628 Produce Degradation: Reaction Pathways and their Prevention
It was observed that protoplasmic streaming ceased in chilling-sensitive plants exposed
to chilling temperatures but was basically unaffected by low temperatures in chilling-
resistant plant tissues. The reasons for the effects of chilling temperatures on stream-
ing were postulated to be related to one or more of the following^33 : (1) cell lipids
and their role in structure and action of the protoplasm; (2) energy supply from
respiration to maintain protoplasmic streaming; (3) energy utilization for streaming;
(4) protoplasmic viscosity; or (5) difference in sensitivity to chilling temperature of
enzyme systems that are responsible for utilizing ATP for streaming.
20.4.1.3.6 Increase in Permeability and Solute Leakage
A number of chilling-sensitive plant tissues have been shown to develop a high rate
of electrolyte leakage in response to exposure to chilling temperatures.^33 For exam-
ple, ion leakage in the mitochondria isolated from chilled sweet potato slices was
as much as five times that of unchilled tissue.^43 This difference has led to postulations
that increased cell permeability and solute leakage are common responses to chilling
temperatures. However, in some chilling-sensitive fruits, such as peaches, electrolyte
leakage remains low and does not appear to be altered while the fruit is held at a
chilling temperature. It does, however, increase rapidly after the fruit is transferred
to ripening temperature.^44 Some chilling-sensitive vegetables, such as bell peppers
and eggplants, do not exhibit any increase in electrolyte leakage at critical chilling
temperatures.^45 These findings have led to the proposal that high rates of electrolyte
leakage are not necessarily a general property of chilling-sensitive plant tissue.
20.4.1.3.7 Accumulation of Metabolites During Cool Storage
Inhibition or disruption of mitochondrial function may lead to an accumulation of
pyruvate.^32 This may be metabolized to such compounds as acetaldehyde, ethanol,
and acetate. Changes in metabolite concentration may occur as a result of the primary
injury of chilling stress or as an outcome of the tissue injury. There is also the
possibility that the metabolite accumulation as a result of chilling stress exerts a
toxic effect on the tissue.
Hulme et al.^46 observed a positive correlation between the accumulation of
oxaloacetate in cool-stored apples and the subsequent development of low-temper-
ature breakdown. Oxaloacetate did not accumulate in varieties of apples that did not
suffer from low-temperature breakdown. This correlation was not sustained by other
studies that found no differences in the levels of TCA cycle intermediates that could
be associated with chill injury.^32
20.4.1.3.8 Fatty Acid Composition of the Membrane Lipids
It is generally agreed that a physical change in the membranes is the primary response
of plants to chilling. However, there is no consensus on whether the composition of
membrane lipids plays a deciding role in determining the sensitivity of plant tissue to
chilling. Lyons et al.^47 reported that the membrane lipids of chilling-resistant tissues
had a higher degree of unsaturated fatty acids than lipids of chilling-sensitive tissues.
However, the relationship of fatty acid composition and chilling sensitivity may
not be as clear-cut. A study by Yamaki and Uritani^48 showed that the white potato,
a temperate-zone plant, had a lower ratio of unsaturated fatty acids to total fatty
acids in the pure mitochondrial fraction than the sweet potato, a tropical plant. As