This research focused on the different approaches to the transport and internal chelation of metals with amino acids and organic acids in plants. Therefore, in the first phase, the plants studied were identified the characteristics of the bioaccumulation factors. Steria pumila, Echium angustifolium, Typha angustifolia, Sisymbrium austriacum were identified as hyperaccumulators (Cd, Ni), accumulators (Pb, Sn, and Se), excluders (Cr, Hg). On the other hand, the Sisymbrium austriacum only showed the characteristic of the accumulator for Cr. In the second phase, the combined effects of amino acids and organic acids on the chelation of heavy metals in plants were tested by a multi-linear regression model. Related to our hypothesis, Amino acids; Gly and Leu (Cd), Trp and Ile (Pb), Asp, Ser, and Leu (Cr), Ser (Hg), Trp and Glu (Ni), Asp, Thr, and Gly (Sn), Asn and Leu (Se), Organic acids; Malonic and Malic acid (Cd), Malonic acid (Pb), Oxalic and Malic acid (Cr), Oxalic, Succinic, Citric and Butyric acid (Hg), Malonic and Malic acid (Ni), Malonic, Maleic, and Malic acid (Sn), Malonic and Citric acid (Se) were concluded that had combined effect for heavy metal’s phytochelation ability into plants.

Iron deficiency, which severely decreases the plant yield and quality, is one of the major problems of calcareous soils. Foliar applications of humic substances and/or Fe fertilizers are environmentally friendly methods to cope with Fe deficiency. The aim of this study was to investigate the combined effect of Fe/nano Fe and humic/fulvic acid-based biostimulant foliar applications on the Fe content and plant growth parameters of spinach. Treatment solutions were prepared either by mixing a common Fe fertilizer, FeSO₄·7H₂O, with different commercial biostimulants (Fulvic-based: Fulvagra^®, Fulvagra^®WSG; Humic-based: HS300^®, Humin Fe^® and Liqhumus^®, Grevenbroich, Germany) or by mixing nano ferrihydrite with different ratios of fulvic substance (FA-50, FA-75, and FA-100) and humic acid (Nano Iron). Growth parameters (plant fresh and dry weights, plant dry matter, root fresh and dry weights, root dry matter, leaf number per plant, and leaf area); chlorophyll reading value (SPAD); chlorophyll (a,b, and total) and carotenoid contents; and leaf and root mineral contents (N, P, K, Ca, Mg, S, Cu, Mn, Zn, B, active Fe, and total Fe) of samples were determined. Our results showed that foliar application of biostimulants together with Fe sources improved the nutrient uptake, chlorophyll contents, growth characteristics, and yield; however, not all humic substances had the same effect. When all parameters were considered, Fulvagra treatment—which contained 17% fulvic acid and microorganisms in its content together with 20 mM FeSO₄·7H₂O—was the most effective application, followed by FA100 treatment containing fulvic acid and 20 mM nano ferrihydrite. This finding indicates that fulvic acid containing biostimulants is more effective in foliar applications than humic-based biostimulants against Fe deficiency due to their low molecular weight which enables better penetration into the leaves. In conclusion, foliar applications of fulvic substances together with Fe fertilizers can be used to increase the Fe uptake of crops and the yields under Fe-deficient conditions.

The use of probiotics to reduce the use of chemical fertilizers in agriculture has proven to be a promising area of research, especially as the agricultural sector searches for sustainable alternatives to synthetic agents. However, there are few studies integrating probiotics into agricultural practices, especially in lettuce cultivation, to minimize the use of chemical fertilizers. Most previous research has focused on the effects of plant growth-promoting rhizobacteria (PGPR) and microalgae on yield, with little consideration of the combined effects of different microorganisms under the same soil and growing conditions. In this study, the effects of microbial biostimulants on the efficiency of fertilizer use in lettuce (Lactuca sativa L.) cultivation were investigated, reducing the use of chemical fertilizers by 25%. The trial was conducted as a randomized block trial in a high tunnel greenhouse and comprised eight treatments: a negative control (T0, no chemical fertilizers or microorganisms), a positive control (T1, 500 kg ha^-1 NPK fertilizer) and treatments with single biostimulants - microalgae (Chlorella vulgaris, T2), plant probiotic microorganisms (PPMs, Lactobacillus spp, Rhodopseudomonas palustris, Saccharomyces cerevisiae, T3), and PGPRs (Bacillus subtilis, Bacillus megaterium, Bacillus amyloliquefaciens, T4). Further treatments combined reduced fertilizer (375 kg ha^-1 NPK) with microalgae (T5), PPMs (T6) and PGPRs (T7). The results showed that T6 (PPMs + reduced fertilizer) achieved the highest plant weight (364 g) and leaf length (43.8 cm), with increases of 152% and 128%, respectively, compared to the control (T0). Chlorophyll content improved significantly in T5 (27.5%) and T7 (29.4%) compared to T0. Biomass production was 78% higher in T6 than in T1, while T5 and T7 showed a moderate increase in biomass of 42% and 47%, respectively. Cluster analysis identified T6 as the most effective treatment, outperforming T1 in growth parameters. These results demonstrate the potential of microbial biostimulants, especially in combination with reduced chemical fertilizers, to increase plant growth, biomass production and fertilizer efficiency. The study underlines their role in promoting sustainable agriculture and provides a basis for future research under different soil and environmental conditions.

In this study, the performance of a novel organic tea compost developed for the first time in the world from raw tea waste from tea processing factories and enriched with worms, beneficial microorganisms, and enzymes was tested in comparison to chemical fertilizers in tea plantations in Rize and Artvin provinces, where the most intensive tea cultivation is carried out in Turkey. In the field trials, the developed organic tea vermicompost was incorporated into the root zones of the plants in the tea plantations in amounts of 1000 (OVT1), 2000 (OVT2) and 4000 (OVT4) (kg ha-1). The experimental design included a control group without OVT applications and positive controls with chemical fertilizers (N: P: K 25:5:10, (CF) 1200 kg ha-1) commonly used by local growers. The evaluation included field trials over two years. The average yields obtained in two-year field trials in five different areas were: Control (6326), OVT1 (7082), OVT2 (7408), OVT4 (7910), and CF (8028) kg ha-1. Notably, there was no significant statistical difference in yields between the organic (at 4000 kg ha-1 ) and chemical fertilizers (at 1200 kg ha-1). The highest nutrient contents were obtained when CF and OVT4 were applied. According to the average values across all regions, the application of OVT4 increased the uptake of 63% N, 18% K, 75% P, 21% Mg, 19% Na, 29% Ca, 28% Zn, 11% Cu and 24% Mn compared to the control group. The application of chemical fertilizers increased the uptake of 75% N, 21% K, 75% P, 21% Mg, 28% Na, 27% Ca, 30% Zn, 18% Cu and 31% Mn compared to the control group. The organic fertilizer treatment had the lowest levels of antioxidants compared to the control groups and the chemical fertilizers. It was also found that the organic fertilizer increased the levels of amino acids, organic acids and chlorophyll in the tea plant. Its low antioxidant activity and proline content prepared them for or protected them from stress conditions. With these properties, the biotechnologically developed organic tea compost fertilizer has proven to be very promising for tea cultivation and organic plant production.