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Plants are often exposed to unfavorable environmental conditions, for instance abiotic stresses, which dramatically alter distribution of plant species among ecological niches and limit the yields of crop species. Among these, drought stress is one of the most impacting factors which alter seriously the plant physiology, ﬁnally leading to the decline of the crop productivity. Drought stress causes in plants a set of morpho-anatomical, physiological and biochemical changes, mainly addressed to limit the loss of water by transpiration with the attempt to increase the plant water use efficiency. The stomata closure, one of the ﬁrst consistent reactions observed under drought, results in a series of consequent physiological/biochemical adjustments aimed at balancing the photosynthetic process as well as at enhancing the plant defense barriers against drought-promoted stress (e.g., stimulation of antioxidant systems, accumulation of osmolytes and stimulation of aquaporin synthesis), all representing an attempt by the plant to overcome the unfavorable period of limited water availability. In view of the severe changes in water availability imposed by climate change factors and considering the increasing human population, it is therefore of outmost importance to highlight: (i) how plants react to drought; (ii) the mechanisms of tolerance exhibited by some species/cultivars; and (iii) the techniques aimed at increasing the tolerance of crop species against limited water availability. All these aspects are necessary to respond to the continuously increasing demand for food, which unfortunately parallels the loss of arable land due to changes in rainfall dynamics and prolonged period of drought provoked by climate change factors. This review summarizes the most updated ﬁndings on the impact of drought stress on plant morphological, biochemical and physiological features and highlights plant mechanisms of tolerance which could be exploited to increase the plant capability to survive under limited water availability. In addition, possible applicative strategies to help the plant in counteracting unfavorable drought periods are also discussed.
Water deficit conditions stimulate several plant responses, such as morphological, physiological, biochemical and molecular alterations, which ultimately result in disturbing plant functioning  (Figure 1). As depicted in Figure 1, drought events limit plant performances in different developmental stages. Limited water availability can indeed reduce the germination rate and the development of young plants . During the progression of plant growth, drought basically influences the plant water relations, which in turn cause severe perturbation to the whole plant metabolism (at physiological, biochemical and molecular levels), depending to the stress severity and duration . Water deficit conditions alter several activities of plant, but one of the main effects is the decline of photosynthetic activity  and finally the plant yield . During drought stress conditions, oxidative stress, directly or indirectly generated in plants, is one of the main drivers of plant responses and results in damage to cell membrane, altering membrane integrity, physiological and biochemical alterations which lead to acute metabolic disorders and eventually alter the plant productivity .
Drought stress is well recognized as a limiting factor which alters multiple aspects of plant growth and development. Germination of seeds, health and coleoptile length are foremost for the plant progression . Seed germination is the primary aspect of growth which is sensitive to drought stress. Noteworthy alterations are observed in the seed germination of a plethora of plant species, including some of the most widely cultivated crops such as maize , sorghum  and wheat .
Visible symptoms of plant exposed to water scarcity in the initial vegetative stage are leaf wilting, decline in plant height and interruption in establishment of buds and flowers . Drought conditions also limit the uptake of nutrients by the plants due to limited soil moisture, leading to decreased stem length . Shoot length was also reduced under water deficit conditions in Lathyrus sativus L. . In conditions of water deficit, plants seek to extract water from deeper soil layers by boosting their root architecture . Moreover, water availability is primarily recognized by roots, which in turn regulates its growth and organization characteristics such as root length, spread, number and length of lateral roots . Roots are crucial for different biological activities and plant yield, for instance nutrient accumulation and water absorption, and they are also involved in rhizosphere symbiotic associations with other microorganisms. Drought stress escalated root length in Crocus sativus L. . Thus, a healthy root apparatus provides the benefit for sustenance of the escalation of plant growth, especially in the course of primary plant growth phase . Escalation in root length is recognized as a useful strategy to increase soil water retention and nutrient accumulation to enhance plant biomass production . Under water deficit, the plant root to shoot proportion generally improves, and, subsequently, the plant biomass decreases substantially .
