## 7 Drought Stress and Mineral Nutrition
### Morphological and Biochemical Mechanisms Involved in Drought Tolerance  
Plants survival to drought encompasses two main strategies: drought avoidance and drought tolerance [53]. Plants have adopted several strategies to increase their drought tolerance at different levels, morphological, physiological, biochemical, and molecular. Conversely, some plant species avoid water deficit situations by accomplishing, for example, their life cycle before or after a drought period while some other plants displayed adaptations to escalate water absorption and decrease water loss to circumvent its adverse consequences [1]. At the morphological level, root is one the major drivers of water; therefore, the root size, its progression rate and density and root proliferation are important features which prompt plant responses to drought stress [5]. Plants with a deep root organization and a perennial development system showed more ability to cope with drought in comparison to plants with shallow-root system [119]. In view of the above, the selection of genotypes with a more developed root apparatus resulted in increased plant yield, as demonstrated for example in rice seedlings [120] and tobacco [121]. When drought stress occurs at initial phases of plant growth, drought-avoidance plants gradually change to succulent types or develop advanced drought tolerance strategies such as generation of compatible solutes, enhancement of antioxidant apparatus and other physiological responses aimed at increase the water use efficiency [122]. [PERSON] et al. [123] demonstrated indeed, that selection of grape varieties with drought tolerance should follow the analyses of water use efficiency increased for example by the proper selection of rootstocks. Plants avoid water loss by stomata closure, thus decreasing evapotranspiration and increasing water use efficiency [124], therefore stomata regulation is of outmost importance in increasing WUE. Drought tolerance, water use efficiency and K\({}^{+}\) content have close  
Figure 4: Diagram showing plants drought tolerance mechanism. TFs, transcription factors; DST, drought and salt tolerance; SERF, serum response factor; SKIP, ski-interacting protein; ZFP, zinc finger TF; SNAC, stress responsive NAC TF; LEA, late embryogenesis abundant; ABA, abscisic acid; SROs, similar to RCD-ONE; CDPKs, Ca\({}^{2+}\) dependent protein kinases; MAPKs, mitogen activated protein kinases; PPs, protein phosphatases; CIPKs, CBL interacting protein kinases (modified from [PERSON] et al. [118]). associations in plants as a sufficient level of K\({}^{+}\) can improve the plant total dry mass and photosynthetic rate; K\({}^{+}\) also regulates the SOD enzyme activity to mitigate the cell membrane injury which is caused by drought-triggered ROS [125]. Besides stomatal movement, drought stress may promote changes of leaf morpho-anatomical traits including vascular bundle per unit leaf area [126], stomata density and leaf thickness [127]. For leaf thickness, especially palisade parenchyma could contain larger numbers of CO\({}_{2}\)-fixation sites [128]. On the other hand, increases of epidermis thickness represent a way to contrast water loss under water limitation and both palisade and epidermis thicknesses can be used to select more tolerant olive genotypes [128]. However, despite drought-promoted morpho-anatomical traits, biochemical limitations might have a greater impact on plant performances [129]. At the biochemical level, plant hormones, secondary metabolites and other key molecules such as carbohydrate, amino acid and polyamines play crucial roles in stress tolerance mechanism and improving the capability of plant adaptation by altering their membrane stabilization, osmoregulation, ROS scavenging, lessening leaf area and its abscission, promoting root development and reducing ion leakage [130]. Osmolytes accumulation is essential for osmo-protection and osmotic adjustment against water deficit conditions which can lead to loss of cell turgor and dehydration.