Europ.J.Hort.Sci., 75 (1). S. 33–41, 2010, ISSN 1611-4426. © Verlag Eugen Ulmer KG, Stuttgart Europ.J.Hort.Sci. 1/2010 The Importance of K + in Ameliorating the Negative Effects of Salt Stress on the Growth of Pepper Plants J. S. Rubio 1) , F. García-Sánchez 1) , F. Rubio 1) , A. L. García 2) and V. Martínez 1) ( 1) Departamento de Nutrición Vegetal, Centro de Edafología y Biología Aplicada del Segura, CSIC. Campus Universitario de Espinardo, Espinardo, Murcia, Spain, and 2) Departamento de Química Agrícola, Univer- sidad de Murcia, Murcia, Spain.) Summary An experiment was conduced to study the response of pepper plants under saline conditions (30 mM NaCl) on growth, cation uptake, and water and ion relations. The physiological role of K + under the saline conditions was studied. Pepper plants were cultivated with five different nutrient solutions that included four K + treat- ments (0.2, 2.0, 7.0 and 14.0 mM K + ) with 30 mM NaCl and an additional treatment with 7 mM K + without salt. Plants were grown in hydroponics in a control- led-environment greenhouse and the experiment was carried out until the start of the fruiting phase. Salinity reduced reproductive organs dry weight and shoot dry weight but not root dry weight. Salinity did not affect water relations at dawn but affected ion relations in leaf sap and decreased relative water content at midday. The concentration in leaf sap of Cl – and the organic solutes fructose, glucose and myo-inositol increased in salt-stressed plants. In addition, salinity produced nutrient unbalance in both leaf and root by increasing the Cl – /NO 3 – ratio. On the other hand, in- creasing K + concentration in the nutrient solution in- creased salt tolerance as indicated by the increases in shoot and root dry weights. This effect was mainly due to the fact that plants under high K + concentrations are able to keep adequate water and ion relations with a high K/Na ratio while plants under low K + decreased the osmotic potential and showed dehydration leaf signs. Leaf sap oxalic acid concentration increased in parallel to leaf sap K + concentration and leaf sap fruc- tose and glucose decreased with the increase of the K + level in the nutrient solution. Key words. Capsicum annuum L. – growth – ion relations – salt stress – pepper – potassium – uptake – water Introduction Pepper is considered a salt sensitive species (MAAS and HOFFMAN 2006) with a salinity threshold about 2.8 dS m –1 in soilless culture (SONNEVELD and VANDERBURG 1991). The adverse effects of salinity in the vegetative growth of pepper plants are well documented (CHARTZOULAKIS and KLAPAKI 2000; DE PASCALE et al. 2003; NAVARRO et al. 2003; LYCOSKOUFIS et al. 2005). Stomatal conductance and photosynthesis rate of pepper plants decrease due to the accumulation of Na + and/or Cl – in the leaf laminas (BETHKE and DREW 1992). In addition to the toxic effect, the osmotic effect contributes to the growth reduction. It has been observed in pepper plants subjected to high NaCl or Na 2 SO 4 concentrations that growth reduction could have been due to leaf turgor potential decrease (NAVARRO et al. 2003), suggesting that pepper plants could not adjust osmotically under severe saline condi- tions. On the other hand, growth reduction at high Na + concentrations in the external medium may be due to a decrease in the K + concentrations and the Na/K ratio in leaves and roots as it has been previously reported in maize (BOTELLA et al. 1997) and in wheat (HU and SCHMID- HALTER 2005). Potassium is an important macronutrient in plants, which carries out vital functions in metabolism, growth and stress adaptation (MARSCHNER 1995). Whereas, most of K + is located within the central vacuole of plant cells, where it functions as an osmoticum (LEIGH and JONES 1984), in the cytosol the requirement for K + appears to be related to its role as an activator of biochemical process- es, particularly protein synthesis, photosynthesis and oxidative metabolism (MAATHUIS 2006). As a major inor- ganic osmolyte, the role of K + in cell osmoregulation and turgor maintenance is crucial in processes such as sto- matal aperture (GRAHAM and ULRICH 1972), cell expansion and turgor-driven movements (MAATHUIS and AMTMANN 1999). K + is not replaceable in its cytoplasmic functions and the plant maintains the cytoplasmic concentration of K + in the range of 100 to 200 mM (LEIGH and JONES 1984). In fact, most of the plants can cope (i. e. show no symp- toms of either deficiency or toxicity) with external K + con- centrations ranging from low µM to tens of mM (ASHER and OZANNE 1967). However, the optimum physiological K + concentrations narrow in the presence of increasing amounts of Na + (MAATHUIS and AMTMANN 1999). While the beneficial effects that the extra supply of K + may play on growth and yield of pepper growth under saline stress