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.