficiency, and a decrease in seed yield up to 80% along with a reduction in seed zinc content has been observed under zinc deficiency conditions (Singh et al., Reference Singh, Natesan, Singh and Usha2005). According to Boostani et al. (Reference Boostani, Najafi-Ghiri, Amin and Mirsoleimani2019), the total amount of zinc in calcareous soils of Iran is sufficient (50 to 300 mg/kg), but its concentration in soil solution and consequently its bioavailability is very low due to the soil conditions such as high pH, low organic matter and high calcium carbonate. These factors have led to the failure of most crops, such as cereals, to achieve high yields. The objectives of this experiment were (1) to study the response of different wheat cultivars introduced in the last 50 years to soil application of zinc sulphate, (2) to identify the cultivar or cultivars with high seed yield under conditions of supplemental zinc supply or zinc deficiency, and (3) to study the effect of zinc on seed yield and its components and seed quality of the cultivars studied.
Materials and methods
Experimental design, plant material and zinc treatment
The field experiment was conducted as a split-block, randomized complete block design with three replications in two consecutive cropping years (2018 and 2019). The experimental treatments included zinc application from zinc sulphate source (33% purity, Kimia Pars Shayankar, Iran) at two levels (0 and 40 kg/ha) as the first factor and 21 wheat cultivars as the second factor. The cultivars studied with their year of release are presented in Table 1. The seeds of these cultivars were obtained from the Seed and Plant Improvement Institute, Karaj, Iran. Zinc sulphate was applied to the field in the form of solution with irrigation water two times with the first and second irrigation after planting.
Table 1. Wheat cultivars' names and years of their release studied under zinc application conditions and two cropping years of 2018–2019
Field preparation and seed cultivation
Seedbed preparation included spring ploughing, disking, and levelling prior to fall planting. Half of the nitrogen (urea fertilizer, Shiraz Petrochemical Co., Iran) was applied at the three to four leaf stage and the rest before stem elongation. Potassium (potassium sulphate, Khadamat hemayti Co., Iran) and phosphorus (triple superphosphate, Khadamat hemayti Co., Iran) fertilizers were completely spread on the field before planting at the rate of 100 kg/ha. The physicochemical characteristics of the soil of the research station are presented in Table 2. The ambrothermic diagrams of the research station of the University of Zanjan are presented in Fig. 1 for two cropping years (2018 and 2019). Seed sowing was carried out using a linear research seed sowing machine (Wintersteiger tool carrier 2700, Austria) in the research farm of the Faculty of Agriculture, University of Zanjan, Zanjan, Iran (located at 36°41′ N latitude, 48°27′ E longitude, and 1620 m above sea level). The sowing depth was 4–5 cm. The date of planting in both years (2018 and 2019) was October 23. Before sowing, the seeds were disinfected with carboxin-thiram fungicide. Each experimental plot consisted of four rows. The distance between rows was 20 cm and the length was 4 m. The distance between plots and blocks was set to 50 and 100 cm, respectively. Irrigation was done once a week with an irrigation brigade tape. Weeds were controlled by frequent manual weeding during the growing season. Deltamethrin (Decis, EC 2.5%, Gyah Co. Iran) and propiconazole (Tilt, EC 25%, Gyah Co. Iran) were applied in late April or May to control sunn pest (Eurygaster integriceps) and rust (Puccinia triticina).
Table 2. Physico-chemical properties of soil at a depth of 0–30 cm in the Research Farm of the University of Zanjan
Figure 1. Monthly mean temperature and monthly total precipitation from August through July for 2018–2019 (a) and 2019–2020 (b).
Leaf area index and flag leaf area (FLA)
At the flowering stage, all plants were removed from 0.5 m2 and the total leaf area was determined using a leaf area metre (Delta T Device LTD, England). Then, the leaf area index (LAI), which is expressed as the ratio of leaf area to ground area (Hunt, Reference Hunt1982), was calculated for each of the experimental units. Also, the FLA of the plants was recorded separately and reported in cm2.
Chlorophyll content index
The greenness of the plants was measured non-destructively using a chlorophyll meter (CCM200-OPTI SCIENCE.UK) at the heading stage. In each experimental plot, 10 plants were randomly selected, and greenness was measured for the main stem flag leaf.
