Silicon Reduces Cadmium Accumulation and Improves Growth and Stomatal Traits in Sea Barley (Hordeum marinum Huds.) Exposed to Cadmium Stress

Silicon (Si) is not deemed sensu stricto as an essential nutrient for higher plants, but it is known for its positive impact under environmental stress conditions. The present study hypothesizes that the addition of silicon (Si) could alleviate cadmium (Cd) toxicity and reduce its accumulation in sea barley. Twenty-day-old plants were hydroponically grown under controlled conditions at two Cd levels (0 and 25 µM) with (1 mM) or without Si supply. Several aspects of the plant morphology, photosynthetic activity, membrane stability, and the status of metals (Cd, Zn, and Fe) were assessed after 17 days. Statistical analyses (Duncan’s test p ≤ 0.05) indicated that most studied parameters were altered by metal treatment, reflecting Cd toxicity. Plant growth, water status, carotenoid content, chlorophyll (Chl. a/Chl. b) ratio, carotenoid/(Chl. a + Chl. b) ratio, gas exchanges, fluorescence parameters (ETR, qP, and NPQ), membrane stability, and Zn concentration in both roots and shoots were significantly impacted by Cd. However, plants simultaneously treated with Cd and Si supply showed a significant restoration in growth parameters, water content, and photosynthetic pigments. The mitigating effect of Si was associated with higher stomatal conductance, transpiration, chlorophyll fluorescence (ETR, qP, and NPQ) values, and better membrane stability. The attenuation of Cd’s toxic effect was also explained by the fact that Si prevented Cd uptake, as Si decreased Cd content in shoot and root tissues. The current study revealed that silicon supply to sea barley, grown under Cd stress, mitigates Cd toxicity and leads the way forward for similar studies on other cereal field crops.