Physiological effects of water stress

Water deficit leads directly to stomatal closure and reduces the potential for CO2 fixation relative to well-watered plants. Closure is caused both by hydraulic effects and by chemical signalling, the latter being an adaptive function that increases transpiration efficiency (Davies et al., 2005) and is the basis of the common practice of deficit irrigation in water-scarce environments (Fereres and Soriano, 2007). A consequence of reduced transpiration rate may be that plant organs experience heat stress (see next section). Increasing water deficit leads to changes in tissue water potentials that may be suboptimal for expansive growth and metabolism (Hsiao, 2003). Osmotic adjustment is commonly observed under water deficit to resist further dehydration and to maintain favourable gradients of water potential that permit growth to continue (Morgan, 2000). If these drought-adaptive strategies are insufficient to maintain growth and development, reproductive behaviour will be impaired leading to floret sterility and/or inadequate levels of assimilation to sustain seed growth (see Barnabas et al., 2008). Cessation of growth may be followed by tissue dehydration if water stress is not relieved, potentially resulting in damage to the photosynthetic apparatus and other metabolic processes (Ghannoum, 2009). A more recently observed phenomenon under drought is that of micronutrient deficiency caused by reduced transpiration rates under water deficit. Zinc is involved in detoxification of reactive oxygen species (ROS) so low rates of passive uptake coupled with increased production of ROS under moisture stress combine to exacerbate drought-stress symptoms in soils that are zinc deficient (Bagci et al., 2007).

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