Plant height and spike length
At maturity, the height of 10 plants from the ground to the end of the spike was measured and recorded. For the same plants, the length of the spike was also measured from the part of the spike node to the end of the spike.
The number of seeds per spike, 1000-seed weight, biological and seed yield and harvest index
The plants were harvested at maturity and when the plants turned yellow. On one m2 of each plot, all plants were cut, and the number of spikelets was counted. After crushing the spike, seeds were counted using a seed counter (Pfeuffer, Germany). The number of seeds per spike was obtained by dividing the number of seeds by the number of spikes. The remaining area of the plot was harvested with a research combine and after weighing all harvested plants, the biological yield (t/ha) was recorded. The harvest index (HI) was calculated by dividing the harvested seeds by the biological yield. The 1000 seed weight was obtained by averaging 1000 seed weight of four samples for each plot.
Seed quality parameters: nitrogen, phosphorus, potassium, zinc, and iron content
Seed nitrogen content was determined by the standard Kjeldahl method (Hanon Auto Kjeldahl Distiller, K9840, Germany). Sulphuric acid and a catalyst mixture of copper sulphate and potassium sulphate were used to digest the sample (0.3 g dry sample). The nitrogen content was expressed as a percentage (Andrews and Newman, Reference Andrews and Newman1968). To determine the content of phosphorus, potassium, zinc and iron, the method of digesting the plant sample (0.3 g of dry sample) with mixed acid (6 g of salicylic acid, 100 ml of 98% sulphuric acid and 18 ml of distilled water) was used (Walinga et al., Reference Walinga, Van Der Lee, Houba, Van Vark, Novozamsky, Walinga, Van Der Lee, Houba, Van Vark and Novozamsky1995). The amount of potassium in the obtained extract was read by the method of flame measurement (flame photometry) using a flame photometer (Jenway, model PFP7/C, UK) and expressed as a percentage (Soloman et al., Reference Soloman, Geldalovich, Mayer and Poljakoff1986). The amount of phosphorus was measured by the yellow calorimetric method using ammonium vanadate molybdate reagent with a spectrophotometer (PerkinElmer, Lambada 25, USA) at a wavelength of 470 nm and reported as a percentage (Chapman and Pratt, Reference Chapman and Pratt1961). Also, the concentration of iron and zinc elements was measured in the obtained extract using an atomic absorption spectrometer (Varian 220 AA, Australia) and the final concentration was determined.
Statistical analysis
A combined analysis of variance was carried out for the split-block experiment in a basic design of randomized complete blocks over two years in all traits. The data were checked for normality before statistical analysis. Statistical analysis and mean comparison were done using SAS 9.1 program. Data mean comparisons were performed by the least significant difference test (LSD, P ≤ 0.05). The linear regression between seed yield and the introduction year of cultivars and between LAI and seed yield was fitted in both conditions of non-application and application of zinc sulphate.
Results
The results of ANOVA showed that LAI, FLA, chlorophyll content index, plant height, spike length, biological yield, seed yield, HI, 1000 seed weight, number of seeds per spike, and amount of nitrogen, phosphorus, potassium, and iron in seed were significantly affected by the simple effects of year, zinc sulphate, and cultivar, but the amount of zinc in wheat seed was only affected by the simple effect of cultivar (Table 3).
Table 3. Analysis of variance of zinc sulphate treatment and cultivar on the studied traits of wheat in two cropping years
*, P ≤ 0.05; **, P ≤ 0.01; ns, not significant.
