Silicon reduces sodium uptake in rice (Oryza sativa L.) in saline conditions and this is accounted for by a reduction in the transpirational bypass flow

Rice is relatively sensitive to salinity and is classified as a silicon acc umulator. There have been reports that silicon can reduce sodium uptake in crop grasses in saline conditions, but the mechanism by which silicon might alleviate salinity damage is unclear. We report on the effects of silicon on growth, gas exchange and sodium uptake in rice genotypes differing in sa lt tolerance. In non-saline media there were no effects of supplementary si licate upon shoot fresh or dry weight or upon root dry weight, indicating t hat the standard culture solution was not formally deficient with respect t o silicon. Plants grown with supplementary silicate had slightly, but signi ficantly, shorter leaves than plants grown in a standard culture solution. Salinity reduced growth and photosynthetic gas exchange. Silicate supplemen tation partly overcame the reduction in growth and net photosynthesis cause d by salt. This amelioration was correlated with a reduction in sodium upta ke. Silicate supplementation increased the stomatal conductance of salt-tre ated plants, showing that silicate was not acting to reduce sodium uptake v ia a reduction in the transpiration rate. Silicate reduced both sodium tran sport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6- pyrenetrisulphphonic acid (PTS). This implies that the mode of action of si licate was by partial blockage of the transpirational bypass flow, the path way by which a large proportion of the uptake of sodium in rice occurs. Mec hanisms by which silicate might reduce the transpirational bypass flow dire ctly are discussed.