Sodium chloride reduces growth and cytosolic calcium, but does not affect cytosolic pH, in root hairs of Arabidopsis thaliana L.

The effects of salinity (NaCl) stress on growth, cytosolic Ca²⁺ gradients and cytosolic pH homeostasis of root hairs of Arabidopsis thaliana are assessed here. Neither cytosolic Ca²⁺ nor pH at the hair apex were significantly affected by 20 min exposure of up to 90 mM NaCl or of up to 5 mM extracellular Ca²⁺. Exposure to increasing NaCl concentrations, up to 90 mM, for 2 d or 6 d reduced hair extension, and this inhibition was relieved by supplemental extracellular Ca²⁺. Such extended salinity stress reduced the magnitude of the Ca²⁺ gradient in the apical 12 μm of hairs at all NaCl concentrations tested (up to 90 mM), including NaCl concentrations that did not reduce hair extension. The magnitude of the tip-focused gradient was also reduced in root hairs of plants grown with low (0.5 mM) extracellular Ca²⁺ when compared to those in 5 mM extracellular Ca²⁺, regardless of the presence of NaCl. Up to 90 mM NaCl did not affect cytosolic pH of root hairs in any of the treatments. It is concluded that NaCl inhibition of root hair extension in the long term may operate via alterations in the tip-focused Ca²⁺ gradient that regulates root hair growth. However, NaCl-induced alterations in this gradient do not always lead to detectably altered growth kinetics. Short-term signalling events in response to NaCl may operate by a means other than altering Ca²⁺ at the root hair apex. Salinity stress in root hairs does not appear to be mediated by effects on cytosolic pH.