Spirulina (Arthrospira platensis) priming mitigates cadmium stress in pearl millet (Pennisetum glaucum) by enhancing photosystem efficiency

Cadmium (Cd) is a highly toxic pollutant that impairs plant growth and photosynthetic performance. While seed priming with Spirulina (Arthrospira platensis) can enhance stress tolerance, its specific role in protecting the photosynthetic machinery from Cd toxicity remains unexplored. This study investigates for the first time the potential of Spirulina (Arthrospira platensis) seed priming to enhance cadmium stress tolerance in pearl millet (Pennisetum glaucum L.). Seedlings derived from P. glaucum seeds primed with Spirulina extract (CPS) or left unprimed (CUPS) were grown for 45 days in control or Cd-supplemented (50 µM CdCl₂) nutrient medium under controlled conditions to evaluate growth, photosynthetic performance, and oxidative stress responses. Cd exposure significantly reduced plant growth, photosynthetic efficiency, and increased oxidative damage, as indicated by elevated hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels (+ 323% and + 497%, respectively). In contrast, Spirulina priming mitigated these adverse effects, promoting shoot and root elongation (+ 25% and + 22%), biomass accumulation (+ 24%), and nutrient uptake. Primed plants exhibited lower Cd accumulation in shoots and roots (- 27% and - 15%, respectively), reduced oxidative stress markers (- 36% H₂O₂; -39% MDA), and higher concentrations of photosynthetic pigments (+ 35% chlorophyll a). A key novel finding was the significant enhancement of photosystem II integrity and function, evidenced by improved chlorophyll fluorescence parameters (F_v/Fₘ +28% and F_v/F₀ +206%).Overall, Spirulina-based seed priming strengthened the physiological and biochemical resilience of pearl millet under cadmium stress by specifically protecting the photosynthetic apparatus. This natural, eco-friendly approach offers a promising strategy to improve crop tolerance to heavy metal toxicity and supports its potential use in sustainable agriculture and phytoremediation.