Excessive nitrogen fertilization results in nitrate accumulation in leafy vegetables. Reducing the dose of mineral nitrogen or using alternate fertilizers lowers the nitrate accumulation; however, a critical minimum level of mineral nitrogen is necessary to maintain yield and nutritional quality. The aim of this study was to evaluate the effect of two levels of mineral nitrogen (100% and 50%) and three levels of an amino acid solution (0, 0.3, and 0.9%) in the nutrient solution of two baby lettuce cultivars (green and red) grown in a floating system. Nitrogen reduction did not affect yield (12.9–13.4 and 11.0–11.3 g/plant, respectively) but reduced nitrate accumulation (by 43 and 19%, respectively) in both green and red lettuce, while enhancing phenolic content (by 28%) and antioxidant capacity (by 69%) in green lettuce and soluble solid (by 7%) and total chlorophyll content (by 9%) in red lettuce. Although nitrate accumulation was prevented (< 355 mg/kg FW) and most nutritional components increased in both lettuce types by amino acids supplementation, plant growth was negatively affected, especially in red lettuce, in both concentrations of amino acids (reduction by 9 and 35% in 0.3 and 0.9%, respectively). In both lettuce types, proline content increased by 0.9% amino acids supplementation (by 45%), implying a probable induction of a stress condition. Mineral nutrients were slightly affected by nitrogen reduction, which was probably perceived as an abiotic stress.

The contribution of non-exchangeable soil K in the total K availability may be essential to K deficient soils; however, less attention has been paid so far for proposing soil tests that assess the bioavailable structural K that is solubilized by soil microbes during a growing season. The objectives of this study were to evaluate cation exchange resins for their performance in predicting K uptake by plants; to investigate the relationships between rhizosphere soil microbes, different soil K pools, and soil parameters; and to assess their exchange capacity with respect to K derived by feldspars. A pot experiment with winter wheat was conducted on K deficient soils, several soil tests were examined, and rhizosphere K solubilizing microbial population was assessed. Single and multiple regressions showed that cation resins performed better in predicting total plant K uptake than the other chemical extractants (r² = 0.64, r² = 0.85, respectively, p ≤ 0.001), whereas the PCA analysis and Pearson correlation tests revealed a positive correlation between K derived by feldspars, K uptake, and the K solubilizing rhizosphere microbial population. The above was further confirmed by the mapping of cation resins of extraction capacity, which showed a significant contribution of K derived by feldspars (15.6%).

The objective of this study was to examine the combined effects of selenium (Se) enrichment and water stress on the accumulation of available macro- and micronutrients in Melilotus officinalis L. aerial parts. Plants of M. officinalis were subjected to three levels of Se addition (0, 1 and 3 mg Se L^-1 water) and to two water treatments: a) full irrigation and b) limited irrigation (water stress). The above ground biomass (stems and leaves) was analyzed for Se, potassium (K), sodium (Na), magnesium (Mg), iron (Fe), copper (Cu), calcium (Ca), manganese (Mn) and zinc (Zn). Se addition differentially affected the K, Mg and Ca content of M. officinalis aerial parts, while it led to the reduction of the micronutrients Cu, Fe and Mn. Water stress resulted in the increase of K, Na, Mg, Ca and Cu, and to the decrease of the Fe, Zn and Mn content. An interaction between selenium addition and water treatment was more notable for Ca and Mg, which decreased under water stress at low Se level and for Zn and Cu, which increased under water stress at high Se level. According to our findings, Se-induced increased accumulation of some inorganic ions in the aerial parts of this species under water stress conditions could serve as a means to alleviate the adverse impact of water deficit on important metabolic processes, enhancing M. officinalis tolerance to water stress.