3.1 Phosphorus and Zn concentrations in wheat grain
At harvest, the average concentrations of P (mg kg− 1) in wheat grain ranged from 2890 ± 694 to 3630 ± 490 for PIR and BAR locations respectively; there was a significant variation (p < 0.05) between locations (Fig. 1). About 40% of wheat grain samples contained higher concentrations than the recommended value of 3570 mg kg− 1 of P (USDA Food Composition Databases, 2018), reflecting the high solubility of P in some of the soils probably due to the application of P fertilizer in each growing season by farmers. Alam and Azam Shah (2002) conducted a greenhouse experiment to study the influence of phosphate fertilization on P concentration in wheat gown in calcareous soils; they found that application of 40 kg kg− 1 of P as SSP increased P concentration in wheat grain from 1978 to 2942 mg kg− 1.
The average Zn concentration in wheat grain (mg kg− 1) ranged from 26.3 ± 7.37 to 35.0 ± 6.11 for CHA and HAL locations respectively (Fig. 2). Similarly, Zn concentrations in bread wheat ranged from 20 to 40 mg kg− 1 in a study of 243 wheat genotypes cultivated in normal agricultural soils (Nikolic et al., 2016). Zhang et al. (2010) reported 29 mg Zn kg− 1 for three main wheat cultivars, grown in calcareous soils in China which did not exhibit Zn deficiency. Liu et al. (2014) analysed 655 wheat grain samples grown in the calcareous soils of China from 2009 to 2011, they found that average Zn concentrations were 30.4 and 30.3 mg kg− 1 for winter and spring wheat respectively. Guttieri et al. (2015) analysed 286 bread wheat cultivars in the USA and observed Zn concentrations ranging from 25 to 33 mg kg− 1. Compared to these extensive reports, the average value of between 26.3 to 35.0 mg kg− 1, suggest Zn in wheat grain grown in Kurdistan is in the range of Zn concentrations reported globally for wheat grown in calcareous soils.
Approximately 88% of the surveyed wheat grain samples contained Zn concentrations below the value recommended for human nutrition of 41.6 mg kg− 1 by the USDA (USDA Food Composition Databases, 2018). According to Cakmak (2008) for populations with a predominantly cereal-based diet, the Zn concentration in wheat grain should be as high as 40–60 mg kg− 1 to provide an adequate level in the diet. Thus, the wheat grain grown in the calcareous soils of Kurdistan falls in the lower range of values recommended for adequate human nutrition. This situation arises because of the low availability of Zn in the Kurdistan soils and because farmers do not normally apply micronutrients to wheat farms in the Kurdistan region when interviewed farmers. Moreover, Nikolic et al. (2016) studied the soil Zn availability and wheat grain Zn status in Serbia and found that wheat grain Zn content was negatively correlated with the application of phosphate fertilizer in calcareous soils. However, in the surveyed wheat grain samples, the concentration of Zn was not negatively correlated with Olsen-P or with the concentration of P in wheat grains. This may be because farmers apply N (as Urea) along with P fertilizers (as TSP and DAP) each growing season which could offset the effect of P on Zn concentration in wheat grains due to the synergistic effect of applied N on Zn uptake by wheat plants. Akram et al. (2017) reported that nitrogenous protein has a major role in Zn uptake from soil to root, mobilization within the plant and accumulation. Svecnjaka et al. (2013) studied the effect of N fertilizer application on trace element uptake by wheat grain in a field experiment; they found that application of 194 kg N ha− 1 as (Urea 46% N) increased Zn concentration from 34.9 to 38.9 mg kg− 1 in wheat grain. Thus, the application of N fertilizer alongside P fertilizer to wheat grown in the calcareous soils of the Kurdistan region may reduce the antagonistic effect of P on Zn uptake.
The principal reason for low concentrations of Zn in wheat grain is probably the presence of large concentrations of calcium carbonate in the soils and high pH values. These are the conditions under which Zn fixation occurs, rendering it unavailable for plant uptake (Cakmak and Kutman, 2017; Li et al., 2010). It is well documented that wheat cultivated in calcareous soils can contain relatively low concentrations of Zn. Thus the application of a Zn biofortification plan could make a substantial contribution to raising the Zn content of wheat in Kurdistan. On the other hand, a critical Zn level of 20 to 24 mg kg− 1 has been suggested (Nikolic et al., 2016) for rain-fed wheat grain grown on alkaline calcareous soils in Pakistan as the lowest Zn concentration in grain needed to deliver 95% of the highest grain yield (Karami et al., 2009). In the present survey, only 12% of surveyed samples had Zn concentrations below 24 mg kg− 1 dry matter, suggesting that Zn deficiency to the crop may actually be a minor consideration for wheat production in the Kurdistan region, compared to the human dietary considerations.
3.2 Phytic acid and PA/Zn molar ratio in wheat grain
Figure 3 presents PA concentrations (g 100g− 1) in wheat grain samples; significant differences (p < 0.05) were found between the ten sampling locations. In the present survey, the average PA content in the whole wheat grains ranged from 0.54 ± 0.16 for HAL to 0.93 ± 0.11 for SSQ locations respectively. An average of 63.1 ± 12.2% of P in the wheat grain was present as PA. Results are compared with other examples in (Table 2) which shows that the Kurdistan PA concentrations fell within the range of other countries.
