Discrimination between the ionic and osmotic components of salt stress in relation to free polyamine level in rice (Oryza sativa)

In order to precise the relative importance of ionic and osmotic components of salt stress on modification of free polyamine level, seedling of two rice cultivars (I Kong Pao (IKP): salt-sensitive and Pokkali: salt-resistant) were exposed to isoosmotic concentrations of NaCl, KCl (50 and 100 mM) or polyethylene glycol (PEG 6000, 16 and 26%) for a maximal period of 12 h in daylight conditions. Both ion and polyamine concentrations were already modified after 3 h of exposure to ionic stresses. At the shoot level, this response occurred independently of any change in the plant water status. Conversely, exposure to 26% PEG during 12 h induced a strong decrease in shoot osmotic potential and water content but had only a limited impact on endogenous free polyamine level. It was thus suggested that the ionic component by itself might trigger short-term polyamine accumulation. This effect might be attributed to monovalent ions, although K+ and Na+ accumulation had different effects on polyamine content. Putrescine may assume differential role in non-photosynthetic organs versus photosynthetic ones since it accumulated to high amounts in the roots of the salt-resistant cultivar Pokkali comparatively to salt-sensitive cultivar IKP, while an opposite trend was recorded in the shoots. Tyramine was also present at higher concentrations in the roots of Pokkali; its level clearly increased in response to ionic stresses while cadaverine level increased in water stress conditions only. No clear relationship was observed between the mean level of salinity resistance and the endogenous concentrations of spermidine or spermine.