Plants experience water stress either when the water supply to their roots becomes limiting
or when the transpiration rate becomes intense. Water stress is primarily caused by the
water deficit, i.e. drought or high soil salinity. In case of high soil salinity and also in other
conditions like flooding and low soil temperature, water exists in soil solution but plants
cannot uptake it – a situation commonly known as ‘physiological drought’. Drought occurs
in many parts of the world every year, frequently experienced in the field grown plants
under arid and semi-arid climates. Regions with adequate but non-uniform precipitation
also experience water limiting environments.
Since the dawn of agriculture, mild to severe drought has been one of the major productionlimiting
factors. Consequently, the ability of plants to withstand such stress is of immense
economic importance. The general effects of drought on plant growth are fairly well known.
However, the primary effect of water deficit at the biochemical and molecular levels are not
considerably understood yet and such understanding is crucial. All plants have tolerance to
water stress, but the extent varies from species to species. Knowledge of the biochemical and
molecular responses to drought is essential for a holistic perception of plant resistance
mechanisms to water limited conditions in higher plants.