Differential tolerance to combined salinity and O2 deficiency in the halophytic grasses Puccinellia ciliata and Thinopyrum ponticum: the importance of K+ retention in roots

Saline environments of terrestrial halophytes are often prone to waterlogging, yet the effects on halophytes of combined salinity and waterlogging have rarely been studied. Either salinity or hypoxia (low O₂) alone can interfere with K⁺ homeostasis, therefore the combination of salinity or hypoxia is expected to impact significantly on K⁺ retention in roots. We studied mechanisms of tolerance to the interaction of salinity with hypoxia in <i>Puccinellia ciliata</i> and <i>Thinopyrum ponticum</i>, halophytic grasses that differ in waterlogging tolerance. Plants were exposed to aerated and stagnant saline (250 mM NaCl) treatments with low (0.25 mM) and high (4 mM) K⁺ levels; growth, net ion fluxes and tissue ion concentrations were determined. <i>P. ciliata</i> was more tolerant than <i>T. ponticum</i> to stagnant-saline treatment, producing twice the biomass of adventitious roots, which accumulated high levels of Na⁺, and had lower shoot Na⁺. After 24 h of saline hypoxic treatment, MIFE measurements revealed a net uptake of K⁺ (~­40 nmol m⁻² s⁻¹) for <i>P. ciliata</i>, but a net loss of K⁺ (~­20 nmol m⁻² s⁻¹) for the more waterlogging sensitive <i>T. ponticum</i>. NaCl alone induced K⁺ efflux from roots of both species, with channel blocker tests implicating GORK-like channels. <i>P. ciliata</i> had constitutively a more negative root cell membrane potential than <i>T. ponticum</i> (−150 versus −115 mV). Tolerance to salinity and hypoxia in <i>P. ciliata</i> is related to increased production of adventitious roots, regulation of shoot K⁺/Na⁺, and a superior ability to maintain negative membrane potential in root cells, resulting in greater retention of K⁺.