Effects of NaCl on cadmium uptake, accumulate in secretor and non-secretor mangrove species.

To explore the NaCl effects on Cd2+ uptake and transport in mangrove plants, Na+, Ca2+, Cd2+ accumulation in shoot and roots, and root Ca2+ and Cd2+ dynamic flux were examined in 3-month-old seedlings of the secretor mangrove Aegiceras corniculatum and non-secretor mangrove Kandelia obovate under NaCl and CdCl2 stress. The results show that Cd2+ content in roots, hypocotyl, stems, and leaves significantly increased with the increase of CdCl2 concentrations, in the two mangrove species. The Cd content in different organs of the secretor mangrove A. corniculatum was higher than in that of the non-secretor mangrove K. obovate, reaching 65% (root), 19% (hycopotyl), 203% (stem), and 96% (leaf) respectively. Using the non-invasive micro-test technique, the Cd2+ influx in roots upon CdCl2 stress was observed in two mangroves. Furthermore, the CdCl2-elicited Cd2+ influx was strongly inhibited by lanthanum chloride (LaCl3), a specific inhibitor of calcium channels. This indicates that Cd2+ enters root mainly through the calcium plasma membrane (PM) channels. We also examined the influence of NaCl on Cd2+ uptake. Low NaCl (100-200 mmol/L) was found to increase the Cd2+ uptake and accumulation, but a high NaCl (400 mmol/L) reduced the Cd2+ uptake in the two mangrove species. The possible explanations are: 1) Under NaCl treatment, the uptake of Na+ was increased by the root of mangrove. Thus, Na+ competed for metal-binding sites with Cd2+ in the PM transporters. As a result, the Cd2+ uptake was reduced in the two mangrove species. 2) NaCl increased Ca2+ influx under NaCl stress. This would inhibit the entry of Cd2+ through calcium channels, leading to a declined Cd2+ entry and accumulation in mangrove roots. Cd2+ accumulation in the secretor mangrove A. corniculatum was higher than the non-secretor mangrove K. obovate, indicating that the PM transporters and Ca2+ channels in A. corniculatum roots had a high capacity for Cd2+ uptake under NaCl and cadmium stress.