Phenotypic Selection Following Gamma Irradiation Improves Zinc Stress Tolerance in Duckweeds

Zinc contamination in aquatic ecosystems poses significant ecological risks, inducing phytotoxicity, disrupting trophic dynamics, and facilitating bioaccumulation within food webs. Duckweed (Lemnaceae) is widely recognized as an efficient phytoremediator due to its rapid growth and high metal uptake capacity. In this study, duckweed fronds of Lemna aequinoctialis and Landoltia punctata were exposed to gamma radiation to generate second clonal irradiated populations, followed by cultivation under increasing ZnSO ₄ concentrations (0, 85, and 170 µM). At a low irradiation dose of 10 Gy, the treatment significantly stimulated new frond formation. In contrast, frond formation declined at higher doses, and no new fronds were formed at ≥50 Gy during the first 7 days. Micro‐X‐ray fluorescence (micro‐XRF) analysis revealed concentration‐dependent Zn signal intensity with pronounced spatial heterogeneity, characterized by preferential localization in central and vascular‐associated frond tissues. Semiquantitative point analyses showed higher relative Zn signal intensity at higher ZnSO ₄ concentrations. Two‐way ANOVA revealed significant line × Zn interactions for all growth and pigment traits in both species ( p < 0.001), showing that the response to Zn depended on the individual line. Of the three lines selected per species, one outperformed the original clone at 170 µM ZnSO ₄ : Lem‐IR‐P1 had higher frond number, frond area, and pigment contents, largely reflecting greater growth at all Zn levels, whereas Lan‐IR‐P2 retained a significantly larger proportion of its daughter‐frond production under the treatment without additional ZnSO ₄ than the original clone (73% vs. 45%) together with higher chlorophyll a and carotenoid contents. Zn stress substantially reduced chlorophylls and carotenoids in the original clones. These results suggest that gamma irradiation followed by phenotypic selection under Zn stress can yield individual lines with improved performance under Zn stress, although the underlying regulatory mechanisms were not directly evaluated and the selected populations have not yet been genetically confirmed as stable mutants. This study indicates that irradiation‐based phenotypic selection combined with micronutrient‐based screening can generate duckweed germplasm with improved Zn tolerance and stable photosynthetic performance, with promising, though yet‐to‐be‐validated, potential for enhanced biomass productivity.