Sodium - calcium interactions under salinity stress 205 CHAPTER 10 SODIUM - CALCIUM INTERACTIONS UNDER SALINITY STRESS

There are a wide range of responses of plants to salinity which involve interactions of Na with Ca. Plant processes such as growth, photosynthesis, mineral nutrition, water and ion transport are affected by these Na-Ca interactions. Many of these responses can be linked to the direct Na-Ca interactions at the surface of the plasma membrane and subsequent Ca signaling events. 10.1. General Effects of Salinity Salinity affects plant growth through ionic and osmotic effects. Sometimes these effects are distinct from each other; sometimes these effects overlap. The difference in a plant’s response to a given level of salinity is dependent upon the concentration and composition of the ions in solution as well as the genotype that is exposed to the salinity. Once the importance of Ca in the external solution was fully appreciated (Epstein, 1961), there was a large increase in the number of experiments that focused on the interactions of Na with Ca in plants. Na-Ca interactions are particularly apparent in plants when the Na:Ca ratio in the external solution is above 17 (Greenway and Munns, 1980), but again the plant response will vary with genotype. 10.2. General Characteristics of Na and Ca 10.2.1. PHYSICAL PROPERTIES OF Na AND Ca