Climate change is predicted to bring regional-scale precipitation extremes, causing both flooding and drought in certain areas (Giorgi et al., 1998). Most model simulations predict decreased precipitation by the end of the 21st century in subtropical regions (IPCC, 2007). Increased precipitation extremes are also likely in major agricultural production areas, e.g., southern and eastern Asia, eastern Australia, and northern Europe (Christensen et al., 2007). The 2003 summer drought in Europe caused a severe reduction in corn yield in eastern Europe (Ciais et al., 2005) and in forest biomass productivity in southern Europe (Gobron et al., 2005).
Water shortage is one of the most important factors limiting crop production worldwide due to geographic limited availability of irrigation water or the occurrence of drought mainly caused by reduced rainfall. Demand for drought-tolerant genotypes will increase due to diminishing water resources and alteration in precipitation patterns under climate change scenarios (Longenberger et al., 2006; Christensen et al., 2007). Understanding the detrimental effects of drought on plant processes and identifying the tolerance mechanisms will help breeders to develop tolerant genotypes.
Drought stress induces several changes in various physiological, biochemical, and molecular components of photosynthesis. Drought can influence photosynthesis through either pathway regulation by stomatal closure and decreasing flow of CO2 into mesophyll tissue or by directly impairing metabolic activities. The main metabolic changes are declines in regeneration of Ribulose bisphosphate (RuBP) and Rubisco protein content, decreased Rubisco activity, impairment of ATP
synthesis and photophosphorylation, and decreased inorganic phosphorus. Effects of drought on whole plant processes are manifold and can influence germination, emergence, leaf, root, tillers, stem development and growth, dry matter production, floral initiation, panicle exertion, pollination, fertilization, seed growth, seed yield, and seed quality. For most crop plants seed is the starting point of the growth cycle. Seeds begin biochemical changes shortly after imbibing water. Water uptake and imbibition of water by seed are dependent upon the soil water availability. Drought delays imbibition and thus can lead to decreased germination rates and total germination percentage. Leaf expansion is one of the growth processes most sensitive to drought (Alves and Setter, 2004; Reddy et al., 2009). This sensitivity is expressed in terms of smaller cells and reductions in the number of cells produced by leaf meristems (Randall and Sinclair, 1988; Tardieu et al., 2000). Drought stress can also influence total leaf area through its effect on the initiation of new leaves, which decreases under drought stress. Drought and heat stress alter the initiation and duration of developmental phases. In most cases, the length of time from floral initiation to anthesis is decreased by moderate drought and/or temperature stress, but is increased by severe stress. Drought stress during panicle development inhibits the conversion of vegetative to reproductive phase, and plants remain vegetative until this stress is relieved. Panicle initiation in sorghum was delayed by as many as 2 to 25 days and flowering by 1 to 59 days under drought stress, with more severe effects when drought was imposed at both early and late stages of panicle development (Craufurd et al., 1993). Drought stress inhibits pollen development and causes sterility. It also shortens the spike development duration (period during which potential kernel or seed numbers are determined) and the grain filling duration (during which the grain or seed weight is determined). Drought stress during later stages of panicle or flower development decreases seed numbers and can also increase the duration from seed set to full seed growth. Drought affects yield by limiting seed numbers caused by either influencing the amount of dry matter produced by the time of flowering (this is particularly true for determinate plant types), or by directly influencing pollen or ovule function which leads o a decreased seed set. Secondarily, drought influences seed filling by limiting the assimilate supply, leading to smaller seed size and lower yields.
Past difficulties have been associated with the identification of physiological traits that could be used as indicators of drought tolerance (Longenberger et al., 2006). However, various plant characteristics such as water use efficiency (Condon et al., 2002), root characteristics (Basal et al., 2003), canopy temperature (Patel et al., 2001), leaf water potential and leaf relative water content (Chiulele and Agenbag, 2004), and stomatal conductance (Bota et al., 2001; Flexas et al., 2002; Medrano et al., 2002) have been used as possible indicators to assess drought tolerance in crop species. Understanding the mechanisms of drought tolerance in crop species, particularly those adapted to dry conditions, will help plant breeders improve agronomic performance of these species by incorporating the superior traits into new species or cultivars (Clavel et al., 2005).
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