Nutrition is crucial factor in reduction of hunger, malnutrition and obesity [1]. Human body requires more than 22 mineral elements, which can be provided by adequate diet. On the other hand, nutritional deficiencies (e.g. in iron, zinc, vitamin A) account for almost two-thirds of the childhood deaths worldwide [2]. These deficiencies can be surpassed by increase of mineral nutrients in food through supplementation, food fortification or plant breeding [3,4].
Iron and zinc are considered to be the most important mineral elements in vegetarian diets. Elimination of meat from diet, along with increased intake of whole grain cereals and legumes rich in anti-nutrients, like phytate, significantly decrease Fe and Zn absorption [5]. The most prevalent among mineral elements deficiencies is Fe deficiency (anemia), affecting approximately 30% of the world’s population. Zn is essential element, involved in the immune system, activation of many enzymes and the growth. Zn deficiency has been detected in cases of inadequate dietary supply, abnormal blood losses or high physiological requirements for growth, as well as during puberty, pregnancy and lactation [4,5]. As a part of the antioxidant system of defense in mitochondria, manganese is also essential element for humans and is involved in metabolism, bone development and wound healing. It has been shown that Mg has protective role against various diseases. However, numerous studies indicated that Mg concentration in human body is usually insufficient [6]. According to Nielsen [7], low level of Mg has been associated with pathological conditions characterized as a chronic inflammatory stress, being widely associated with obesity, atherosclerosis, hypertension, osteoporosis, diabetes mellitus, and cancer.
According to present knowledge, it is necessary to increase content of mineral nutrients in edible parts of plants. Accumulation of mineral elements in seeds and grains is controlled by a number of processes including root-cell uptake, root-shoot transfer, and the ability to deliver these nutrients to developing seeds and grains [8]. Designing of cultivation systems, in order to improve nutrition and health, should become an integral part of goals in modern agriculture. It is mainly concerned to cultivation on poor soils, where micronutrient element enhancement can contribute to increased crop yield. According to Graham et al. [9], probably half of all soils are deficient in micronutrients and even though plant production is not limited, humans and animals whose diets are mainly based on crops can be potentially deficient in essential micronutrients. Incorporation of important mineral elements into soil by fertilizers could be problematic due to their pathway in soil. For instance, Fe from fertilizers could be quickly oxidized and became insoluble in soil, so Fe deficiency is mainly a consequence of Fe deficient soils [9]. Welch [8] reported significant impact of fertilizers containing N, P, K, S and Zn on accumulation of nutrients in edible plant products, including grains. Other micronutrient fertilizers were shown to have very small effect on the amount of micronutrients accumulated in edible seeds and grains when applied to soils.
Increased content of mineral elements in crops presents only the first step in making them improved sources of nutrients for humans [10], since not all mineral elements in plant foods are bio-available to humans and animals. Plant food can contain anti-nutrients, which interfere with the absorption of mineral nutrients in humans and animals. The question of bio-availability must be taken into consideration when enrichment of plant food with mineral elements was employed. This also takes into account enhancing substances - promoters (e.g. ascorbic acid, S-containing amino acids, etc.) that promote micronutrient bioavailability and/or suppress anti-nutrient substances (e.g. phytate, polyphenolics, etc.) that inhibit micronutrient bioavailability [2,11]. Thus, it is essential to decrease content of various anti-nutrients in foods and to increase content of promoters [9].
Phytic acid - Phy (myo-inositol 1,2,3,4,5,6-hexakisphosphate) is the major phosphorus storage compound in grains (accounting for up to 80% of total P) and it can acts as anti-nutritional factor that chelate essential elements including Ca, Zn and Fe [12]. As content of phytic acid in diet increases, the intestinal absorption of Zn, Fe and other mineral nutrients decreases [12], while the reduction in phytic acid content in food is likely to result in improved Fe, Zn and Mn content [3,13]. β-carotene is considered to be a promoter due to positive effect on mineral nutrients absorption. Lönnerdal [3] stated that β-carotene can enhance Fe absorption in humans. Luo and Xie [14] found that addition of food rich in β-carotene or pure β-carotene, can significantly enhance Fe and Zn bio-availability from the grains. Moreover, Noh and Koo [15] reported that low β-carotene absorption is associated with low Zn intake or slight Zn deficiency. Different cultivation practices, including macronutrient treatments (N, P and Mg), can result in increased concentration of β-carotene (by 42%) and micronutrients in carrots [8].
