Abstract:
Effects of sodium chloride and sodium chloride acclimation on proline accumulation and expression of the P5CS gene were determined in 4 soybean cultivars; SJ.5, KKU.35, ST.2, and CM.60. Salt stress treatments during the seed germination were found to lower the germination percentage and inhibit the shoot and root growth of the soybean seedlings. Neither the salt acclimation with 40 mM NaCI nor the salt acclimation with 80 mM NaCI at the seed germination stage enhanced the salt tolerance in all soybean tests. Moreover, both treatments resulted in the reduction of shoot and root dry weight when compared to that without the salt acclimation. Leaf interveinal bleaching was also detected in the seedlings treated with salt acclimation during the seed germination. Two conditions of salt acclimation, Acclimation I (gradually increased salt concentration from 10, 20, to 40 mM NaCI for 3, 3, and 4 days, respectively, before being subjected to 80 mM NaCI) and Acclimation II (10, 20, 40 and 0 mM NaCI for 3, 3, 2 and 2 days, respectively, before being subjected to 80 mM NaCI), were applied to 5 day-old seedlings. Shoot and root growth of the acclimated seedlings were similar to those of the non-acclimated ones, which were lower than those of the non-stressed controls. The 80 mM NaCI treatment in 15 days-old seedlings induced significant proline accumulation in the ST.2 cultivar after 12 days of the salt treatment whereas no proline accumulation could be detected in the others. Both conditions of the salt acclimation did not affect the proline content of the ST.2 seedlings. However, Acclimation I caused the significant increase in proline content in the KKU.35 seedlings. The increase in proline accumulation in these salt-stressed soybean seedlings occurred together with the necrosis and downward curling of the leaf margin suggesting that it might be the symptoms of injury rather than the salt tolerance. No significant change in proline content was detected in any of the tested SJ.5 and CM.60. Using the Southern Blot Hybridization with the mothbean P5CS as a probe, only one DNA fragment was detected which implied that the soybean P5CS is possibly a single gene. Signals of the P5CS mRNA found in the salt stressed KKU.35 leaves correlated with the leaf proline content suggesting that the higher level of proline accumulation during the salt stress condition in KKU.35 was due to the increase of the P5CS gene expression.