LAI and FLA
In the first year (2018), the highest LAI was associated with Sirvan, Alamoot, Bayat, Falat, Chanab and Khalij cultivars (Table 4). In the second year (2019), the highest LAI was observed in Sirvan, Alamoot, Moghan2 and Moghan1 cultivars. The lowest LAI in the first year was observed in cultivar Pishtaz. In the second year, Karaj2 and Golestan showed the lowest LAI (Table 4). In general, the highest LAI was found in the high yielding cultivars in both years and under either zinc sulphate application or no application conditions (Tables 4 and 5; and Fig. 3). In both years, Falat and Moghan3 cultivars had the highest and lowest FLA, respectively. When comparing the two years, the highest FLA (34.7 cm2) was associated with cultivar Falat in the first year (Table 4). In all cultivars, the values of LAI and FLA showed a decremental trend in the second year compared to the first year (Table 4). Zinc sulphate application increased LAI and FLA in all wheat cultivars compared to the non-application conditions (Table 5). The highest values of LAI and FLA were obtained with the application of zinc sulphate in cultivar Falat. The lowest LAI and FLA were observed in Chenab and Moghan3 cultivars, respectively, under the non-application of zinc sulphate condition (Table 5). Linear regressions between seed yield and years of experiment showed that leaf area was higher in the first year than in the second year and the highest LAI was above 5 in the first year. This increase in LAI was achieved with the increase in seed yield and the maximum seed yield was over seven t/ha. But in the second year as a hot and dry year, LAI decreased in all cultivars and the maximum LAI was near to 4.7, therefore the maximum seed yield was reduced to near to 6.4 t/ha (Figs 3(a) and (b)). A similar trend was found between LAI and zinc sulphate application condition (Figs 3(c) and (d)). In fact, the application of zinc sulphate increased the LAI values, and in high yielding varieties, LAI showed a greater increase than for other varieties.
Table 4. Mean comparison of the leaf area index, flag leaf area, chlorophyll index, plant height, spike length, biological yield, and seed yield of different wheat cultivars in two cropping years
Table 5. Mean comparison of the leaf area index, flag leaf area, chlorophyll index, plant height, spike length, biological yield, and seed yield in different wheat cultivars was affected by zinc sulphate treatment
Chlorophyll content index (CCI)
Golestan and Khalij cultivars had the highest CCI in the first year, and in the second year, Khalij again had the highest CCI compared to other cultivars. The lowest CCI was observed in Moghan2 in the first year and in Moghan3 in the second year. In the second year, the CCI decreased in all the varieties studied except Toos (Table 4). The application of zinc sulphate increased the CCI values in the wheat cultivars compared to the non-application of zinc sulphate (Table 5).
Plant height and spike length
In the first year, the highest plant height belonged to the cultivars Inia (99 cm) and Khalij (98 cm) (Table 4). In the second year, Karaj2 (91 cm) showed the highest height. The lowest height was associated with the cultivar Bayat in both years (Table 4). In the second year, the height of Moghan3, Alamoot, Sirvan, Chenab, Navid, Karaj2, Karaj3 and Hirmand cultivars increased compared to the first year, while it decreased in other cultivars (Table 4). Plant height of cultivars increased with zinc sulphate application (Table 5).
In the first year, the spike length of Moghan1, Kouhdasht, Golestan and Sirvan was higher compared to the other varieties, among which Moghan1 had the longest spike. In the second year, the highest spike length was observed in Toos, Kouhdasht and Golestan cultivars. The lowest spike length was obtained in the first year in cultivar Falat and in the second year in cultivar Seymareh (Table 4). Spike length decreased in the second year (Table 4). Application of zinc sulphate increased the spike length in all wheat cultivars compared to non-application of zinc sulphate (Table 5). The maximum spike length was obtained in cultivar Kouhdasht with zinc sulphate application. The lowest spike length was obtained in Seymareh and Falat cultivars in non-application of zinc sulphate condition (Table 5).
Biological yield, seed yield, and HI
In both years, Sirvan had the highest biological yield, which was not significantly different from Alamoot and Moghan2. In both years, Pishtaz showed lower biological yield than other varieties (Table 4). The biological yield of all cultivars in the second year showed a decrease of about 3.5 to 7% compared to the first year (Table 4). The results of the interaction of zinc sulphate and cultivar showed that under no zinc sulphate application conditions, the highest and lowest biological yields were associated with Alamoot and Moghan3 cultivars, respectively. While under zinc sulphate application conditions, the highest and lowest values were observed in Sirvan and Karaj3 cultivars, respectively (Table 5).
In both years, Moghan2 had the highest seed yield compared to other varieties and there was no significant difference with Moghan1 variety. In contrast, Pishtaz had the lowest seed yield in both years (Table 4). Seed yield in the second year was about 8–16% lower than in the first year (Table 4). Zinc sulphate application resulted in an increase in seed yield in all wheat cultivars. The highest rate of increase (118.5%) was observed in cultivar Sirvan (Table 5).
