Effects of silicon on absorbed light allocation, antioxidant enzymes and ultrastructure of chloroplasts in tomato leaves under simulated drought stress

• Silicon improved the absorbed light allocation between PSII and PSI in tomato leaf chloroplasts under drought stress.
• Silicon regulated the activities of antioxidant enzymes and decreased the levels of ROS in tomato leaf chloroplasts under drought stress.
• Silicon contributed to maintenance of the chloroplast membrane structure in tomato leaves under drought stress.

The impact of silicon (Si) on chlorophyll fluorescence, antioxidant enzymes and the ultrastructure of chloroplasts of drought-stressed tomato (Solanum lycopersicum L.) was investigated. Si reduced the accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA) in chloroplasts. Si-induced improvement of absorbed light allocation in leaves reduced the risk of ROS generation. Drought stress increased the relative deviation from full balance between the two photosystems (β/α – 1) and PSII excitation pressure (1 – qP). Application of Si restrained chlorophyll degradation and increased optimal photosynthetic efficiency of PSII (Fv/Fm) and the electron transport rate (ETR), which contributed to improvement of the net photosynthetic rate (Pn), (β/α – 1) and (1 – qP). On the other hand, Si-mediated enzymatic systems contributed to ROS elimination. The dual nature of ROS was detected during 12 days of drought stress. While Si played an important role in suppressing the decline of the activities of ROS scavenging enzymes in chloroplast, such as superoxidase dismutase (SOD) and enzymes in the ascorbate–glutathione pathway. Therefore, Si protected the structure of the chloroplast from severe oxidative damage, such as the distortion of the grana lamellae and stroma lamellae. This study suggested that Si might be involved in metabolic or physiological activities in Si non‑accumulating plants under drought stress.