## 1 Introduction  
Plants experience continuous fluctuations of environmental conditions and are often exposed to abiotic stresses, for instance shortage of available water, salinity, excess light, high/low temperatures and nutrient imbalance, all leading to impairment of plant performance [1]. The capability of plants to respond to abiotic stress is associated with their plasticity as well as the adaptableness of plant traits to the fluctuating conditions of water availability [2]. Amongst these limiting abiotic factors, drought (or water deficit) stress is extensively studied given that it is likely the main constraint for crop productivity in many arid and semi-arid areas worldwide [3]. Water deficit occurs when the plant water requirement cannot be fully satisfied and this situation takes place when the level of transpired water exceed the water taken up by the roots, which is caused by inadequate precipitation, decreased ground water level or the retention of water by soil particles [4; 5]. As a result of water stress, plants respond with morpho-anatomical, physiological and biochemical adjustments aimed at counteracting the loss of water with the attempt to preserve their hydric status [2]. Being sessile organisms, plants have to face several adverse factors in natural environments, and, for this reason, they possess numerous defense strategies and have evolved several resistance mechanisms through which they cope with abiotic stresses [6]. Enduring severe water deficit periods, which relies on plant-genotype-specific features, also depends upon stress intensity, duration, speed and recovery effectiveness to regulate plant performance [7; 8]. In the case of water scarcity, plants need to respond quickly, thus virtually all biological functions are altered by water deficit conditions at whole plant level [9; 10]. Plants have to stimulate different strategies that benefit them to absorb water through their roots and to uphold cell turgor, i.e., evade the water loss [11]. Declined frequency of cell division and cell enlargement, root differentiation, foliage dimensions, shoot length, altered stomatal movements, water and mineral nutrition association with decreased plant yield and water usage efficacy are major outcomes of drought in plants [12]. Photosynthesis activity is decreased primarily by closing of stomata, membrane injury and altered functioning of several enzymes, specifically those which are associated with ATP synthesis [12; 13]. Drought stress conditions also result in increased generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which disturb the cell redox regulatory functioning [8; 14]. Plants that are able to tolerate drought stress for extended periods and sustain their vigor and yield represent one of the foremost exploration fields in agriculture studies [15]. As detailed below, tolerant plants may benefit from different features which allow them to tolerate better than others the effect of water scarcity. For example, among morpho-anatomical features, a well-developed root apparatus ensures the plant a deeper exploration of the soil thereby increasing the capability of water uptake [16]. Other physiological (e.g., rapid stomata closure and water use efficiency) and/or biochemical responses (e.g., synthesis of osmolytes, aquaporins and a powerful antioxidant apparatus) may contribute in increasing the drought tolerance of some plant individuals [17], thereby supporting the use of those drought-tolerant genotypes/varieties. Besides the exploitation of plant tolerant genotypes/varieties based on classic breeding selection, some applicative strategies have also been applied to attempt to overcome drought effects in crop species. For example, under controlled circumstances, regulated deficit irrigation may allow to obtain positive results in plant growth, likely due to a significant overproduction of advantageous moieties such as sugars, organic acids and antioxidant compounds [18; 19].