Effects of Fe-chelate and iron oxide nanoparticles on some of the physiological characteristics of alfalfa (Medicago sativa L.)

Introduction Iron is an essential micronutrient for plant growth that plays an important role in plant metabolism. Iron deficiency is an abiotic stress that is often found in plants grown in calcareous and alkaline soils. The solubility of Fe+3 decreases dramatically with increasing pH. 30% of the arable land worldwide consists of calcareous and alkaline soils. Common iron fertilizers used to reduce deficiency syndromes contain iron(II) sulfate heptahydrate (FeSO4.7H2O) or iron chelates. Iron chelate (for example Fe-EDTA) is absorbed by plants, which however depends on soil conditions especially soil pH. Nowadays , nano-Fe fertilizer can be used as a rich source of iron for plants ,because it gradually releases Fe in a wide pH range (pH 3– 11). Nanofertilizer usage leads to increase element efficiency, reduce soil toxicity and negative effects caused by the excessive consumption of chemical fertilizers and reduce the fertilizer’ s application . This research was carried out to determine the suitable type of iron fertilizer and to evaluate the effects of different concentrations of nano-Fe fertilizer on Medicago sativa Materials and methods In order to investigate the effects of Fe-deficiency and different levels of Fe2O3 nanoparticles compared to Fe-EDTA on leaf growth, photosynthetic pigments and antioxidative activity of alfalfa (Medicago sativa cv.Hamadani), an experiment was conducted based on completely randomized design with three replications in Arak University during 2015. After germination of sterilized seeds of alfalfa, 1-day seedlings were cultured in plastic vases contains perlite. Plants were maintained under 25/18°C day/night temperatures with 12-hr photoperiod. Irrigation was done weekly with 100ml complete Hoagland solution (containing iron chelate (Fe-EDTA) for control plants) or 100ml Hoagland solution without iron chelate and containing different concentrations of ironoxide nanoparticles (0, 5, 10, 20 and 25µM). Plants treated with 0 µM iron nanoparticles did not receive iron in 45day period. Ironoxide nanoparticles were prepared from Pishgaman Company located in Mashhad, Iran . After the final harvest of 45-day plants, leaf fresh weights were measured. Dry weight of leaf were obtained by drying samples in an oven for 24h at 75°C until constant weight. Chlorophylla, chlorophyllb, total chlorophyll and carotenoids contents by spectrophotometry method at 663 and 645 nm were determined. Contents of proline, activities of catalase(CAT), guaiacol peroxidase(GPOX) and superoxide dismutase(SOD) and 2,2diphenyl-1-picrylhydrazyl (DPPH) scavenging activity were measured. All data were analyzed by variance analysis using SPSS16. Mean comparisons were conducted using Duncan’s test. Results and discussion Iron treatment had a positive effect on the growth parameters and photosynthetic pigments. The highest and lowest values of leaf growth and photosynthetic pigments were obtained at 25μM and 0µM of Fe2O3 nanoparticles, respectively. Treatment of iron nanofertilizer at different concentrations (even at 5µM) caused significant increase in leaf growth and photosynthetic pigments content compared to iron chelate. Nanoparticles have high reactivity because of more specific surface area, more density of reactive areas, or in creased reactivity of these areas on the particle surfaces. These features streamline the absorption of fertilizers , which are produced in nano scale. Iron nanofertilizer can be considered as an enriched source of bivalent iron for plant because of its high stability and gradual release of Fe in a wide pH range (pH 3–11). Iron is a necessary element for the formation of chlorophyll, therefore deficiency of iron in plants reduces chlorophyll content, so photosynthesis decreases The highest values of proline and antioxidants activity were measured in 0μM ironoxide nanoparticles. Thus, iron concentration of 0μM is considered stressful for alfalfa. Under abiotic stresses, reactive oxygen species (ROS) content increases more than the normal condition. ROS are highly reactive and toxic and damages cell membranes. Therefore, the concentration of ROS in cell must be controlled. In such conditions, plants develop a high efficient antioxidant defense system to increase tolerance to different stress factors. Overexpression of ROS scavenging enzymes like SOD, CAT, and GPOX resulted in abiotic stress tolerance in various crop plants due to efficient ROS scavenging capacity. It has been reported that proline act as an osmolyte, a metal chelator, ROS scavenger/an antioxidative, membrane stabilizer, and a signaling molecule Thus, the proline content is a good marker for screening tolerant varieties under stress condition  In this study , There was no statistical difference between different levels of Fe2O3 nanoparticles and Fe-EDTA based on values of proline and antioxidants activity, because stress conditions are not created in levels of Fe2O3 nanoparticles and Fe-EDTA. Conclusions Therefore, the suitable type of iron-fertilizer for alfalfa is iron nano-fertilizer and the concentration of 25μM Fe2O3 nanoparticles is the optimum value.