Soybean is important dietary source of proteins, lipids, minerals, vitamins, fiber and bioactive compounds. However, commonly high levels of phytate in soybean grain could negatively affect its nutritive value. Variability of mineral elements in soybean grain is significant and it also depends on applied cultivation systems [16,17]. Since Zn bioavailability from some soya products is low, application of an adequate cultivation system becomes important. However, compared to other plant foods with lower phytate contents, the Fe availability from soya flour and soya isolates is higher.
The aim of this experiment was to investigate the effect of applied foliar fertilizers on mineral nutrients content (i.e. Mg, Fe, Mn and Zn), along with contents of phytate as anti-nutritive factor and β-carotene as promoter, in chosen soybean varieties differing in chemical composition of grain.
Experimental
Plant material
Two commercial soybean varieties with standard grain composition - ZP-015 and Nena, and the variety lacking in Kunitz trypsin inhibitor - Laura, were the objectives of the present study.
Soil
The field trial was carried out in Zemun Polje (44°52'N 20°20'E), vicinity of Belgrade, Serbia (in rain-fed conditions). Soil was a slightly calcareous chernozem with 0.0 % coarse, 53.0 % sand, 30.0 % silt, 17.0 % clay, 3.3 % organic matter, 7.0 pH KCl and 7.17 pH H2O. The texture was silty clay loam, containing: 37.45 mg kg−1 N, 10.70 37.45 mg kg−1 P, 107.40 37.45 mg kg−1 K, 327.95 37.45 mg kg−1 Mg, 0.65 37.45 mg kg−1 Fe and < 0.02 37.45 mg kg−1 Zn in 0–30 cm layer, before fertilizer application. A split-plot experimental design in four replications was used in the experiment. Size of elementary plot was 5 m x 5 m.
Foliar fertilizers
Experimental trial included application of different foliar fertilizers in recommended doses, at the beginning of flowering (first half of June): 1. Zlatno inje (liquid extract of cow’s manure, with 0.8% of organic matter, 0.004% N and 0.0004% P), in amount of 4 L ha−1; 2. Bioplant Flora (organic fertilizer with 8% humic acids, isolated from vermicompost, with 1.0% N, 1.5% P, 48.35 mg L−1 Mg, 2.41 mg L−1 B, 13.14 mg L−1 Cu, 212.8 mg L−1 Zn, 1.64 mg L−1 Co, 462 mg L−1 Mn, 775.6 mg L−1 Mo and 500 mg L−1 Fe), in the amount of 1 L ha−1; 3. AlgarenBZn (organic fertilizer based on Ecklonia maxima algae extract with 2% of B and 3% of Zn), in an amount of 0.834 L ha−1; 4. Zircon (extract of medicinal plant Echinacea purpurea L., that contains a mixture of 0.1 g L−1 of phenolic acids: 3,4-dihydroxycinnamic (caffeic) acid (IUPAC: 3-(3, 4-dihydroxyphenyl)-2-propenoic acid; CAS No 331-95-5), chlorogenic acid (IUPAC: (1S,3R,4R,5R)-3-{[(2Z)-3-(3,4-dihydroxyphenyl)prop-2-enoyl]oxy}-1,4,5-trihydroxycyclohexanecarboxylic acid; CAS No 327-97-9), сichoric acid (IUPAC: (2R,3R)-2,3-bis{[(E)-3-(3,4-dihydroxyphenyl)prop-2-enoyl]oxy}butanedioic acid; CAS No 327-97-9), as active ingredients identical to Echinacea purpurea L. plant extract), in the amount of 0.12 L ha−1; 5. plant growth regulator Epin Extra (based on 0.025 g L−1 of 24-epibrassinolide (IUPAC: (22R 23R 24S)-2α, 3α, 22, 23 tetra hydroxy-24-metyl 5α-holestan-6-on; CAS No 72962-43-7), in the amount of 0.136 L ha−1. All these organo-mineral fertilizers were applied with a dose of 400 L ha−1 of water.