The highest HI in both years was found in Moghan1 and had no significant difference from Moghan3 and Falat cultivars. On the other hand, the lowest HI was obtained from Khazar1 cultivar, which had no significant difference from Pishtaz cultivar in both years. In the second year, the HI of all cultivars showed a decreasing trend compared to the first year (Table 6). In general and with some exceptions, zinc sulphate application showed an incremental effect on HI (Table 7). The highest HI was related to the cultivar Moghan3 with the application of zinc sulphate and the lowest value was obtained in the cultivar Khazar1 under non-application of zinc sulphate (Table 7).
Table 6. Mean comparison of harvest index, 1000-seed weight, number of seeds per spike, and seed N, P, K, Zn, and Fe contents of different wheat cultivars in two cropping years
Table 7. Mean comparison of harvest index, 1000-seed weight, number of seeds per spike, and seed N, P, K, Zn, and Fe contents in different wheat cultivars was affected by zinc sulphate treatment
The number of seeds per spike and the 1000-seed weight
In both years, the maximum number of seeds per spike was obtained in Moghan1 and Seymareh cultivars and the lowest value without significant differences was found in Pishtaz and Falat cultivars in both years. In all cultivars, the number of seeds per spike decreased in the second year compared to the first year (Table 6). Application of zinc sulphate increased the number of seeds per spike by 2–20% in the cultivars studied (Table 7).
In the first year, Khalij and then Sirvan cultivars had more 1000 seed weight compared to other cultivars, while in the second year, higher 1000 seed weight was obtained in Sirvan, Kouhdasht and Seymareh cultivars, respectively. On the other hand, the lowest 1000 seed weight in the first year was associated with Karaj3 cultivar, while in the second year Toos showed the lowest 1000 seed weight and had no significant differences with Alamoot and Karaj3 cultivars (Table 6). In all cultivars studied, the 1000 seed weight decreased by 4–40% in the second year compared to the first year (Table 6), but the seed weight increased with zinc sulphate application in all cultivars (Table 7). The highest 1000-seed weight was related to the variety Sirvan with zinc sulphate application and the lowest value was obtained in the variety Karaj3 without zinc sulphate application.
Seed nitrogen, phosphorus, potassium, zinc and iron content
The results of elemental analysis of seeds showed that the amount of these elements was higher in Karaj2 cultivar than in other cultivars in both years. In the first year, the maximum amount of zinc in wheat grains was associated with Karaj3 and Karaj2 cultivars, and in the second year it was associated with Karaj2 cultivar (Table 6). In both years, the lowest amount of seed nitrogen was obtained in Khalij and the lowest content of phosphorus and zinc was found in Falat cultivar. Also, Hirmand, Chenab and Khazar1 cultivars showed the lowest amount of potassium and iron content in both years (Table 6). In all cultivars, the amount of nitrogen, phosphorus, potassium, zinc and iron decreased in the second year compared to the first year (Table 6). Zinc sulphate application caused an increase in seed nitrogen content in cultivars Falat, Moghan1, Moghan2, Sirvan, and Karaj2, while there was no change in cultivar Moghan3 and a tendency to decrease in other wheat cultivars (Table 7). The highest amount of seed phosphorus was associated with the Karaj2 cultivar under zinc sulphate application, and the lowest amount was obtained in the Falat cultivar without zinc sulphate application conditions (Table 7). Zinc sulphate application in all wheat cultivars except Pishtaz, Bayat, Moghan3 and Golestan cultivars had a positive effect on the amount of potassium and iron elements (Table 7). The interaction effect of zinc and cultivar on the seed zinc content was significant. This shows wheat cultivars' different responses to zinc sulphate application in terms of seed zinc content. Surprisingly, application of zinc sulphate caused a decrease in seed zinc content in Bayat, Toos, Inia, Kouhdasht and Golestan cultivars compared to non-application of zinc sulphate, while there was no change in Khalij, Karaj3 and Hirmand cultivars and an increasing trend in other wheat cultivars, Moghan1, Karaj2, Moghan2, Chenab, Falat, Navid, Seymareh, Zagros, Pishtaz, and Moghan3 (Table 7). This could be due to different genetic potential in wheat cultivars. In general, the highest amount of nitrogen, phosphorus, potassium, zinc and iron in seeds was obtained in cultivar Karaj2 with zinc sulphate application and their contents were increased compared to the non-application of zinc sulphate (Table 7).
