The effects of iron and zinc spraying in sulfate and nano forms on morphological and biochemical properties of peppermint (Mentha piperita L.) under salinity stress

Introduction Salinity after drought stress is one of the important stress around world including Iran. Salinity causes interrupt in cell division and growth and affect all metabolic reactions in plant. Consuming micronutrient could increase the resistance of the plant against negative effects of salinity. The medicinal plant needs micronutrient enough for producing essential oil and among micronutrients, iron is more needed. Iron is one of essential nutrients, micro, and immobile. This nutrient is required a cofactor for many oxidant and reduction enzymes and the synthesis of chlorophylls. Zinc is also one of micronutrients that involves in tryptophan synthesis, auxin precursor, obsceneness of leaves, carbohydrate metabolism and protein synthesis. One of the important applications of the non-technology is nano-fertilizers in agriculture. Nanoparticle inters into the plant through stomata. Peppermint (Mentha piperita L.) from family Lamiaceae grows in the sandy-acidic soil and medium light and high humidity conditions. Peppermint is a rich source of polyphenol that has the antioxidant effect. Regarding importance of peppermint, this research was aimed to investigate effects of nano-Fe and nano-Zn on some biochemical characteristics of peppermint. Material and methods This work conducted for evaluating the effects of iron and zinc application in two forms, nano and sulfate, on some morphological and biochemical characteristics of peppermint under salt stress, at research greenhouse of Ferdowsi University of Mashhad, Mashhad, Iran. The experiment arranged as factorial based on completely randomized design with three replications. The first factor was salinity in four levels (0, 40, 80, and 120 mM NaCl) and the second factor was foliar application Fe and Zn fertilizers in five levels (0, Fe-sulfate and Zn-sulfate at 1500 mg.l-1 and nano-particles of Fe and Zn at 300 mg.l-1). Rhizomes with 10 cm length were cultured within 5 cm depth in pots with 40 cm height and 30 cm diameter. The pots filled with an equal amount of soil, sand, and peat (1:1:1) in each pot. The physio-chemical characteristic of soil was as follows: 1.2 dS/m EC, 7.9 pH, 29% sand, 41% clay, 30% silt, and loam-clay tissue. The first spraying was applied one week before exerting salt stress and two spraying after three weeks exerting salt stress in two intervals. Traits measured were chlorophyll, SPAD, soluble carbohydrate, and some morphological traits. Results The results indicated that salt stress had significant effect on the growth and biochemical characteristics of peppermint including shoot length, number of branches, number of tiller, length and width of leaves, fresh and dry weight of aerial part, chlorophyll a and b, total chlorophyll, carotenoid, and SPAD. It decreased all of these traits. analysis of interaction effects of treatments indicated that the highest levels of growth and chlorophyll content were obtained in control treatment (without of salt stress) and plants grown under mild stress (40 mM NaCl) treated with nano-Fe and Zn. As the highest plant high (78 cm), number of branches (36 branches per plant), and leaf length (6 cm) were related to control treatment (without of salt stress) and foliar application with nano-Fe. While the highest width of leaf (2.16 cm) was obtained in plants treated with nano-Fe and grown under 40 mM NaCl which there was no significant difference with application of nano fertilizers in control treatment. The highest amount of soluble carbohydrates with the amount of 1.28 and 1.212 mg/g dry weight were obtained under 40 and 80 mM salinity conditions with the application of nano-Fe fertilizer, respectively. Conclusion The results of this study imply that application iron and zinc nanoparticles in comparison to its sulfate forms improves the growth characteristics, chlorophyll contents, and carbohydrate under control and mild salinity stress conditions (40 mM). Considering pivotal role of iron and zinc in the growth of the plant and expanding of nano-Fe and nano-Zn could have important role in supporting micronutrient for plants.