236 P. BISWAS ET AL.
refuted that argument by observing a phase transition at 15◦C for mito-
chondria from soybean hypocotyls, at 16◦C for tomato, and at 15◦Cfor
cucumber and asserted that sensitivity to chilling injury is related to a
temperature-induced alteration in the structure of cell membranes.
The reduced flexibility of mitochondrial membranes from sensitive
tissues below a critical temperature leads to a loss of membrane integrity
following phase transition and altered oxidative rate. This depresses
the rate of mitochondrial oxidation that could lead to an accumulation
of metabolic intermediates or reduced ATP supply and ultimately be
responsible for decoupling of oxidative phosphorylation. Mitochondria
would then exhibit altered respiration rate, decreased cytochrome c oxi-
dase activity, and enhanced alternative oxidase activity (Prasad et al.
1994). However, these effects are not seen if the cold period is relatively
short—only a slight decrease in respiration is observed in this case,
but in general, oxidative phosphorylation is not affected (Lyons and
Raison 1970). The metabolic dysfunction and disintegration of respira-
tory activity result in the generation of ROS leading to oxidative stress
(Shewfelt and del Rosario 2000). The induced ROS react with different
cell components and cause a cascade of oxidative reactions including
lipid peroxidation, protein degradation, and DNA damage (Scandalios
1993). Finally, loss of cell membrane integrity causes cell rupture, cell
autolysis, and ultimately cell death (Parkin and Kuo 1989).
A transitory burst of respiration is usually observed, specifically fol-
lowing transfer of fruit to non-chilling temperature after being exposed
to low temperature for a certain period of time (Lyons and Breidenbach
1990). An increase in respiration at warmer temperature after a chilling
period could be attributed to greater accumulation of oxidizable inter-
mediates (Eaks 1980). Moreover, this increase may be a result of the
need for more energy to repair cellular damage from chilling (Lueng-
wilai et al. 2012).
- Role of Ethylene in Inducing or Preventing CI.Ethylene is a sim-
ple gaseous hormone in plants. Methionine, an amino acid serves as a
precursor of ethylene in higher plants. S-adenosylmethionine (SAM),
an intermediate in ethylene biosynthesis synthesized from methionine,
is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC
synthase (ACS). In the presence of oxygen, ACC is then converted to
ethylene by ACC oxidase (ACO) (Kende 1993). Lowering the tempera-
ture usually results in reduced ethylene biosynthesis in fruit by reduc-
ing ACC concentrations and decreasing the activity of the enzyme ACO
that becomes irreversible if the chilling period is too long (Etani and
Yoshida 1987; Cabrera and Saltveit 1990). Mung beans, for instance,