The Physiological Mechanisms Underlying Exogenous Nitric Oxide Mediated Stress Responses Of Populus× Canescens To Cadmium

Nitric oxide?NO?is a bioactive molecule in plants which mediates a variety of physiological processes and responses to biotic and abiotic stresses including heavy metals.Cadmium?Cd?is a non-redox toxic heavy metal present in the environment and negatively affects plant growth and development.Many reports indicate that exogenous NO improves plant tolerance to cadmium stress.Nitric oxide functions in Cd challenged plants in many different pathways,such as stimulating seed germination,regulating antioxidant system,and inhibiting Cd translocation from root to the shoot.Nevertheless,little is known about the mechanisms of NO mediated Cd stress in plants.Besides,relative researches in woody plants is still lacking.In the present study,the effects of exogenous NO?sodium nitroprusside,SNP,as a donor?on widetype Populus × canescens on cadmium?Cd?toxicity were investigated.Populus × canescens was pretreated with and without 200 ?M or / and500 ?M Nw-Nitro-L-arginine methyl ester hydrochloride?L-NAME,NO synthase inhibitor?for 24 hours,then treated together with 100 ?M CdCl2 for another 24 hours.The results show that:?1?Net Cd²⁺ influx in root apex was observed after 100 ?M CdCl2 treatment for24 h,which dramatically increased accumulation of Cd in tissues,decreased H⁺-ATPase activity in roots and leaves,increased H2O2 and MDA content,induced free proline accumulation,decreased activities of catalase?CAT?,superoxide dismutase?SOD?,peroxidase?POD?,and disturbed mineral absorption,decreased Zn?Mn content,increased Fe?Mn and K content;?2?Exogenous NO directly inhibited Cd²⁺ influx in root apex.Although addition of SNP did not change the maximum absorption site of Cd²⁺,the Cd²⁺ uptake rates in root apex was decreased.In addition,exogenous NO decreased Cd accumulation in both roots and shoots,increased Cd accumulation in leaves,decrased total Cd amount;?3?Exogenous NO affects Cd uptake,which may related to H⁺-ATPase.Exogenous NO increased H⁺-ATPase activity in roots,changed the inhibition of sodium orthovanadate to H⁺-ATPase during the Non-invasive Micro-test.Moreover,the absorption of Mg,Fe decreased,indicating that exogenous NO stimulated H⁺-ATPase activity to influence uptake of ions;?4?Exogenous NO significantly diminished Cd-induced H2O2 and MDA accumulation.It indicated a reactive oxygen species?ROS?scavenging activity of NO.SNP increased CAT and SOD activities,induced proline accumulation,indicating that exogenous NO improves antioxidative capacity under Cd stress.