The trend of seed yield during the years of introduction
Linear regressions were fitted between seed yield and cultivar release year to evaluate the trend of seed yield of cultivars during the release years. The linear regression in zinc sulphate-free conditions showed that seed yield decreased significantly from about 5.5 to about 4.2 t/ha for cultivars released from 1960 to 2006. In fact, newer cultivars (e.g., Sirvan and Moghan3) had lower seed yields and older cultivars (e.g., Khalij and Inia) had higher seed yields under low zinc soil conditions. Under these conditions, the estimated reduction was 28 kg/ha per year (Fig. 2(a)). In contrast, the fitted linear regression between seed yield of wheat cultivars and year of cultivar adoption under zinc sulphate application conditions showed that seed yield increased linearly from about 6 t/ha to about 6.8 t/ha during the 45 years of cultivar adoption. In fact, under zinc sulphate application, relatively new varieties (Sirvan and Moghan3) had higher seed yields than older varieties (Khalij and Inia). Under these conditions, the estimated increase was 18 kg/ha per year (Fig. 2(b)).
Figure 2. Relationship between seed yield production of 21 wheat cultivars and year of cultivar release from 1960 to 2006 in the conditions without (a) and with (b) zinc sulphate application.
Discussion
After the Green Revolution, wheat yield showed a dramatic increase, half of which was due to genetic improvement and the other half to appropriate agronomic practices (del Pozo et al., Reference del Pozo, Yáñez, Matus, Tapia, Castillo, Sanchez-Jardón and Araus2016). In addition to genetic differences between wheat cultivars, which can also show significant variations in productivity potential, wheat growth can be affected by several environmental factors. Changes in growth conditions, such as nutrient application, can have a significant effect on plant growth and performance (Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). Our results indicated high genetic diversity among wheat cultivars in terms of productivity potential and seed element concentrations (Tables 4–7), which was consistent with some reports. For example, significant differences were reported among wheat cultivars in terms of plant height, spike length, biological and seed yield (Ram et al., Reference Ram, Rashid, Zhang, Duarte, Phattarakul, Simunji, Kalayci, Freitas, Rerkasem, Bal, Mahmood, Savasli, Lungu, Wang, de Barros, Malik, Arisoy, Guo, Sohu, Zou and Cakmak2016; Sharifi-soltani et al., Reference Sharifi-soltani, Alavi-Kia and Shariatipour2016; Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022), and seed zinc and iron content (Sharifi-soltani et al., Reference Sharifi-soltani, Alavi-Kia and Shariatipour2016; Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017; Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022).
Meteorological data showed that there were differences in air temperature and rainfall in the two years of the experiment (Fig. 1).The lower rainfall and higher average temperature in the second year compared to the first year caused a decrease in plant height, leaf area, CCI, seed element content, spike length and yield components (Tables 4 and 6). Also, the reduction in HI in the second year compared to the first year (Table 6) indicates a deeper effect of adverse environmental factors on seed yield compared to biological yield. The negative effect of adverse environmental conditions such as decrease in soil moisture and rainfall on leaf characteristics such as area, dimension and number, chlorophyll content, plant height, yield components, seed yield and HI has been reported (Fahad et al., Reference Fahad, Bajwa, Nazir, Anjum, Farooq, Zohaib, Sadia, Nasim, Adkins, Saud, Ihsan, Alharby, Wu, Wang and Huang2017; Ramazan et al., Reference Ramazan, Hafeez, Khan, Nadeem, Rahman, Batool and Ahmad2020; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021; Mannan et al., Reference Mannan, Tithi, Islam, Al Mamun, Mia, Rahman, Awad, ElSayed, Mansour and Hossain2022). It seems that during the second year, warmer and drier conditions induced heat and drought stress in the plants. Also, seed quality was affected by adverse conditions in the second year and seed elemental content was reduced compared to the first year (Table 7). An increase in air temperature with a decrease in soil moisture content in spring and in the pre-anthesis stage of wheat can cause a decrease in genetic expression to produce more seeds per unit area and a decrease in seed yield (del Pozo et al., Reference del Pozo, Yáñez, Matus, Tapia, Castillo, Sanchez-Jardón and Araus2016). Heat and drought stress have been reported to decrease the uptake of minerals from the soil and disrupt plant functions (Giri et al., Reference Giri, Heckathorn, Mishra and Krause2017). Fahad et al. (Reference Fahad, Bajwa, Nazir, Anjum, Farooq, Zohaib, Sadia, Nasim, Adkins, Saud, Ihsan, Alharby, Wu, Wang and Huang2017) also stated that drought and heat stress affect the cycling, absorption, and availability of nutrients to plants. As shown in Fig. 1, after anthesis in the second year, the temperature increased sharply and perhaps there was not enough opportunity for remobilization of elements from the secondary source organs to the seeds. This may explain the decrease in the concentration of elements in the second year compared to the first year.
