Abstract
Salinity and drought are two important abiotic stresses that limit the production of food crops worldwide. Plant responses to salinity and drought are often similar. For example, the first phase of salinity stress, the osmotic effect, is quite similar to that of drought stress. Plant roots have effective mechanisms to sense low water potential that result either from an actual lack of water in the environment, due to low precipitation, or to the presence of excess salt ions in the water present. In both cases plants cannot uptake sufficient water for normal growth and development and common stress related signal transduction pathways are activated, for example increased levels of ABA. Immediately following the induction of these stress related signal transduction pathways, under drought or salinity, shoot growth, especially leaf growth, is greatly reduced. A significant contributor to this growth reduction is associated with a change in cellular oxygen and carbon dioxide levels due to the partial closure of stomata. Plant cells also produce various reactive oxygen species (ROS) in response to both salt and drought stress. Increased ROS levels are found in both the apoplast and the cytoplasm. Elevated ROS levels under drought and salinity result in changes in normal cellular metabolism and cellular redox homeostasis, and also ROS dependent modification to plant cell‐wall structure, which either restricts cell expansion by cell‐wall strengthening, or favours cell expansion by loosening the wall structure. This chapter reviews the similarities and differences between the salinity and drought‐induced oxidative state of cells that account for the inhibition of shoot growth.