The effect of different levels of silicon sources and irrigation regimes on photosynthetic parameters of rice (Tarom Hashemi cultivar)

Photosynthetic parameters are the most sensitive process in the physiological metabolism of plants that are affected by irrigation regime and silicon fertilization. Thus, silicon is one of the effective elements in increasing the net rate of leaf photosynthesis, water use efficiency, stomatal conductance and intercellular carbon dioxide. Therefore, the aim of this study was to determine the effects of different levels of silicon sources and irrigation regimes on rice.

A pot experiment was conducted to investigate the effect of different levels of silicon sources and irrigation regimes on rice photosynthetic parameters at Sari Agricultural Sciences and Natural Resources University in 2018. The experiment was laid out in a split-factorial design with three replications. In this design, soil texture was considered as the main-plot factor, and irrigation regime and different levels of silicon sources as factorial-subplot factors. Factors included two Si doses of 60 and 120 mg Si Kg–1 as potassium silicate (Ps), sodium silicate (Ss), calcium silicate (Cs) and biochar (Bi) sources under two irrigation regimes (continuous flooding (W1) and periodic wetting-drying (W2)) in two soil series with different textures (Sandy-loam (S1) and Silty-clay (S2)). After plant growth, dry matter weight of straw and photosynthetic parameters such as relative leaf moisture content, transpiration intensity, stomata conductance, chlorophyll index and photosynthesis were measured in flag leaf at the flowering stage of rice.

The results showed that the highest leaf relative moisture content (63.80%), transpiration intensity (12.03 mmolm-2s-1), stomata conductance (384.79 mmolm-2s-1), chlorophyll content (55.00), photosynthesis rate (24.33 µmolm-2s-1) and dry matter weight of straw (33.3 gr/pot) were observed in plants treated with potassium silicate at a concentration of 120 mg Si Kg–1 of silty-clay soil under continuous flooding irrigation regime. The lowest dry matter weight of straw and photosynthetic parameters were observed in sandy-loam soil under periodic wetting-drying irrigation regime without using silicon fertilizer. Also, in sandy-loam soil, the leaf relative moisture content (58.95%), transpiration intensity (11.20 mmolm-2s-1), stomata conductance (340.32 mmolm-2s-1), chlorophyll content (51.80), photosynthesis rate (19.55 µmolm-2s-1) and dry matter weight of straw (18.6 gr/pot) was the highest in biochar treatment at a concentration of 120 mg Si Kg–1 under continuous flooding conditions. Also, a decrease in dry matter weight of straw and photosynthetic parameters was observed at the higher Si rates (120 mg Si Kg–1 compared to 60 mg Si Kg–1) in treatments of potassium silicate and sodium silicate in sandy-loam soil under periodic wetting-drying irrigation regime. This may be due to the inhibitory effect of high concentrations of silicon on photosynthetic function.

Although most of the photosynthetic parameters of plants and dry matter weight of starw in periodic wetting and drying irrigation regime were somewhat reduced compared to continuous flooding irrigation regime, application of different sources of silicon to both soil textures under both irrigation regimes improved relative leaf moisture content, transpiration rate, stomata conductance, chlorophyll content, photosynthesis rate and dry matter weight of straw were compared to treatments without silicon fertilizer application. This reflects the protective effect of silicon against low irrigation conditions. Therefore, in the scope of this experiment, it seems that the use of silicon to improve the photosynthetic parameters of rice plant and dry matter weight of straw under low-irrigation regime was satisfactory. But, the amount of use of this element is very important to maintain the balance of soil and plant properties.