Table 2
Comparison of Phytic acid content in wheat grain from several countries
Phytic acid
|
Wheat
|
Country
|
References
|
(g 100g− 1)
|
No.
|
|
|
0.42–1.12
|
120
|
Kurdistan region
|
Current study
|
0.46–0.95
|
46
|
Mexico
|
Magallanes-Lopez et al., (2017)
|
1.46–1.69
|
15
|
Serbia
|
Brankovic et al., (2015)
|
0.71–1.11
|
65
|
Pakistan
|
Hussain et al., (2012)
|
0.52–0.98
|
186
|
China
|
Liu et al., (2006)
|
0.60-1.00
|
100
|
Canary Islands
|
Febles et al., (2002)
|
The regional average PA/Zn molar ratio for whole wheat grain ranged between 15.7 ± 5.06 and 30.6 ± 6.18 for HAL and BAR locations respectively (a 2-fold variation) as shown in (Fig. 4); a significant difference was found between locations (p < 0.05). Hussain et al. (2012) and Tavajjoh et al. (2011) reported PA/Zn molar ratios ranging from 23.9 to 41.4 and 26.5 to 26.9 in 65 and 17 bread wheat and bread wheat genotypes grown in the calcareous soils of Pakistan and Iran respectively. Those PA/Zn molar ratios are broadly comparable to the current survey.
Reasons for differences in PA and PA/Zn ratios may include location, soil type, variety, climatic conditions, phosphate fertilizer application and the presence or absence of Zn fertilization. Magallanes-Lopez et al. (2017) studied PA content and PA/Zn molar ratio in a worldwide collection of commercial durum wheat and found that environmental factors caused a difference in wheat grain PA concentration between 0.46 and 0.95 g 100g− 1. Erdal et al. (2002) measured PA in twenty wheat cultivars grown with, and without, Soil application of Zn to calcareous soils in 55 different wheat growing locations in Central Anatolia, Turkey. They reported that the application of 23 kg Zn ha− 1 to soils reduced the PA contents of wheat grain from 1.07 to 0.91 g 100g− 1 and PA/Zn molar ratio from 126 to 55. These results were due to a reduction in P concentration in wheat grain from 3900 to 3500 mg kg− 1 and increasing Zn concentration from 14 to 23 mg kg− 1. Probably in the calcareous soils of Kurdistan, P deficiency occurs due to high calcium content. Therefore, when samples collected and farmers interviewed. They apply phosphate fertilizer to improve yield which causes an increase in PA content in wheat grain and in the resulting PA/Zn ratio.
The PA/Zn molar ratio has been used as an indicator of Zn bioaccessibility by the International Zinc Nutrition Consultative Group (IZiNCG). Considering the PA/Zn molar ratios observed in the current survey, and in previous studies on calcareous soils, it is apparent that ratios are not ideal when durum wheat is the staple food and principal source of dietary Zn. The relatively high PA/Zn molar ratio in the current survey (average of 24.3 ± 8.59) suggests that wheat biofortification with Zn and decreasing the application of P fertilizer are urgent priorities in the Kurdistan region. Developing cereal cultivars with low PA contents and a high affinity for Zn would also be useful strategies.
3.3 Estimated Zn bioavailability in wheat grain
Estimated Zn bioavailability (Eq. 3), in mg d− 1 assuming consumption of 300 g wheat per day, ranged from 1.84 ± 0.08 for BAR to 2.65 ± 0.05 for HAL locations respectively, as shown in (Table 3). A significant difference (p < 0.05) was found between locations. The results indicate that in all the wheat grain samples, bioavailable Zn is lower than the optimum level of 3 mg from daily consumption of 300 g of wheat (Hussain et al., 2013) for communities dependent on wheat grain for Zn intake. Maqsood et al. (2014) estimated Zn bioavailability in 58 wheat cultivars grown in calcareous soils in the wheat-cotton zone of the Punjab, Pakistan, using a trivariate model. They reported that bioavailable Zn ranged from 0.8 to 2.4 mg 300 g− 1 with an average value of 1.5. Hussain et al. (2012) also analysed 65 wheat grains grown in calcareous soils of Pakistan and found that estimated Zn bioavailability ranged from 1.52 to 2.25 mg 300g− 1. Thus, the current Kurdistan survey shows that the estimated bioavailable Zn in wheat grain falls within the range of Zn bioavailability reported globally for wheat grown in calcareous soils.
On average, about 23.3 ± 3.48% of grain Zn was actually bioavailable. Low bioavailability of Zn in the wheat was probably due to high PA/Zn molar ratio caused by a high concentration of P in wheat grains and low Zn concentrations. Li et al. (2015) conducted a two-year field experiment to investigate the effects of P and Zn fertilization on Zn bioavailability in wheat grain. They found that estimated Zn bioavailability in whole grain was greater in the Zn-alone treatments than in combined P + Zn treatments.
Table 3
Estimated Zn bioavailability in mg d-1 for consumption of 300 g wheat grain (Eq. 3). Samples were collected from ten different locations (Table 1) in Sulaimanyah province.
Location
|
SHA
|
KHO
|
BAR
|
HAL
|
SSQ
|
SIR
|
DAR
|
PIR
|
KAL
|
CHA
|
Average
|
2.13
|
2.19
|
1.84
|
2.63
|
2.04
|
2.45
|
2.06
|
2.10
|
2.30
|
2.05
|
SD
|
0.06
|
0.11
|
0.08
|
0.05
|
0.04
|
0.10
|
0.07
|
0.10
|
0.06
|
0.08
|
Figure 5 shows the relationship between estimated bioavailable Zn and measured PA concentration for all the surveyed wheat grains (whole wheat flour), as determined by the trivariate models used. According to the data and calculations, the estimated bioavailable Zn decreased by 19.7%, when the concentration of PA intake increased from 885.6 to 3526 mg 300g− 1 in wheat grain (using the full range of measured PA data). Khoshgoftarmanesh et al. (2017) and many other studies have reported that dietary Zn intake decreases with increasing PA concentration. This present survey indicates that Zn bioavailability in wheat from all regions of Kurdistan, estimated from the trivariate models, is sub-optimum for human health.