As emerging pollutants, microplastics (MPs) pose serious threats to the terrestrial ecosystems, and the long-term presence of aged MPs in soil results in toxic effects on plant growth. However, the phytotoxicity mechanisms of aged MPs remain unclear. To understand the toxic effects of aged MPs and the response mechanism of lettuce plants, we selected polyethylene (PE) and polypropylene (PP) (commonly found in soil), and then studied the effects of the two phytotoxins on the soil-plant system before and after aging of the MPs. We found that aging enhanced the toxicity of the MPs to the plants. Compared with the original MPs-treatment group, aged PE and PP particles reduced plant biomasses by 26.19%-28.44% and 25.58%-26.13%, respectively, potentially due to the effects of aged MPs on the rhizosphere soil, which further inhibited nutrient absorption in lettuce. The metabolic response of lettuce to MPs was also different. Aged PE significantly attenuated malic acid and proline concentrations in lettuce, and the reduction in these two products inhibited photosynthesis, energy metabolism, and cellular homeostasis, thereby aggravating the damage caused by aged PE. Aged PP principally affected the metabolic pathways of phenylalanine, tyrosine and tryptophan, which was postulated to be the reason why aging enhanced the phytotoxicity of PP. This study provides new insights into the assessment of the toxic effects of MPs, as well as the environmental behavior and ecological risks of aged MPs.

Backround The utilization of high-quality water in agriculture is increasingly constrained by climate change, affecting availability, quality, and distribution due to altered precipitation patterns, increased evaporation, extreme weather events, and rising salinity levels. Salinity significantly challenges salt-sensitive vegetables like lettuce, particularly in a greenhouse. Hydroponics water quality ensures nutrient solution stability, enhances nutrient uptake, prevents contamination, regulates pH and electrical conductivity, and maintains system components. This study aimed to mitigate salt-induced damage in lettuce grown via the floating culture method under 50 mM NaCl salinity by applying biostimulants. Results We examined lettuce's physiological, biochemical, and agronomical responses to salt stress after applying biostimulants such as amino acids, arbuscular mycorrhizal fungi, plant growth-promoting rhizobacteria (PGPR), fulvic acid, and chitosan. The experiment was conducted in a greenhouse with a randomized complete block design, and each treatment was replicated four times. Biostimulant applications alleviated salt's detrimental effects on plant weight, height, leaf number, and leaf area. Yield increases under 50 mM NaCl were 75%, 51%, 31%, 34%, and 33% using vermicompost, PGPR, fulvic acid, amino acid, and chitosan, respectively. Biostimulants improved stomatal conductance (58-189%), chlorophyll content (4-10%), nutrient uptake (15-109%), and water status (9-107%). They also reduced MDA content by 26-42%. PGPR (1.0 ml L-1), vermicompost (2 ml L-1), and fulvic acid (40 mg L-1) were particularly effective, enhancing growth, yield, phenol, and mineral content while reducing nitrate levels under saline conditions. ConclusionsBiostimulants activated antioxidative defense systems, offering a sustainable, cost-effective solution for mitigating salt stress in hydroponic lettuce cultivation.