The leaf is the chief part of the plant where most of the photosynthetic products are synthetized. The number of leaves decreased when subjected to water stress in Andrographis paniculate . Optimal leaf development and the maintenance of an adequate leaf area is vital for photosynthesis, which in turn is the main driver of plant growth. Water stress causes reduction in leaf area, which results in decreased photosynthesis, hence reducing the crop yield. Leaf area declined under water stress conditions in Petroselinum crispum L. and in Stevia rabaudiana plants to achieve stability among the water absorbed by roots and the water status of various plant parts . Reduction in leaf area is a drought avoidance strategy because declining leaf area results in a decreased water loss by the process of transpiration and this reduction in leaf area is attributable to the inhibition of leaf expansion by declined rate of cell division, which results in loss of cell turgidity . Decrease in soil moisture causes a parallel reduction of leaf water content, which, in turn, induces a decline of turgor pressure of guard cells due to stomata closure . Of note, the rate of premature leaf senescence is enhanced in drought environments .
Major consequence of water deficit in plants is the decrease or suppression of photosynthesis (Figure 2). Reduced leaf area, increased stomata closure and consequent reduced leaf cooling by evapotranspiration increases osmotic stress leading to damages to the photosynthetic apparatus are among the major constraints for photosynthesis . Among these, the decrease in photosynthetic process in plants under drought is mainly attributable to the decline in CO2 conductance via stomata and mesophyll limitations . Decrease in photosynthetic activity due to drought may also be due to reduced ability of stomatal movement . Declined activity of photosynthesis is triggered by the loss of CO2  uptake, whose drop has been shown to affect Rubisco activity and decrease the function of nitrate reductase and sucrose phosphate synthase and the ability for ribulose bisphosphate (RuBP) production. Supportively, CO2 enrichment eliminated many early responses of maize metabolites and transcripts attributable to drought stress .
Figure 2. Schematic representation of effect of drought stress on photosynthesis .
Water deficit also resulted in decreased leaf area per shoot, and, thus, modification in canopy architecture, and this feature can alter gas exchange, water relations, vegetative growth and sink development (e.g., fruits or grains) , altering, for example, berry sugar concentration in grape  and biomass partition in maize (i.e., kernel number and 100-kernel dry weight decreased with increasing water stress duration) .
Chlorophyll content, which is of outmost importance for photosynthesis , is another photosynthetic attribute strongly influenced by water deficit that has been recognized as a distinctive indication of photo oxidation and degradation of chlorophylls . For example, leaf chlorophyll synthesis and chlorophyll a/b proportion in soybean is altered by drought stress . Decline in photosynthetic activity, amount of chlorophylls, loss of photosystem II photochemical efficiency, alteration in stomatal movement and disturbance in water status of plants resulted in declined plant productivity . Among others, a major cause for decline in amount of chlorophyll due to drought stress is the drought-promoted O2− and H2O2, which results in lipid peroxidation and ultimately chlorophyll degradation . The decrease of plant development and yield in several plant species under water deficit is often associated with decline in photosynthetic action and chlorophyll content impairment . Water deficit alters the action of photosynthetic moieties and chlorophyll pigments, which ultimately resulted in reduced photosynthetic activities in Vigna mungo .
Drought stress induces a decreased net photosynthesis and also changes the plant carbon allocation and metabolism, which ultimately results in energy dissipation and declined yield . For example, drought stress decreased the physiological metabolic disorders by suppressing the photosynthetic products production and disrupting the carbon balance in soybean. Drought stress also caused a reduction in the abundance of several Calvin cycle proteins, including Rubisco downregulation in olive . Acute drought stress conditions also cause the damage to Rubisco enzyme and other enzymes associated with photosynthesis and are responsible for the loss of photosynthetic pigment content .