Mechanical Injury of Fresh Produce 81
molecules play regulatory^ roles in “wound signaling,” including the oligopeptide
system,^7 oligosaccharides released from the damaged^ cell wall,^8 and molecules with
hormonal activity such as jasmonates,^9 ethylene,^10 and abscisic acid.^11 A molecular
response is evoked in the local vicinity of a stimulus and in other parts of the plant,
often at distances far removed from the initial site of stimulation. Thus, a stimulus
can cause local and long-range effects. The long-range or systematic effects can be
rapid and involve a massive amplification of response, as local changes occurring
within one cell or a small group of cells producing systemic changes throughout the
entire plant.^12 Wounding of plant cells and tissues always causes bioelectric
responses. The vicinity of a wound, for example, becomes transiently polarized.^13
Wounded cells induce at least one of three kinds of responses in the receptor cell:
(1) slow depolarization lasting more than 10 min, (2) action potentials, and (3) small
spikes. The first of these response types is ubiquitous. This transient depolarization
is usually the first step in a chain of reactions that could be propagated over large
distances in the plant. Cell turgor pressure is needed for the depolarization that is
generated only at the nodal end of the receptor cell, not at the flank. The “death
message” from killed cells contains information that turgor pressure has been lost.^14
Wound signals may also be transmitted rapidly over considerable distances through
propagated electrical signals at speeds that could range from 0.5 cm/h in cut lettuce
to 0.13 cm/sec in wounded Pueraria lobata leaves.^15
Inward current at the wound site and outward current through the unwounded
parts, resulting in a change of polarity in cells, involve ions in the plant cell. Calcium
ions, for example, are involved in many signal transduction pathways.^16 The control
of numerous cellular reactions by calcium occurs with high spatial and temporal
precision in the cytoplasm. Many of the effects of calcium are exerted through
proteins containing EF-hand domains for Ca2+-binding as a common structural
motif.^17 Calmodulin, one of the calcium-binding proteins, is found in all eukaryotes
and is known to be a primary transducer of the intracellular calcium signal. It
mediates a number of calcium-regulated events in eukaryotic cells. It has been
observed that when the plant cell is activated by external stimuli, the intracellular
levels of free calcium are increased.^18 Calcium activates calmodulin by binding to
its four calcium-binding domains, thus causing a conformational change. The cal-
cium–calmodulin complex can then regulate the activity of many plant enzymes,
including NAD kinase, Ca2+-ATPase, H+-ATPase, and protein kinase.^18
Calcium levels within the cells are also important because they profoundly affect
stability of cellular membranes. The essential role of calcium in delaying plant
senescence is largely associated with its stabilizing influence on cell membranes^19
and the induction of membrane lipid catabolism released as a result of wounding as
in fresh-cut processing.^20 Abscisic acid (ABA), a plant hormone involved in the
response of plants to reduced guard^ cell turgor, diminishes the aperture of the
stomatal pore^ and thereby contributes to the ability of the plant to conserve^ water.
Cytosolic Ca2+ is involved in the signal transduction pathway that mediates^ the
reduction in guard cell turgor elicited by ABA, and ABA uses a Ca2+-mobilization
pathway that involves cyclic adenosine 5′-diphosphoribose^ (cADPR).^21 A number
of compounds that participate in the transmission of stress signals such as nicotina-
mide, an inhibitor of cADPR action, may offer defensive actions in wounded plant