Silicon Affects Rice Growth, Superoxide Dismutase Activity and Concentrations of Chlorophyll and Proline under Different Levels and Sources of Soil Salinity

Purpose Silicon (Si) application shows beneficial effects on growth and salt tolerance of plants; however, its effects on the rice ( Oryza sativa L. var. Hashemi) growth, superoxide dismutase (SOD) activity and concentrations of chlorophyll and proline have not been fully understood under different levels and sources of soil salinity. Methods In this research, the effects of four salinity levels (0.46, 2, 4, and 8 dS m −1 ), two salt compositions (NaCl and four-salt combination) and four Si rates (8, 100, 200 and 300 mg kg −1 soil) as silicic acid (H 4 SiO 4 ) on rice growth characteristics and SOD activity were studied in a greenhouse. Salt composition included NaCl, Na 2 SO 4 , CaCl 2 and MgSO 4 at a molar ratio of 4:2:2:1. The experiment was arranged in a factorial framework in a completely randomized design with three replications. Results Increasing soil salinity level significantly decreased dry matter (about 40%), number of tillers (about 12%), leaf area (about 55%), concentrations of leaf chlorophyll (about 45%) and Si (about 40%), whereas caused a severe increase in proline concentration (more than 200%) and SOD enzyme activity (up to 65%). Deleterious effect of salinity on dry matter and leaf area as well as its stimulating influence on proline accumulation and SOD activity was more intense in NaCl-treated plants than those subjected to a combination of salts. Source of salinity had no significant effects on dry matter, number of tillers and chlorophyll concentration. Si treatment strongly enhanced these parameters, except for proline that declined in plants subjected to Si. Conclusions The suppressing effect of salinity on the rice growth characteristics can be alleviated by soil Si fertilization. The stimulating effects of soil Si fertilization on dry matter as well as chlorophyll concentration became more pronounced at the higher salinity levels. Consequently, when rice plants are to be grown in salt-affected soils, it is recommended to supply them with adequate Si.