Silicon mitigates cold stress in barley plants via modifying the activity of apoplasmic enzymes and concentration of metabolites

Apoplasm is the first compartment encountering environmental stresses and is important for plant’s tolerance to low temperature. The effect of silicon (Si) on tolerance to low-temperature stress, e.g., chilling and freezing, has been shown in some plant species, while the involvement of leaf apoplasm in the Si-mediated amelioration of cold stress has not been investigated to date. In this work, one winter and one spring barley (Hordeum vulgare L.) cultivars were grown without or with Si (56 mg L−1 as Na2SiO3) and exposed to gradual temperature decrease from + 25 to + 5 °C for cold acclimation and then treated with freezing (− 5 °C) temperatures. Growth inhibition, photosynthesis reduction, and loss of membrane integrity under low-temperature conditions were higher in the spring than in the winter cultivars, but these parameters were similarly alleviated by Si in both cultivars. Cold acclimation decreased the lethal temperature (LT50) and increased survival (%) of plants exposed to freezing temperatures. The effect of acclimation treatment on reduction of LT50 was substituted completely by Si in both cultivars, while in the case of survival rate, Si substituted for acclimation treatment only in the winter cultivar. The activity of antioxidative enzymes and concentrations of soluble carbohydrates and proteins in the leaf apoplasm were increased upon cold acclimation and particularly on Si treatment; the highest values were observed in Si–cold-acclimated plants. Our data demonstrated an ameliorative effect of Si under both chilling and freezing stresses in barley via modification of biochemical properties in the leaf apoplasm.