## 7 Drought Stress and Mineral Nutrition  
Water deficit situations usually reduce the overall soil nutrient accessibility, root nutrient translocation and ultimately lessen the ion content in various plant tissues [102]. Water deficit conditions decreased plant potassium (K) uptake [103]. This decline in K was attributable to reduced K mobility, declined transpiration rate and weakened action of root membrane transporters [103; 104]. Decreased K amount was also found in drought-stressed plants of _Malus hupehensis_[105]. Resistant genotypes of _Tritcium durum_ had the maximum amount of K and susceptible genotypes had the maximum amount of sodium (Na) [69]. Genes encoding K transporters were inhibited by water deficit [106] and inner K channels are stimulated by a protein kinase, CIPK23, which in turn cooperates with calcineurin B-like calcium sensors. This K channel was inhibited in roots but activated in leaves of grapevine [107]. Leaf nitrogen (N) level did not change in drought-stressed _Mentha piperita_, _Salvia lavandulifolia_, _Salvia sclarea_ and _Thymus capitatus_, whereas, in _Lavandula latifolia_ and _Thymus mastichina_ plants, N content decreased while leaf phosphorus (P) level reduced in all species except _S. sclarea_ whose concentration remained the same [100]. This reduction in N was considered as the main responsible factor for photosynthesis decline and leaf senescence [108]. There was a significant reduction in leaf P amount in _Ocinum gratissimum_[109] and decline in K level in _Thymus daenensis_ under water deficit conditions [110]. K level also decreased in _Ocimum basilicum_ and _Ocimum americanum_ plants subjected to limited water availability [111]. Principally, decrease of K amount occurs in leaves because water scarcity disturbs stomata movement and guard cell turgidity, which results in decreased photosynthesis and, finally, the plant biomass production [112]. Drought-stress conditions increased the accumulation of manganese (Mn), molybdenum (Mo), P, K, copper (Cu), calcium (Ca) and zinc (Zn) in soybean [113].  
Plant Tolerance Mechanisms Against Drought Stress to Increase Crop Tolerance: How to Exploit These Mechanisms to Increase Crop Tolerance  
The intimal meaning of drought tolerance or drought resistance is still under debate. It is conceivable that water-saving plants mainly refer to the effective use of water resource in the process of growth and development of plants, thereby increasing crop water use efficiency (WUE) [114]. WUE is defined as the economic production per unit water consumption and it may or may not be related to drought resistance [115]. On the other hand, the main accepted definition of drought resistance is the ability of an individual to survive or grow in a water-stressed environment due to dehydration avoidance, dehydration tolerance or drought recovery, where dehydration is considered as the progressive loss of water content in plant tissue [115]. Discerning between drought tolerance or drought resistance can be very complex and is out of the scope of the present review, as there are already excellent papers dealing with this topic [116; 117]. Therefore, in the next paragraphs of the present review, plants able to tolerate drought stress conditions better than others are referred to as \"tolerant\" without any distinctions between drought tolerant or drought resistant.  
Plant drought tolerance encompasses alterations at morphological, biochemical and molecular levels (Figure 4). Exhibition of single or multiple tolerance factors governs the plant capability to survive under adverse drought conditions. From an applicative point of view, an in-depth knowledge of these mechanism can be exploited to select crop species/varieties/genotypes with a lower degree of sensitivity to limited water availability. Below, physiological, biochemical and molecular mechanisms which allow tolerant plants to tolerate better drought conditions are described with the attempt to propose some of them as suitable features for crop selection in the context of reduced water availability.