Mechanical Injury of Fresh Produce 99
Plants degrade cellular materials during senescence and under various stresses.
Hayashi et al.^201 reported the precursors of two stress-inducible cysteine proteinases,
RD21 (product of responsive-to-desiccation gene 21) and a vacuolar processing
enzyme, that are specifically accumulated in transgenic Arabidopsis thaliana
(ecotype Columbia) plants. They are surrounded with ribosomes and thus are
assumed to be directly derived from the endoplasmic reticulum (ER). These ER
bodies are considered to be proteinase-sorting systems that assist the plant cell under
various stress conditions.^202 Heat shock in mechanically wounded plants appears to
lead to the disruption of ER lamellae that has been hypothesized to cause destabi-
lization of otherwise stable mRNA associated with ER-bound polyribosomes.
Putrescine N-methyltransferase (PMT) catalyzes the first committed step in the
biosynthesis of pyrrolinium ring-containing alkaloids. Using mechanically damaged
plums, Pérez-Vicente et al.^81 demonstrated the activation of the polyamine biosyn-
thesis pathway, showing an accumulation of cell wall putrescine and spermidine as
a result of wounding. Infiltration of exogenous polyamines can ameliorate mechan-
ical damage, increase fruit’s firmness, and reduce ethylene production and respiration
rate. Fruits under putrescine and calcium treatments maintain higher firmness values
and are more resistance to peel rupture than control fruits during storage.^84 They
also show less deformation when the compression force to induce mechanical dam-
age is applied. Exogenous polyamines can inhibit ethylene production in several
climacteric fruit and prolong the postharvest shelf life of whole and mechanically
damaged fruit. This has been reported for plums,^81 tomatoes,^203 , peaches,^204 avocados,
and pears.^205 Inhibition of ethylene production after polyamine treatment is due to
the inhibition of ACC synthase.^205 The contents of the diterpene alkaloid lappaconitin
in various parts of Aconitum septentrionale plants (leaves, stems, whole above-
ground part, roots) when studied in relation to the extent of mechanical damage
indicate that lappaconitin appears to be synthesized mainly in roots and that the
change of its content in plant parts after damage to the above-ground part is con-
nected with an activation of growth processes.^206
4.10 TEXTURE
Lignification of injuries is an important component of a plant’s defense against
postharvest diseases. Wounding triggers a variety of biochemical and developmental
pathways in plants that collectively harden injury sites against infection. A response
commonly observed in plants is the accumulation of aldehyde-selective reagents in
cells adjacent to injuries. This material has been called lignin, lignin-like, phenolic
polymers, and wound gum.^207
Mechanical injury to avocado (Persea americana Mill.) pericarp will initiate a
meristem and the production of periderm. Injury to tissues deep within the pericarp
results in cellular differentiation of parenchyma with various degrees of cell wall
thickening. Sclereid-like cells with thick, lignified walls and prominent pits can be
formed in tissue normally occupied by thin-walled, oil-filled parenchyma.^208 Wound
healing in developing apple fruit has been largely associated with wound periderm
formation, which is lacking in fruit wounded after harvest.^209 Wounds in mature