Zinc application can affect plant functions and performance. For example, zinc application in rice and wheat increased chlorophyll and carotenoid content, improved the activity of phosphoenolpyruvate carboxylase and ribulose bisphosphate carboxylase enzymes, and increased the efficiency of macronutrient use (Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021; Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021). Zinc sulphate application (foliar and soil) also increased the number and area of leaves and CCI in wheat (Ma et al., Reference Ma, Sun, Wang, Ding, Qin, Hou, Huang, Xie and Guo2017; Mannan et al., Reference Mannan, Tithi, Islam, Al Mamun, Mia, Rahman, Awad, ElSayed, Mansour and Hossain2022). The increase in leaf area may be due to the effect of zinc element on auxin biosynthesis, cell division and elongation, and the increase in meristem activity (Taiz and Zeiger, Reference Taiz and Zeiger2003). The results of the present research on the increase in CCI, LAI and FLA of wheat cultivars with zinc application were in accordance with the results of the above studies. The linear relationship between LAI and seed yield (Figs 3(a)–(d)) shows that more increase in seed yield is possible by selecting for higher LAI. On the other hand, zinc has an indirect effect on chlorophyll content, so it can have a positive effect on iron uptake, which is necessary for chlorophyll biosynthesis, and increase the chlorophyll content (Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). Also, it can be due to the positive role of zinc in chlorophyll synthesis and the enhancement of photosynthesis, which increases the CO2 assimilation and, increases the protein and carbohydrate production, and finally, leaf area enlargement and plant growth (Sharma et al., Reference Sharma, Patni, Shankhdhar and Shankhdhar2013; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021).
Figure 3. Relation between seed yield and LAI at the flowering stage of 21 wheat cultivars under non-application of zinc sulphate in 2018 (a) and 2019 (b); and with application of zinc sulphate in 2018 (c) and 2019 (d).
Zinc is an essential element and plays many roles in plant functions such as height increment, biomass accumulation, fertilization and pollen viability (Taiz and Zeiger, Reference Taiz and Zeiger2003). The increase in spike length of wheat with soil and foliar application of zinc (Mannan et al., Reference Mannan, Tithi, Islam, Al Mamun, Mia, Rahman, Awad, ElSayed, Mansour and Hossain2022) and height of rice with application of different sources of zinc (Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021) have been reported. Zinc also significantly increased the number of spikelets per spike, number of seeds per spike, and 1000 seed weight in different wheat cultivars (Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021; Mannan et al., Reference Mannan, Tithi, Islam, Al Mamun, Mia, Rahman, Awad, ElSayed, Mansour and Hossain2022). The results of our study regarding the positive effect of zinc on plant height and yield components were consistent with the above reports. There was a direct and positive relationship between the number of spikelets per spike and the number of seeds per spike. Thus, the number of seeds per spike increased with the increase in the number of spikelets per spike. Similarly, zinc can increase seed formation and the number of seeds through the effects on pollination and fertilization and also by increasing the number of spikelets per spike (Taiz and Zeiger, Reference Taiz and Zeiger2003; Panday et al., Reference Panday, Pathak and Sharma2006; Afzal et al., Reference Afzal, Ibni Zamir, Ud Din, Bilal, Salahuddin and Khan2017). Elshayb et al. (Reference Elshayb, Farroh, Amin and Atta2021) stated that the increase in seed number and 1000 seed weight in rice may be related to the role of zinc in improving chlorophyll content and enzyme activity, which stimulate photosynthetic rate and photoassimilate translocation to seeds. In addition, zinc affects the uptake of other elements and enhances the remobilization of photoassmilates from the secondary sources to the sink (Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021; Mannan et al., Reference Mannan, Tithi, Islam, Al Mamun, Mia, Rahman, Awad, ElSayed, Mansour and Hossain2022). Environmental stresses such as high temperature can occur and damage wheat plants during growth stages such as reproductive and seed filling stages (Li et al., Reference Li, Wang, Tian, Li, Chen, Jia, Liu and Zhao2016). Zinc application by increasing heat tolerance can protect the photosynthetic system and improve crop yield under heat and moisture deficit conditions and may affect seed growth (Chattha et al., Reference Chattha, Hassan, Khan, Chattha, Mahmood, Chattha, Nawaz, Subhani, Kharal and Khan2017; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021).
