Agricultural water scarcity has become a global issue. Optimizing irrigation water quality and effectively utilizing non-conventional water resources are essential strategies to alleviate pressure on agricultural water use and achieve sustainable development. This study employed Italian lettuce as the test crop to explore the effects of magnetization treatment (M) at a magnetic field strength of 0.2 T and various irrigation water sources (T) on its growth. The following six treatments were established: fresh water irrigation (M0T1), recycled water irrigation (M0T2), saline water irrigation (M0T3), magnetized fresh water irrigation (M1T1), magnetized recycled water irrigation (M1T2), and magnetized saline water irrigation (M1T3). The results showed that the magnetization treatment increased the electrical conductivity (EC), power of hydrogen (pH), and dissolved oxygen (DO) of the three water sources compared to the non-magnetized treatment. Furthermore, magnetized irrigation with fresh water, recycled water, and saline water increased the contents of nitrogen (N), potassium (K), calcium (Ca), and magnesium (Mg) in lettuce. It also led to increases in the contents of soluble proteins (by 9.27% to 22.25%), soluble sugars (by 13.45% to 20.50%), and vitamin C (VitC) (by 4.18% to 19.33%) in lettuce. Additionally, it enhanced the above-ground fresh weight of lettuce (by 9.36% to 8.81%) and water productivity (WPc) (by 5.85% to 10.40%), while reducing water consumption. Among these treatments, magnetized fresh water irrigation was the most effective in improving quality, fresh weight, and WPc, followed by magnetized recycled water. Gene expression analysis revealed that differentially expressed genes (DEGs) were primarily enriched in metabolic pathways such as the MAPK signaling pathway—plant, phytohormone signaling, and cysteine and methionine metabolism. In summary, magnetized irrigation significantly enhanced DO levels in irrigation water, along with the fresh weight, quality, and WPc of lettuce, demonstrating its effectiveness as an efficient method for agricultural irrigation.

Biostimulants, particularly single amino acids, can increase plant growth and crop quality, gaining significant attention. This study investigates the effects of 10 amino acids via root/foliar application on the growth, quality, taste, and volatile flavor of mini-watermelons and compares the differences between the application methods. Here, we employed electronic noses, electronic tongues, and gas chromatography–ion mobility spectrometry to investigate these effects. Root application excels in fruit growth and pectin accumulation, while foliar application boosts soluble protein and specific nutrients. Specifically, root application (except for Val) significantly increases fruit weight, with Gly being most effective for longitudinal diameter, while most amino acids (except Val/Lys) promote transverse diameter. Pectin content shows bidirectional regulation: root application of Glu/Gly/Lys/Pro/Trp/Val enhances pectin, whereas foliar application inhibits it. For taste indices, most treatments improve soluble solids (except Glu root/Arg-Leu foliar), and Ala/Asp/Glu/Gly reduce titratable acids, optimizing the sugar–acid ratio. Foliar application is more efficient for soluble protein accumulation (Ala/Glu/Gly/Pro/Leu). For nutritional quality, except for Lys, all treatments increase vitamin C and widely promote total phenolics and lycopene, with only minor exceptions, and only Arg foliar application enhances ORAC. Additionally, the results revealed that root-applied lysine and valine greatly raised the levels of hexanal and 2-nonenal, whereas foliar-applied valine significantly increased n-nonanal and (Z)-6-nonenal. Overall, we found that amino acids can considerably improve mini-watermelon production, quality, taste, and antioxidant capacity, providing theoretical and practical references for their widespread use in agriculture.