Different methods of silicon application attenuate salt stress in sorghum and sunflower by modifying the antioxidative defense mechanism

Soil salinization is the most common abiotic stress limiting agricultural productivity worldwide. Recent research has suggested that the application of silicon (Si) has beneficial effects against salt stress in sorghum (Sorghum bicolor L. Moench) and sunflower (Helianthus annuus L.) by regulating the antioxidant system, mineral nutrients, and other important mechanisms. However, whether these effects can be achieved through foliar application of Si, or whether Si application affects Si-accumulating (e.g., sorghum), and intermediate-Si-accumulating (e.g., sunflower) plant species differently, remains unclear. This study investigated different methods of Si application in attenuating the detrimental effects of salt stress, based on the biological responses of two distinct species of Si accumulators, under greenhouse conditions. Two pot experiments were designed as a factorial (2 × 4), randomized complete blocks design (RCBD) with control and salt-stress groups (0 and 100 mmol.L⁻¹ NaCl), and four Si-treatment groups: control (no Si), foliar application (28.6 mmol.L⁻¹), root application (2 mmol.L⁻¹), and combined foliar and root applications. Our results showed that the harmful effects of salt stress were attenuated by Si treatments in both plant species, which decreased Na⁺ uptake and lipid peroxidation, and increased Si and K⁺ uptake, relative leaf water content, antioxidant enzyme activities, leaf area, and shoot dry matter. These results were more prominent when Si was applied via nutrient solution in the sorghum plants, and the combined foliar and root applications of Si in sunflower plants. In addition, foliar application of Si alone is an efficient alternative in attenuating the effects of salinity in both plant species when Si is not available in the growth medium. These results suggest that the Si application method plays an important role in Na⁺ detoxification by modifying the antioxidative defense mechanism, which could actively mediate some important physiological and biochemical processes and helps to increase the shoot dry matter production in sorghum and sunflower plants under salt stress.