The effects of zinc application on crop yield have been reported previously. For example, Sharifi-soltani et al. (Reference Sharifi-soltani, Alavi-Kia and Shariatipour2016) reported that the application of zinc increased the seed yield of wheat genotypes by 16%. Maleki et al. (Reference Maleki, Fazel, Naseri, Rezaei and Heydari2014) and Ramazan et al. (Reference Ramazan, Hafeez, Khan, Nadeem, Rahman, Batool and Ahmad2020) observed a significant increase in HI of maize and wheat with zinc sulphate application. Our results regarding the increase in biological yield, seed yield, and HI with zinc application were consistent with the results of the above studies. Since zinc is an important component of many enzymes and can increase chlorophyll content and photosynthetic rate, it can affect growth and seed yield (Sharma et al., Reference Sharma, Patni, Shankhdhar and Shankhdhar2013; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021; Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021).
In our research, zinc sulphate application affected all measured seed element contents. Jaksomsak et al. (Reference Jaksomsak, Tuiwong, Rerkasem, Guild, Palmer, Stangoulis and Prom-U-Thai2018) found that zinc application increased zinc content in seeds of rice cultivars. Our study showed that in some cultivars, zinc content increased with zinc application. Sharifi-soltani et al. (Reference Sharifi-soltani, Alavi-Kia and Shariatipour2016) reported the effects of zinc application on wheat genotypes and concluded that the amount of zinc in seeds increased by 18% compared to no zinc application. They stated that zinc application decreased the zinc content of seeds in one cultivar, while it increased it in other cultivars. They also stated that seed iron content was increased by zinc treatment in most cultivars, but decreased in one cultivar (Sharifi-soltani et al., Reference Sharifi-soltani, Alavi-Kia and Shariatipour2016). Similarly, Sher et al. (Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022) stated that the response of wheat cultivars to zinc application can be highly variable, indicating genetic differences in the uptake potential of the elements. For example, one of the cultivars studied had the highest amount of zinc in the seed under zinc application and non-application conditions. The effect of the zinc fertilization on the efficiency of the fertilizer and increasing the zinc concentration of the seeds depends on the cultivar, the type of used fertilizer, and the fertilization time can be different (Hidoto et al., Reference Hidoto, Worku, Mohammed and Taran2017). It has been observed that if the application of zinc is done in the final or near to final growth stages of wheat, its effect on zinc concentration of seeds will be greater. On the contrary, fertilization in the early stages of plant life will be effective on growth and yield rather than seed quality (Mutambu et al., Reference Mutambu, Kihara, Mucheru-Muna, Bolo and Kinyua2023). However, there are no similar results from different researches (Cakmak, Reference Cakmak2008).
In the present study, zinc application showed a decremental effect on phosphorus content of wheat seeds in some cultivars such as Pishtaz, Bayat and Toos (Table 6). This finding may indicate a negative correlation between zinc and phosphorus uptake, which is known by the antagonistic effect of zinc and phosphorus (Bostick et al., Reference Bostick, Hansel, la Force and Fendorf2001). Our results also indicated that zinc plays a role in the accumulation of the high concentration of potassium in wheat seeds. It has been noted that zinc application can also affect macroelement uptake and lead to an increase in nutrient use efficiency of macro- and microelements (Sher et al., Reference Sher, Sarwar, Sattar, Ijaz, Ul-Allah, Hayat, Manaf, Qayyum, Zaheer, Iqbal, Askary, Gharib, Ismail and Elesawy2022). In wheat, foliar application of zinc increased potassium, zinc and iron contents in shoots and seeds. As the concentration of zinc in the shoot increased, the remobilization and accumulation of zinc in the seed also increased (Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). In rice, zinc application increased the amount of nitrogen, potassium, and zinc in seeds and decreased the amount of phosphorus in seeds (Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021). Adequate zinc nutrition has been shown to be important in controlling potassium and iron accumulation by leaves and seeds (Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). This increase in potassium with zinc application may be due to the synergistic relationship between zinc and potassium, resulting in greater availability of potassium and an increase in potassium flux from the root and shoot to the seed (Elshayb et al., Reference Elshayb, Farroh, Amin and Atta2021). A positive relationship between seed zinc and iron concentrations in cereals has been reported (Xia et al., Reference Xia, Kong, Wang, Xue, Liu, Zhang, Yang and Li2019; Anwar et al., Reference Anwar, Khalilzadeh, Khan, un-Nisa, Bashir, Pirzad and Malik2021). The accumulation of zinc and iron in seeds may be due to pleiotropic effects or linkage between genes responsible for zinc and iron accumulation in seeds (Xia et al., Reference Xia, Kong, Wang, Xue, Liu, Zhang, Yang and Li2019).
