2.1 Drought stress
Human-induced changes in the atmosphere have led to increasing incidence of drought in many areas of the world, such as Central and South America, Southeast Asia, and the Mediterranean basin (IPCC, 2021). The incidence of drought in these regions is expected to worsen in the future (IPPC, 2021). By 2020, 5 billion people will be living in water scarce regions where crop production will be threatened by drought (UN-DESA, 2011). As a result of an increasing population and intensification of agriculture in drought prone areas, water demand for agriculture will double by 2050, while freshwater resources are expected to drop by 50% (Gupta et al., 2020). Drought significantly decreases crop growth and yield, and in the past decade has produced a loss in crop income of approximately $30 billion (Gupta et al., 2020). For example, the drought of summer 2012, is the most severe global drought recorded in recent years and caused $18 billion in crop losses (Schnoor, 2012). This drought caused production losses in corn (Zea mays ) (52%) and sorghum (Sorghum bicolor ) (51%) (Lal et al., 2012) led to significant yield reductions of 24-26% for these crops (Schnoor, 2012). Water is crucial for human and plant survival, and its deficit limits plant growth, development, and ultimately yield. Drought negatively affects plant growth from the cellular to the whole-plant level. This section will review the main effects of drought on plant physiological characteristics focusing on those that may be relevant for plant-biotic interactions.
With drought, the soil water potential decreases, leading to drop in root and leaf water potential (Liu et al., 2004). This drop in water potential is accompanied by tissue dehydration and loss in turgor (Reddy et al., 2004). To adapt to this lower water potential, plants tend to accumulate osmotolerant substances such as sugars, proline and other active amino acids that decrease the water potential in the cell allowing for retention of water (Roosens et al., 2002). The decrease in turgor is associated with a reduction in cell elongation and division. This results in stunted growth, observed as a reduction in leaf area and overall aboveground biomass (Farooq et al., 2009; Asrar & Elhindi, 2011).
At the leaf level, the cuticle is one barrier that protects the leaf from desiccation and limits water loss through the stomata. Drought increases cuticle thickness as an adaptation to reduce leaf transpiration (Bi et al., 2017). In fact, cultivars that show higher increases in cuticle thickness under drought are considered drought tolerant (Bi et al., 2017). Additionally, work in Arabidopsis has shown mutant lines (aldh2c4 ) with a 50% reduction in leaf cuticle thickness have higher water loss than wild type plants (Liu et al., 2022). Alterations in leaf thickness under drought stress also works to better regulate the balance between CO2 acquisition and water loss (Li et al., 2021). For this reason, some species respond to drought by thickening their leaves while others become thinner (Wellstein et al., 2016).
As the soil dries and soil and plant water potential decreases, the hormone abscisic acid (ABA) is synthesized in roots and leaves, and consequently, the stomata close and transpiration is reduced (Buckley, 2019). Additional drought adaptations include reductions in total stomatal number and size (Casson & Gray, 2008; Pitaloka et al., 2022). It has been documented that rice mutants with decreased stomatal density and size show higher yield and water use efficiency due to a reduced transpiration without any yield penalty under drought (Pitaloka et al., 2022). As the stomata are one of the main points of entry of leaf pathogens, drought acts to decrease pathogen entry through the stomatal pore, making the interaction of pathogens and drought a key area of research for future climate resiliency in plants.