[Effects of NaCl stress on free polyamines content and reactive oxygen species level in pumpkin roots]

Taking Cucurbita moschata Duch hybrid 360-3 x 112-2 and C. ficifolia Bouche as test materials, the effects of NaCl stress on their plant growth and the O2*- production rate and H2O2 and free polyamines (PAs) contents in their roots were studied with hydroponic culture. The results showed that after 10 d NaCl stress, the plant growth of the two pumpkin varieties was strongly inhibited, compared with the control, and C. ficifolia was more injured than hybrid 360-3 x 112-2. Under NaCl stress, the root O2*- production rate and H2O2 content of the two pumpkin varieties were increased, but their absolute values were lower in hybrid 360-3 x 112-2 than in C. ficifolia. The contents of PAs, putrescine (Put), spermidine (Spd) and spermine (Spm) and Put/PAs ratio in the roots of the two pumpkin varieties were always higher than the control and had a trend of increased first and decreased then; while the (Spd + Spm)/Put ratio was lower than the control and decreased first and increased then. Compared with C. ficifolia Bouche, hybrid 360-3 x 112-2 always had a lower Put/PAs ratio and a lower Put content in its roots, but the (Spd + Spm)/Put ratio and Spd and Spm contents were always higher. It was concluded that under NaCl stress, the increasing PAs content in the roots of test materials played an active role in decreasing or scavenging reactive oxygen species (ROS). The conversion of Put to Spd and Spm was advantageous to the increase of plant salt tolerance. The higher salt tolerance of hybrid 360-3 x 112-2 was closely related to the lower Put/PAs ratio and the higher (Spd + Spm)/Put ratio and PAs content in its roots, and thus, the stronger capacity to scavenge ROS.