Under non-application of zinc sulphate, the highest seed yield (8.3 t/ha) was obtained in cultivars introduced from 1971 to 1976 years, indicating the high potential yield of old cultivars in soils with lower available zinc and/or without applying zinc sulphate (Fig. 2(a)). On the other hand, with the application of zinc sulphate, the highest seed yield (10.1 t/ha) was observed in the cultivars introduced in the last years, which indicates the positive response and high yield of the newer cultivars with the application of zinc sulphate and the dependence of the newer cultivars on more fertilizers (Fig. 2(b)). It seems that the performance of newer cultivars is more sensitive to conditions that reduce leaf area and photosynthesis rate than older cultivars. This difference explains the yield gap between new and old cultivars in fertile and less fertile environments. del Pozo et al. (Reference del Pozo, Jobet, Matus, Méndez-Espinoza, Garriga, Castillo and Elazab2022) by studying the regression analysis of seed yield in 25 wheat cultivars and advanced lines introduced from 1959 to 2017 years determined that the increment amount of seed yield between these years was equal to 128.8 kg/ha per year. In another study, durum wheat seed yield showed a positive and linear relationship with the year of cultivar introduction between 1964 and 2010 (del Pozo et al., Reference del Pozo, Matus, Ruf, Castillo, Méndez-Espinoza and Serret2019). Senapati and Semenov (Reference Senapati and Semenov2019) also reported an increase in wheat yield from 3.8 mg ha−1 in the 1980s to 5.7 mg ha−1 after 2010. In Mexico, the seed yield improvement for spring bread wheat cultivars was found to be 30 kg/ha per year (0.59%) from 1966 to 2019 (Aisawi et al., Reference Aisawi, Reynolds, Singh and Foulkes2015). However, in our study, there was a positive and linear relationship between seed yield and release year of wheat cultivar only in the conditions of zinc sulphate application, and in contrast, a negative and linear relationship between seed yield and release year of cultivar was observed in the conditions without zinc sulphate application. It seems that the breeding of new cultivars was done in research stations that had regular annual fertilization and soils with suitable fertility. Therefore, newer cultivars are highly dependent on the application of fertilizers such as zinc sulphate to increase seed yield.
Conclusion
Our results showed that zinc sulphate improved growth, seed yield and yield components. Cultivars had significant differences in response to zinc sulphate. In non-application of zinc sulphate, Moghan2 and Moghan1 cultivars had the highest seed yield, while in application of zinc sulphate, Sirvan, Alamoot and Moghan2 can be introduced as high yielding cultivars. Among the cultivars in both years and under application or non-application of zinc sulphate, Moghan2 showed priority over the other cultivars. In terms of seed quality and nutritional value, Karaj2 had higher elemental concentrations under zinc sulphate application than others. Our data show that zinc sulphate application not only affects seed yield and quality, but also reduces the effect of environmental stress in hot and dry years. The results showed that high-yielding varieties had higher LAI values at flowering. It appears that maintaining greenness at the flowering stage can increase seed yield in wheat cultivars.
Authors’ contributions
PQ, FS and KA conceived and designed the study. AT, RS and KS conducted the field experiments and laboratory analysis. FS and AM performed the statistical analyses. FS and PQ wrote the manuscript. FS and AM reviewed the final manuscript.
Funding statement
This research received no specific grants from any funding agency, commercial sector, or not-for-profit sector.
Competing interest
None.
Ethical standards
Not applicable.
Open access
