Plant biomass is primarily a product of photosynthesis, a process needing carbon dioxide, water as bi-products and solar radiation as the energy source and mineral nutrients as basic blocks. In majority of the instances carbon dioxide and solar radiation never limit biomass production while abiotic stresses like water deficit and soil salinity very often do. Plant response to abiotic stress is one of the most active research topics in plant biology due to its practical implications in agriculture, since abiotic stresses (mainly drought and high soil salinity) are the major cause for the reduction in crop biomass and yield worldwide, especially in the SAT.

Plants are extremely sensitive to changes resulting from drought or salinity, and do not generally adapt quickly (Lane and Jarvis 2007). Plants also adapt very differently from one another, even from a plant living in the same area. When a group of different plant species was prompted by a variety of different stress signals, such as drought or cold, each plant responded uniquely. Hardly any of the responses were similar, even though the plants had become accustomed to exactly the same home environment (Mittler 2006). Abiotic stresses can come in many forms. The occurrence of many of these abiotic stresses is unpredictable, however, in agricultural management point of view, drought and soil salinity are relatively more predictable and common in occurrence demanding focused research. Therefore, the scope of this chapter is limited to drought and soil salinity.