Effects of NaCl and mannitol iso-osmotic stresses on the free polyamine levels in leaf discs of tomato species differing in salt tolerance

Short-term changes in free polyamines induced by salt (200 mmol/L NaCl) and osmotic (400 mmol/L mannitol) stresses were determined in leaf discs of the salt-sensitive cultivated tomato species (Lycopersicon esculentum) and its wild salt-tolerant relative L. pennellii with the objective of checking whether these metabolites could be used as early traits of tolerance. In L. pennellii, a putrescine and spermidine accumulation was found in the osmotically-stressed leaf discs from 30 min until the end of the experiment (2h). Under salt stress, the free polyamine accumulation in L. pennellii only took place during the first 15 min, mainly due to putrescine, whereas its levels decreased after this time. These results suggest that the initial polyamine accumulation was due to the osmotic effect induced by salinity, but the levels rapidly began to decrease as the saline ions were accumulated from the medium. In a similar manner to the wild species, the leaf discs of L. esculentum submitted to salt stress also accumulated Na+ from the beginning of the experiment, although the accumulation was slower. Nevertheless, the patterns of free polyamines were similar under both salt and osmotic stresses in L. esculentum, which suggests that the results only show the osmotic shock. In this species, the polyamine levels tended to decrease at the beginning of the experiment, whereas the opposite response was observed at the end of the period (between 1 and 2 h). The results obtained show that the polyamine levels change in a very different way in the leaf discs of both tomato species. Moreover, NaCl stress and non-ionic osmotic stress caused by mannitol are separable only in the salt-tolerant species in terms of their effects on free polyamine levels.