Exogenous hydrogen peroxide, nitric oxide and calcium mediate root ion fluxes in two non-secretor mangrove species subjected to NaCl stress

Using 3-month-old seedlings of Bruguiera gymnorrhiza (L.) Savigny and Kandelia candel (L.) Druce, we compared species differences in ionic homeostasis control between the two non-secretor mangrove species. A high salinity (400 mM NaCl, 4 weeks) resulted in a decline of the K+/Na+ ratio in root and leaf tissues, and the reduction was more pronounced in K. candel (41-66%) as compared with B. gymnorrhiza (5-36%). Salt-altered flux profiles of Na+, K+, H+ and Ca2+ in roots and effects of exogenous hydrogen peroxide (H2O2), nitric oxide (NO) and Ca2+ on root ion uxes were examined in seedlings that were hydroponically treated short term with 100 mM NaCl (ST, 24 h) and long term with 200 mM NaCl (LT, 7 days). Short term and LT salinity resulted in Na+ efflux and a correspondingly increased H+ influx in roots of both species, although a more pronounced effect was observed in B. gymnorrhiza. The salt-enhanced exchange of Na+ with H+ was obviously inhibited by amiloride (a Na+/H+ antiporter inhibitor) or sodium orthovanadate (a plasma membrane H+-ATPase inhibitor), indicating that the Na+ efflux resulted from active Na+ exclusion across the plasma membrane. Short term and LT salinity accelerated K+ efflux in the two species, but K. candel exhibited a higher flux rate. The salt-induced K+ efflux was markedly restricted by the K+ channel blocker, tetraethylammonium chloride, indicating that the K+ efflux is mediated by depolarization-activated channels, e.g., KORCs (outward rectifying K+ channels) and NSCCs (non-selective cation channels). Exogenous H2O2 application (10 mM) markedly increased the apparent Na+ efflux and limited K+ efflux in ST-treated roots, although H2O2 caused a higher Na+ efflux in B. gymnorrhiza roots. CaCl2 (10 mM) reduced the efflux of K+ in salinized roots of the two mangroves, but its enhancement of Na+ efflux was found only in B. gymnorrhiza. Under ST treatment, sodium nitroprusside (SNP) (100 proportional to M, an NO donor) increased Na+ efflux at the root apex of the two species; however, its inhibition of K+ loss was seen only in K. candel. Of note, NaCl caused an obvious influx of Ca2+ in B. gymnorrhiza roots, which was enhanced by H2O2 (10 mM). Therefore, the salt-induced Ca2+ benefits B. gymnorrhiza in maintaining K+/Na+ homeostasis under high external salinity.