Effect of Micronutrient Micnobit and Salt Fertilization on Lettuce

Plants can absorb nano- and micro-size particles containing nutrients, opening the possibility for instantaneous uptake of nutrients supplied through roots or leaves. Here, we studied the uptake by lettuce (Lactuca sativa L.) of particulate nutrients from a micnobit (mixture of nanoparticles producing different nano and micro-scale sizes) formulation composed of ZnO, CuO, Fe3O4, MnO, and B2O3 used as fertilizers, in comparison to a similar mixture of their ionic equivalents from salts of Zn, Cu, Fe, Mn and boric acid at levels optimal for growth, yield, metabolism and nutritional quality attributes in food crops. In the case of Zn, the effect of a double dose compared to the basic fertilization was also studied to evaluate the possibility of increasing Zn levels in vegetable crops, as a potential strategy for alleviating Zn deficiency in human/animal diets. A greenhouse pot experiment was conducted using a sandy soil (pH 7.1) poor in most nutrients, including micronutrients but rich in calcium, and with very low content of organic matter to study the effects of micronutrient fertilization on lettuce. For application of the micronutrients, four fertilizer treatments were used: i) traditional (ionic) fertilizer; ii) micnobit particle coated seeds, iii) micnobit foliar application (spraying treatment), and iv) micnobit soil application. The lettuce plants were grown for 73 days and harvested periodically to study fresh and dry weight production and leaf greenness by SPAD measurements over time. Shoot or root materials from the final harvest was used to determine contents of micronutriënts, leaf chlorophyll and biomolecules (vitamins, flavonoids, phenolics, and antioxidants). Values from SPAD measurements significantly differed between the harvests and showed an increase with time up to 59 days after sowing. Differences between the treatments were less distinct, but showed the highest values between the Priming and Control treatments. Chlorophyll contents did not significantly differ between the treatments. The lettuce plants showed S-shaped growth under all treatments, although the foliar applications, both with micnobits and ions, caused necrosis at the leaf edges in the long run. Also, Foliar application with micnobits caused deposition and showed black spots on the leaves that could not be removed by rinsing methods described in the literature. Growth analysis, i.e. production of fresh weight, dry weight and root-total weight ratio over time, showed that for both shoot and root, the Control treatment resulted in the best growth. Priming of seeds resulted in similar growth as at the Control treatment. Addition of micronutrients to soil or leaves, both applied as ions or as micnobits, even decreased fresh weight and dry weight production of lettuce. Moreover, the shoots and roots of plants in the Control treatment also showed accumulation of micronutrients, while no micronutrients had been applied. No significant differences were found between the Foliar application and Soil treatments. Moreover, even no significant differences were found between application of micnobits or application of ions. Thus, we concluded that the soil - although very poor with a low content of micronutrients - was not lacking in the tested nutrients and the low availability of micronutrients in the soil was already sufficient for an optimal growth. The determination of micronutrient contents showed that ions and micnobits were taken up by both roots (Soil treatments) and by leaves (Foliar application). However, the amounts of micronutrients found in the leaves appeared to be higher than those described in literature for plant shoots with adequate growth. The contents of the micronutrients other than Fe varied between the treatments in both leaves and roots, suggesting excessive uptake that could have inhibited biomass production in these treatments. Foliar application with ions resulted in a higher micronutrient content than foliar application with micnobits. The fact that all treatments, whether applied to the leaves or to the soil or via priming, resulted in an accumulation of micronutrients in both shoots and roots, proved that the micronutrients had been transferred through the plants from the shoot to the roots (Foliar application) and from the roots to the shoot (Soil application). However, it could not be determined in which form, i.e. whether as ions, micnobits or metabolically processed, the micronutrients were transported through the plant. The presence of micnobits could only be detected for Fe2O3, CuO and MnO3. However, due to a high noise level in the ICP-MS sample analyses, it was not possible to quantify the content of the elements properly. Vitamin C content in leaves was highest in plants from the Control treatment, but not significant different from other treatments. No clear distinction could be made between the Soil and Foliar application, nor between treatments with ions or with micnobits. LCMS profiles of the lettuce leaves showed a relative intensity of 257 compounds present in the leaves. In the Soil and Foliar application, 80 out of the 237 metabolites (25%) had been changed significantly, although not annotated, from which 66 of them revealed a more than 2-fold difference. We conclude that this study with composite nanoparticles could not endorse the hypothesis that micnobits will be taken up more efficiently than ions, or that they will enhance growth in lettuce as reported in the literature for other crops. Furthermore, this study also clearly demonstrates the need for plant tissue testing as an important yardstick for supporting soil-based nutrient testing, prior to fertilizer recommendations.
Lettuces, Nanoparticles, Micronutrients, Foliar application
Blom-Zandstra, M., W. De Visser, A.K. Van der Werf, M.K. Van der Lee, C.H. Vos, R.E.E. Jongschaap, C. Dimkpa, and P.S. Bindraban. 2017. Effect of Micronutrient Micnobit and Salt Fertilization on Lettuce, VFRC Report 2017/1, Virtual Fertilizer Research Center, Washington, D.C., USA.