and disrupts xanthophyll cycle. When exposed to excessive light stress, plants may
sense high photon flux. Shade loving plants get damaged even by brief exposure of
strong light. In plants the excessive light damage photosystem II by destroying
photosensitive pigments and thylakoid structures, thus inhibiting photosynthesis.
There is disruption in electron transport chain and breaking of protein sub-units
(Hopkins and Huner 2009). To avoid damage by strong light, plants show some
adaptive features such as orienting leaves at an angle to receive less radiation, rolling
up of shoots, dense covering of trichomes on upper surface of leaves, thickened cell
walls, and production of anthocyanin pigment to shield mesophyll (Balfagón et al.
2019).
In tomato (Solanum lycopersicum) lipidomics analysis identified lipophilic antioxidant molecules, which protects PS II from photodamage (Spicher et al. 2017).
The study conducted by Balfagón et al. (2019) showed that in Arabidopsis, in high
light intensity combined with heat stress, the stomata opens to increase transpiration
and cool the leaf instead of reducing stomatal aperture (Devireddy et al. 2018).
1.2.3 Water Stress
Water which is present in 80–90% in non-woody plants is key molecules for all
metabolic processes. It is essential and when it becomes limiting and is not available
to roots or when transpiration rate is very high it becomes a stress factor. Water stress
may arise when there is water deficit or excess of water (flooding). During flooding
oxygen becomes limiting and is not supplied to roots as per requirement. Due to
water deficit stress there is desiccation; protoplasm dries up, solute concentration
increases, leading to loss in integrity of membranes. Selectivity of membranes is lost.
Proteins get denatured and displaced, and finally leading to loss of cellular compartmentalization. Due to water deficit the water potential inside the cells decreases
leading to stomatal closure. It is also known that hydropassive stomatal closure
occurs in plants to avoid transpirational loss and is evident in all tropical, temperate,
or dessert plants (Hopkins and Huner 2009). Stomatal closure limits gaseous
exchange, reduced transpiration and photosynthesis, uptake of mineral nutrients
and disruption of homeostasis and ions. Drought stress also causes various other
adverse effects such as decreased water potential leading to hydro active closure of
stomata, lesser number of stomata, thickening of leaf cell wall, decreased cell
enlargement, slow or inhibited growth and reproduction, cutinization of leaf surface,
poorly developed conducting system, increase in root–shoot ratio, production of
osmolytes such as proline, formation of reactive oxygen species (ROS), ascorbate,
and glutathione accumulates in cell and further aggravates the condition (Lisar et al.
2012). Water deficits also alter the cell wall confirmations, non-enzymatically
through the interaction of pectate and calcium (Boyer 2009; Cramer et al. 2011).
Although photosynthesis gets disrupted in both C3 and C4 plants but studies show
that C4 plants are more sensitive to drought. Water stress also alters the mRNA
expression and thus new proteins are synthesized. Different proteins which are
1 Abiotic Stress in Plants: An Overview
7
sense high photon flux. Shade loving plants get damaged even by brief exposure of
strong light. In plants the excessive light damage photosystem II by destroying
photosensitive pigments and thylakoid structures, thus inhibiting photosynthesis.
There is disruption in electron transport chain and breaking of protein sub-units
(Hopkins and Huner 2009). To avoid damage by strong light, plants show some
adaptive features such as orienting leaves at an angle to receive less radiation, rolling
up of shoots, dense covering of trichomes on upper surface of leaves, thickened cell
walls, and production of anthocyanin pigment to shield mesophyll (Balfagón et al.
2019).
In tomato (Solanum lycopersicum) lipidomics analysis identified lipophilic antioxidant molecules, which protects PS II from photodamage (Spicher et al. 2017).
The study conducted by Balfagón et al. (2019) showed that in Arabidopsis, in high
light intensity combined with heat stress, the stomata opens to increase transpiration
and cool the leaf instead of reducing stomatal aperture (Devireddy et al. 2018).
1.2.3 Water Stress
Water which is present in 80–90% in non-woody plants is key molecules for all
metabolic processes. It is essential and when it becomes limiting and is not available
to roots or when transpiration rate is very high it becomes a stress factor. Water stress
may arise when there is water deficit or excess of water (flooding). During flooding
oxygen becomes limiting and is not supplied to roots as per requirement. Due to
water deficit stress there is desiccation; protoplasm dries up, solute concentration
increases, leading to loss in integrity of membranes. Selectivity of membranes is lost.
Proteins get denatured and displaced, and finally leading to loss of cellular compartmentalization. Due to water deficit the water potential inside the cells decreases
leading to stomatal closure. It is also known that hydropassive stomatal closure
occurs in plants to avoid transpirational loss and is evident in all tropical, temperate,
or dessert plants (Hopkins and Huner 2009). Stomatal closure limits gaseous
exchange, reduced transpiration and photosynthesis, uptake of mineral nutrients
and disruption of homeostasis and ions. Drought stress also causes various other
adverse effects such as decreased water potential leading to hydro active closure of
stomata, lesser number of stomata, thickening of leaf cell wall, decreased cell
enlargement, slow or inhibited growth and reproduction, cutinization of leaf surface,
poorly developed conducting system, increase in root–shoot ratio, production of
osmolytes such as proline, formation of reactive oxygen species (ROS), ascorbate,
and glutathione accumulates in cell and further aggravates the condition (Lisar et al.
2012). Water deficits also alter the cell wall confirmations, non-enzymatically
through the interaction of pectate and calcium (Boyer 2009; Cramer et al. 2011).
Although photosynthesis gets disrupted in both C3 and C4 plants but studies show
that C4 plants are more sensitive to drought. Water stress also alters the mRNA
expression and thus new proteins are synthesized. Different proteins which are
1 Abiotic Stress in Plants: An Overview
7