Effects of cesium accumulation on chlorophyll content and fluorescence of Brassica juncea L

Highlights: • The chlorophyll fluorescence parameters of F_v/F₀, F_v/F_m, ΦPS II, qP, and NPQ decreased significantly under Cs stress. • Excessive Cs can damage PS II in leaves, decreasing photochemical activity and the energy conversion rate. • Cs interfered with the expression of chloroplastic metabolic genes. • Cs inhibited the expression of PsaB, psbC, PetF, LHCA1, and LHCB5. - Abstract: The aim of our study was to investigate the toxicological mechanism of cesium on Indian mustard (Brassica juncea L.). The impact of cesium toxicity to plants was evaluated using phytophysiology and genetic methods. In this study, Brassica juncea was grown on Cs-contaminated Hoagland's nutrient solution, and chlorophyll content, chlorophyll fluorescence, and Cs bioaccumulation were measured. Transcriptome data was used to perform an in-depth analysis of the molecular mechanisms underlying the effects of Cs accumulation. The results showed that Cs accumulated up to 3586.70 mg kg⁻¹ in B. juncea treated with 100 mg L⁻¹ Cs. The chlorophyll content and several chlorophyll fluorescence parameters (F_v/F₀, F_v/F_m, ΦPS II, qP, and NPQ) significantly decreased under Cs exposure. The starting process of PSII was also inhibited under higher Cs conditions. These results indicate that excessive Cs can damage PS II in leaves, decreasing photochemical activity and the energy conversion rate. Further analysis revealed that Cs interfered with the expression of chloroplastic metabolic genes (25 up and 36 down) and inhibited the expression of PsaB, psbC, PetF, LHCA1, and LHCB5. The results indicate that stable Cs leads to abnormal expression of genes related to photosynthesis pathway, blocking the electron transport process from plastoquinone-QA to plastoquinone-QB, resulting in abnormal photosynthesis, which leads to abnormal growth of B. juncea.