Effect of Soil Water Deficits on Plant–Water Relationship: A Review

Plants are often subjected to periods of soil and atmospheric water deficit during their life cycle. The frequency of such phenomena is likely to increase in the future even outside today’s arid/semi-arid regions. Under the climatic changing context, soil water deficit has been, and is becoming an acute problem most constraining plant growth, terrestrial ecosystem productivity, in many regions all over the world, particularly in arid and semi-arid area. With global warming, it is expected that soil water deficit would be escalated by increasing evapotranspiration, increasing the frequency and intensity of soil water deficit with an increase from 1% to 30% in extreme water deficit land area by 2100, which would offset the beneficial effect from the elevated CO2 concentration, further limiting the structure and function of the terrestrial ecosystem. Thus, an understanding of stress due to soil water deficit and water use in relation to plant growth is of importance for sustainable agriculture. Soil water deficit affects the growth, dry mater, and harvestable yield in a number of plant species, but the tolerance of any species to this menace varies remarkably. Plant responses to water scarcity are complex, involving deleterious and/or adaptive changes, and under field conditions these responses can be synergistically or antagonistically modified by the superimposition of other stresses. A ramified root system has been implicated in the tolerance to water deficit and high biomass production primarily due to its ability to extract more water from soil and its transport to above-ground parts for photosynthesis. In addition to other factors, changes in photosynthetic pigments are of paramount importance for tolerance to soil water deficit. Of the two photosynthetic pigments classes, carotenoids show multifarious roles in tolerance to water deficit including light harvesting and protection from oxidative damage caused by soil water deficit. Thus, increased contents specifically of carotenoids are important for stress tolerance. Differences among species that can be traced to different capacities for water acquisition, rather than to differences in metabolism at a given water status, are described. Changes in the root: shoot ratio or the temporary accumulation of reserves in the stem are accompanied by alterations in nitrogen and carbon metabolism, the fine regulation of which is still largely unknown. At the leaf level, the dissipation of excitation energy through processes other than photosynthetic carbon metabolism is an important defense mechanism under conditions of water stress and is accompanied by down-regulation of photochemistry and, in the longer term, of carbon metabolism.