Transfer of Sodium Ion from Soil to Rice Seedling (Oryza sativa L.).

Soil salinity is one of the major environmental stresses for crops. Effects of salinity stress on plant growth are attributed to osmotic stress and ionic toxicity. Plants need to maintain a high K⁺/Na⁺ ratio. Excess sodium ion at the root surface, however, may inhibit potassium ion adsorption. This may result in sodium ion toxicity. The salinity tolerance of rice plants varies among varieties. The IR66946-3R-230-1-1 (FL530) is a salinity tolerant variety whereas Khao Dawk Mali 105 (KDML105) is prone to salinity stress. The backcross inbred line (BIL), 221-3, from the cross between KDML105 and FL530 is tolerant in hydroponic culture but sensitive in clay soil culture. We hypothesized that the different appearance of salinity tolerance in soil and hydroponic cultures may attribute to clay particles adsorbing sodium ion. In contrast, it is less likely for sand particles to adsorb sodium ion. The objective of this study was to compare the amount of sodium ion transferred from culture media to rice seedlings in different rice varieties with 150 mM/L NaCl solution  applied to hydroponic solutions, clayey and sandy soils. Roots, shoots and soil solution samples were collected at 0, 2, 6, 24 and 120 h after adding the NaCl solution. After root and shoot samples were dried at 70°C in a drier, they were used for sodium ion extraction using acetic acid (100 mM m^-3, 90°C, 2h). Sodium ion concentration was determined in the extract and soil solution by an ion chromatograph. Bulk Electrical Conductivity (EC_b) in clayey and sandy soil cultures was measured every 1 minute using four-electrode probes. The EC of the hydroponic solution and water in a container in which clayey and sandy soil culture pots were soaked was measured at 0 and 48 h after adding salt. Results will be discussed at the meeting.