Effects of cadmium (Cd) on photosynthetic characteristics and chlorophyll fluorescence parameters in the ornamental Plant Salvia splendens Ker-Gawl.

Salvia splendens Ker-Gawl. (scarlet sage), widely used in urban landscaping, it is frequently exposed to cadmium (Cd) contamination resulting from industrial and vehicular emissions. However, its tolerance and adaptability to Cd stress remain poorly understood. A soil experiment was conducted to investigate the effects of Cd on the growth and the photosynthetic performance of S. splendens by measuring photosynthetic pigments, gas exchange and chlorophyll fluorescence parameters. Four weeks-seedlings were treated with 0 (CK), 0.5, 2.5, 5, 10, 25 and 50 mg·kg^-1 Cd for 60 days. Results showed significant reductions in root length and biomass of leaves, stems, and roots, with shoot and root biomass notably decreasing by up to 46.3% and 28.5% at higher Cd levels, respectively. The translocation factor remained low (TF < 1.0), and the bioaccumulation factors (BCF < 1.0) decreased when Cd higher than 5 mg·kg^-1, indicating limited Cd uptake. Cd stress (> 5 mg·kg^-1) caused a decrease in Chl a and Chl b content, but increased the Chl a/b ratio, thereby disrupting photosynthesis and causing significant declines in photosynthetic parameters. Cd exposure (> 2.5 mg·kg^-1) significantly decreased net photosynthetic rate (Pn) by 18.94-52.91%, stomatal conductance (Gs) by 35.77-58.53%, and transpiration rate (Tr) by 24.63-48.83%, accompanied by only a slight reduction in inter-cellular CO₂ concentration (Ci) of just 7.0%, indicating non-stomatal factors in Pn decline. Cd concentrations (> 5 mg·kg^-1) caused a reduction in initial fluorescence (Fo) by 7.44-31.58% and maximal fluorescence (Fm) measurements by about 20%, indicating damage to photosystem II (PSII). At 50 mg·kg^-1, further decreases were observed in photochemical quenching (qP) by 40.31%, the quantum yield of photochemical energy dissipation (ΦPSII) by 44.77%, and the electron transport rate (ETR) by 25.11%, while non-photochemical quenching increased by 42.66%, signifying significant PSII inhibition and enhanced photoinhibition. Decrease in ΦPSII, along with the increase in the quantum yield of regulated non-photochemical energy loss in PSII (ΦNPQ) and the quantum yield of non-regulated energy loss in PSII (ΦNO) as Cd levels rise, indicates enhanced non-photochemical energy dissipation and greater photoinhibition. S. splendens shows high sensitivity to Cd stress, with reduced growth and disrupted photosynthesis, highlighting its potential as a bioindicator for Cd contamination in urban areas.