Abstract: To explore the proline and polyamine metabolism-responsive mechanisms of naked oat (Avena nuda) under saltalkaline mixed stress, a naked oat cultivar ‘Dingyou No. 6’ was cultured with perlitesandy pot in greenhouse. The contents of proline and polyamine and the activities of proline and polyaminemetabolizing enzymes of naked oat leaves on 0, 1, 3, 5 and 7 days by the treatments using Hoagland nutrient solution added with mixed saltalkali (molar ratio of NaCl∶Na₂SO₄∶NaHCO₃∶Na₂CO₃=12∶8∶9∶1) at 25 and 75 mmol·L^-1 were investigated after the seedlings had three leaves. The results showed that both low and high levels of saltalkaline stress significantly increased the proline contents in leaves during the whole stress compared with the control, and the increment was higher under the high concentration stress. Activities of pyrroline-5-carboxylate synthetase (P5CS) in leaves were enhanced significantly under the low concentration stress, while the activities of ornithine-δ-aminotransferase (δ-OAT) and proline dehydragenase (ProDH) were inhibited obviously. The δ-OAT activities were enhanced significantly, but P5CS and ProDH activities were decreased obviously under the high concentration stress. These changes demonstrated that the proline in naked oat seedlings was mainly metabolized through the pathway of glutamate under low concentration stress, and an ornithine pathway was predominant under high concentration stress. Moreover, the contents of free putrescine (Put), free spermidine (Spd), free spermine (Spm), conjugated Spd, bound Spd and bound Spm in leaves were increased significantly in the whole period of both levels of stress, and the contents of conjugated Put, conjugated Spm and bound Put were increased significantly in the 1-5 days of saltalkaline stress, but they were obviously decreased on day 7 under the same stress. All of these changes were more obvious under the high concentration stress than that under the low concentration stress. The values of Put/PAs and (cPAs+bPAs)/fPAs were increased significantly in the first 3 days, but decreased or without significant changes in the 5-7 days under the same stress, while the changes of (Spd+Spm)/Put demonstrated the contrary trend. Low concentration stress had lower Put/PAs value and higher (Spd+Spm)/Put value in the whole period, and the (cPAs+bPAs)/fPAs value was lower in the first day under the low concentration stress compared with the high concentration stress; during the 3-7 days, there were no significant differences between the two concentration treatments. Furthermore, the activities of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) were increased significantly in the whole period under each level of stress, and the diamine oxidase activities in the first 3 days of the stress decreased or slightly changed while a significant increase was observed in the 5-7 days. The change of polyamine oxidase (PAO) activities demonstrated the contrary to the DAO. These changes were much greater under the high concentration stress than under the low concentration stress. All of the results mentioned above suggested that salt-alkaline stress enhanced the capabilities for the transformation of free polyamine to conjugated and bound polyamines in the early period (1-3 days), and the low-concentration stress had stronger capabilities for the transformation of Put to Spd and Spm in the late period (5-7 days) compared with the high concentration stress, while the rate of PAs synthesis and decomposition in the whole period under the high concentration stress was faster than that under the low concentration stress.

Key words: spring wheat, irrigation and nitrogen application, permanent raised bed, grain yield., root