Tree Physiology

Tree Physiology Advance Access originally published online on July 28, 2009
Tree Physiology 2009 29(9):1175-1186; doi:10.1093/treephys/tpp048

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Calcium mediates root K⁺/Na⁺ homeostasis in poplar species differing in salt tolerance

Jian Sun^1,*, Songxiang Dai^1,*, Ruigang Wang^1,*, Shaoliang Chen^1,2,3,**, Niya Li¹, Xiaoyang Zhou¹, Cunfu Lu¹, Xin Shen¹, Xiaojiang Zheng², Zanmin Hu⁴, Zengkai Zhang⁵, Jin Song⁵ and Yue Xu⁵

¹ College of Biological Sciences and Technology, Beijing Forestry University, P.O. Box 162, Beijing 100083, P.R. China
² Key Laboratory of Biological Resources Protection and Utilization in Hubei Province, Hubei University for Nationalities, Enshi 445000, P.R. China
³ Corresponding author (lschen{at}bjfu.edu.cn)
⁴ Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, P.R. China
⁵ Xuyue (Beijing) Science and Technology Co., Ltd., Yingzhi Dasha 601, Suzhoujie Street 49-3, Haidian District, Beijing 100080, P.R. China

	Abstract

Using the non-invasively ion-selective microelectrode technique, flux profiles of K⁺, Na⁺ and H⁺ in mature roots and apical regions, and the effects of Ca²⁺ on ion fluxes were investigated in salt-tolerant poplar species, Populus euphratica Oliver and salt-sensitive Populus simonii x (P. pyramidalis + Salix matsudana) (Populus popularis 35-44, P. popularis). Compared to P. popularis, P. euphratica roots exhibited a greater capacity to retain K⁺ after exposure to a salt shock (SS, 100 mM NaCl) and a long-term (LT) salinity (50 mM NaCl, 3 weeks). Salt shock-induced K⁺ efflux in the two species was markedly restricted by K⁺ channel blocker, tetraethylammonium chloride, but enhanced by sodium orthovanadate, the inhibitor of plasma membrane (PM) H⁺-ATPase, suggesting that the K⁺ efflux is mediated by depolarization-activated (DA) channels, e.g., KORCs (outward rectifying K⁺ channels) and NSCCs (non-selective cation channels). Populus euphratica roots were more effective to exclude Na⁺ than P. popularis in an LT experiment, resulting from the Na⁺/H⁺ antiport across the PM. Moreover, pharmacological evidence implies that the greater ability to control K⁺/Na⁺ homeostasis in salinized P. euphratica roots is associated with the higher H⁺-pumping activity, which provides an electrochemical H⁺ gradient for Na⁺/H⁺ exchange and simultaneously decreases the NaCl-induced depolarization of PM, thus reducing Na⁺ influx via NSCCs and K⁺ efflux through DA-KORCs and DA-NSCCs. Ca²⁺ application markedly limited salt-induced K⁺ efflux but enhanced the apparent Na⁺ efflux, thus enabling the two species, especially the salt-sensitive poplar, to retain K⁺/Na⁺ homeostasis in roots exposed to prolonged NaCl treatment.

Keywords: K⁺ flux, NaCl, Na⁺/H⁺ antiport, Populus euphratica, Populus popularis, the scanning ion-selective electrode technique, X-ray microanalysis

Received February 20, 2009; Accepted June 11, 2009

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^* These authors contributed equally to this work as first authors.

^** Present address: College of Biological Sciences and Technology, Beijing Forestry University, P.O. Box 162, Beijing 100083, P.R. China.

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