Effects of Ca 2 ,K  and cGMP on Na  uptake in pepper plants Francisco Rubio *, Pilar Flores, Josefa M. Navarro, Vicente Martı ´nez Departamento de Fisiologı ´a y Nutricio ´n Vegetal, Centro de Edafologı ´a y Biologı ´a Aplicada del Segura (CSIC), Apartado de Correos 4195, 30080 Murcia, Spain Received 2 April 2003; received in revised form 22 May 2003; accepted 1 July 2003 Abstract The effects of Ca 2 ,K  and cGMP on Na  influx in pepper plants (Capsicum annum L.) were studied. Calcium and K  inhibited Na  net uptake and had no effect on Na  efflux. The inhibition of Na  uptake by Ca 2 showed a K i of 0.46 mM and micromolar concentrations of the membrane permeable cyclic nucleotide 8-bromo-cGMP inhibited Na  uptake. These results suggest that transporters with low discrimination between K  and Na  , sensitive to Ca 2 and to cyclic nucleotides, may constitute important pathways for Na  influx. As suggested for other plant species, non-selective voltage-independent cation channels may mediate a large amount of Na  influx into pepper plants. # 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Calcium; Cation channel; cGMP; Pepper; Potassium; Sodium influx 1. Introduction Pepper is one of the most important vegetable crops produced under irrigated agriculture. In many areas, the water used for irrigation contains NaCl, which reduces production. Sodium ions are toxic for plants, and Na  exclusion is crucial for salt tolerance of salt-sensitive plants such as pepper [1,2]. Reduced Na  influx via the root system is one of the key processes in Na  exclusion. In maize, two cultivars that differ in their capacity to exclude Na  show different rates of Na  influx [3]. Unfortunately, the mechanisms for Na  influx across the plant plasma membrane have not been completely established. Cal- cium has been demonstrated to alleviate the toxic effects of Na  in plants [4,5]. The beneficial effect of Ca 2 has been related to an increased K  /Na  selectivity of the plasma membrane and it has been observed, in several plant species, that the presence of Ca 2 in solutions containing Na  and K  inhibits Na  and increases K  absorption [6  /8]. In barley roots, Ca 2 is essential for selective cation transport [9] and Ca 2 and K  inhibit Na  influx via the mechanism two of alkali cation transport described by Epstein [10]. Electrophysiological studies demonstrated the exis- tence of several types of cation channels that reside in the plasma membrane of plant cells and it has been proposed that non-selective cation channels greatly contribute to Na  entry [11  /13]. This type of channel shows a similar permeability for K  and Na  and is inhibited by Ca 2 [11]. In animals, non-selective ion channels are gated via binding to cyclic 3?,5?-adenosine monophosphate (cAMP) or cyclic 3?,5?-guanosine monophosphate (cGMP), and in Arabidopsis thaliana , cyclic nucleotides have been shown to regulate Na  uptake [14]. The molecular cloning and characterization of high- affinity plant K  transporters of the TRK-HKT and HAK-KT-KUP families showed that, at high external Na  ,K  uptake is inhibited and Na  is taken up through these transporters [15  /17]. Members of these two families may exclusively transport Na  [18,19] and others may equally transport K  or Na  [20]. In addition, cation transporters homologous to LCT1 of wheat may constitute an important pathway for Na  entry into the plant [21]. Abbreviations: cAMP, cyclic 3?,5?-adenosine monophosphate; cGMP, cyclic 3?,5?-guanosine monophosphate. * Corresponding author. Tel.:  /34-968-39-6234; fax:  /34-968-39- 6213. E-mail address: frubio@cebas.csic.es (F. Rubio). Plant Science 165 (2003) 1043  /1049 www.elsevier.com/locate/plantsci 0168-9452/03/$ - see front matter # 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S0168-9452(03)00297-8