A salt-tolerant cultivar of wheat maintains photosynthetic activity by suppressing sodium uptake

The effects of NaCl treatment on the photosynthetic machinery in wheat (Triticum aestivum L.) cultivars differing in salt tolerance were investigated by comparison with iso-osmotic PEG treatment. Both cultivars similarly reduced the photosystem 2 (PS2) energy conversion efficiency (Phi(PS2)) rapidly when plants were exposed to a 100 mM NaCl solution, though no decline was detected under the iso-osmotic PEG treatment. There was no correlation between the reduction of the leaf relative water content (RWC) and the Phi(PS2) in the two iso-osmotic stress treatments. In contrast, a decline of Phi(PS2) along with the increase of the leaf sodium content above 4% dry matter was detected under the NaCl treatment, while no such correlation was detected with other cations. The recovery Of Phi(PS2) after photoinhibitory irradiation was repressed by the NaCl treatment as the increase of the duration of the treatment. Norin 61 subjected to the 100 mM NaCl treatment for 10 d showed a decline of the Phi(PS2) after 1 h moderate irradiation of 400 mumol m(-2) s(-1) PPFD. Thus the concentrated Na+ within a leaf under salinity treatments may decrease the stability of PS2 functions and lead to photochemical inactivation.