*4.4. Increasing Ascorbic Acid Decreases Tolerance to Drought Stress*

While ROS are generally detrimental they also serve as important signaling cues about the external environment, e.g., the role of H2O2 in guard cells in regulating gas exchange and transpiration in response to water availability [98]. Abscisic acid (ABA) can promote H2O2 production during periods of water limitation which signals for stomatal closure [99]. Although tobacco overexpressing DHAR grew normally under well-watered conditions, the higher Asc content in guard cells not only reduced their responsiveness to ozone but also their responsiveness to the onset of water stress which normally triggers stomatal closure to prevent further water loss [55]. The reduction in responsiveness can be understood through the role of Asc as a scavenger of H2O2 and the balance between H2O2 production and Asc establishes whether H2O2 rises to a level that triggers stomatal closure. As a consequence, increasing Asc in DHAR-overexpressing tobacco maintains H2O2 at a lower level which delays stomatal closure upon onset of water stress, resulting in greater open stomatal area, increased transpiration and water loss, and ultimately decreased tolerance

to water stress [55]. Reducing Asc content through suppressing DHAR expression results in an elevated accumulation of H2O2 in guard cells and a greater degree of stomatal closure even under non-stress conditions [55]. This hyperresponsiveness enables such plants to reduce transpiration during drought conditions resulting in up to 30% less water loss [55]. Thus, increasing Asc content throughout a plant confers protection against environmental ROS while reducing drought tolerance whereas reducing Asc content reduces CO2 assimilation under normal growth conditions as a consequence of the reduction in the open stomatal area but also reduces water loss resulting in improved drought tolerance. A strategy to increase foliar Asc content while maintaining normal levels of Asc in guard cells may improve nutritional value and tolerance to environmental ROS without increasing sensitivity to drought conditions.
