Adaptive plasticity of salt-stressed root systems

Salinity can cause several challenges for plants, including water stress, mal-nutrition and accumulation of excess ions to potentially toxic levels. While salt exclusion, compartmentation and osmoregulation are the mechanisms particularly considered to increase the salt tolerance of plants, tolerance is determined by the integrating effects of several mechanisms at the cell, tissue and organ level. Because roots are in direct contact with the soil solution, they are first to encounter excess salinity and are potentially the first sites of damage or line of defence. However, despite the likelihood that differences among root systems may (partially) underlie distinct salt tolerances, information on the phenotypical and physiological plasticity of root systems under salt stress is scant compared to aboveground organs. This chapter reviews modifications among root size and architecture, morphological and anatomical root traits and root physiology under salinity. Furthermore, root elongation, halotropism and carbon metabolism of roots under salinity are addressed. The review explores the question of whether changes among roots are caused by ion toxicity or whether they could be an active response of plants, which may be of potential adaptive significance. A short overview of the chemical and physical properties of saline soils is given.