Extreme Temperature Responses, Oxidative Stress and Antioxidant Defense in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 170

stresses are together termed low temperature or cold stress: the damage due to cold stress can
range from chilling injury and freezing injury to suffocation and heaving. In general, plants
from temperate climatic regions are considered to be chilling tolerant to variable degrees, and
their freezing tolerance can be increased by exposing to cold, but non-freezing, temperatures;
this process is known as cold acclimation. However, generally the plants of tropical and
subtropical origins are sensitive to chilling stress and lack this mechanism of cold acclimation
[9]. Low temperature may affect several aspects of crop growth; viz ., survival, cell division,
photosynthesis, water transport, growth, and finally crop yield.
The cellular changes induced by either HT or LT include responses those lead to the excess
accumulation of toxic compounds, especially reactive oxygen species (ROS). The end result of
ROS accumulation is oxidative stress [10-12]. In response to HT, the reaction catalyzed by
ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) can lead to the production of
H 2 O 2 as a consequence of increases in its oxygenase reactions [13]. On the other hand, LT
conditions can create an imbalance between light absorption and light use by inhibiting the
activity of the Calvin-Benson cycle. Enhanced photosynthetic electron flux to O 2 and over-
reduction of the respiratory electron transport chain (ETC) can also result in ROS accumulation
during chilling which causes oxidative stress [14]. Plants have evolved a variety of responses
to extreme temperatures those minimize damages and ensure the maintenance of cellular
homeostasis [15]. A considerable amount of works have explored that there is a direct link
between ROS scavenging and plant stress tolerance under temperature extremes [12]. Thus,
the improvement of temperature stress tolerance is often related to enhanced activities of
enzymes involved in antioxidant systems of plants. Plants exposed to extreme temperatures
use several non-enzymatic and enzymatic antioxidants to cope with the harmful effects of
oxidative stress; higher activities of antioxidant defense enzymes are correlated with higher
stress tolerance. Different plant studies have revealed that enhancing antioxidant defense
confers stress tolerance to either HT or LT stress [16-19].
In this chapter, we review the recent research findings those revealed variable responses of
plants to extreme temperatures. We also focus on the oxidative stress and antioxidant defenses
that are invoked by plants for survival under temperature stress conditions.
2. Plant responses to high temperature
2.1. Seed germination and emergence
Seed germination and seedling vigor are important traits for obtaining a good plant stand and
subsequent high yields of a crop. Seed germination is highly dependent on temperature as
temperature is one of the basic requisites of this process. However, the range of temperature
in which seeds perform better germination depends largely on crop species (Table 1). Soil
temperature is one of the major environmental factors that influences not only the proportion
of germinated seeds, but also the rate of emergence and the subsequent establishment, even
under optimum soil and irrigation conditions [20].