Responses in polyamine titer under osmotic and salt stress in sorghum and maize seedlings

Changes in leaf water potential, K+/Na+ ratio and in polyamine titers in maize (Zea mays L. cv. Pioneer 3950) and sorghum (Sorghum bicolor (L.) Moench cv. ICSV 112) were investigated as the function of increasing equi-osmotic concentrations of NaCl or polyethylene glycol 6000 applied for 3 days in the nutrient solution. For both treatments, sorghum responded with a more intensive decrease in leaf water potential compared with maize. Plant analysis showed that under salinity, a higher level of Na+ was accumulated in the roots of maize than in those of sorghum while shoot Na+ concentrations were nearly the same. Internal K+ concentration was always higher in sorghum, leading to a K+/Na+ ratio 2 to 4-fold higher in the roots of sorghum compared with that in maize. In maize, NaCl and osmotic stresses evoked similar rates of polyamine accumulation, while in sorghum, osmotic stress was more effective in comparison to salt treatment. In maize, spermidine was apparently absent, indicating its fast turnover. In sorghum, the accumulation of di- and polyamines was proportional to the strength of the osmotic stress and the impairment of polyamine synthesis was also observed at high degrees of stress. The present data support the idea that the initiation of polyamine accumulation needs an osmotic signal; however, when a permeable ion is present, salt accumulation can contribute to the osmotic adjustment and thus the onset of polyamine biosynthesis is delayed or does not take place. Some enzymes in the polyamine biosynthetic pathway may be sensitive for high salinity and the biosynthetic processes shift towards oxidative degradation.