Agriculture Reference
In-Depth Information
ethylene production (Thompson
et al
. 1982, 1997; Faragher
et al
. 1987; Paliyath & Thompson 1990; Borochov
et al
.
1997). Perhaps the enzymic processes leading to membrane
changes (phospholipase, decreased phospholipid synthesis,
increased lipoxygenase), loss of protective antioxidants
and membrane attack by free radicals, are some of the
critical early events in senescence, leading to a cascade of
subsequent degradative processes.
The green pigment chlorophyll and the chloroplast orga-
nelles that contain it are usually degraded during flower
development and senescence (Halevy & Mayak 1979;
Mayak & Halevy 1980). Orange and yellow carotenoids
may accumulate in lipid chromoplast organelles that arise
from chloroplasts. Water-soluble red and blue anthocya-
nins accumulate in vacuoles. Co-pigments and pH can
influence the shade of anthocyanin-based colours. Blueing
of senescing red cut flowers is thought to be associated
with a pH shift from more to less acid due to the release of
free ammonium from proteins during their catabolism
(Halevy & Mayak 1979; Mayak & Halevy 1980).
Respiration rates tend to be high in young developing
buds and flowers and then fall, often to be followed by a
further, climacteric increase in association with senescence
(Halevy & Mayak 1979; Mayak & Halevy 1980; Wu
et al
.
1991a). The respiratory climacteric may be associated with
a more or less co-incident ethylene climacteric.
Carbohydrates and other respirable substrates (e.g.
organic acids) provide energy for flower development
and senescence (Halevy & Mayak 1979; Mayak & Halevy
1980). Some flowers, such as rose, accumulate starch
reserves (Gorin & Berkholst 1982; Berkholst & Navarro
Gonzales 1989; Shellabear
et al
. 1993). In roses, starch is
hydrolysed during petal expansion, which is presumably
driven by the osmotic potential of the released sugar (Evans
& Reid 1988). Other flowers, like gladiolus and other
harvested spike flowers, may depend relatively more upon
the continuous import of sugars from vase solutions to
maintain vase life (Reid 2002). Sugars are also important
as osmotically active molecules to maintain water balance
and as components of cell walls. There is also increasing
evidence for sugars acting as signalling molecules in
senescence (O'Donoghue
et al
. 2002b; Eason 2006).
Changes in petal cell walls after harvest have not been
extensively studied, despite the fact that wilting is an
important visual result of ageing. There are significant
alterations in cell wall enzymes and polymers during
flower growth and ageing (e.g. in sandersonia and daylily)
(Rubenstein 2000; O'Donoghue
et al
. 2002a). However,
O'Donoghue
et al
. (2002a, 2002b) believe that wilting is
not governed by those events alone.
(A)
3
_
500
_
400
2
_
300
1
_
200
(B)
100
0
28
24
20
16
12
01234567
Time (days)
Figure 19.5
Ethylene production rates (top panel
(A),
°
), microsomal membrane phospholipids (top
panel (A),
l
) and plasma membrane diacylglycerol
(phospholipid breakdown product; bottom panel
(B),
l
) levels during cut petunia flower senescence
(Borochov
et al
. 1997). (Reproduced with permission
from the authors and the publisher.)
Senescence is associated with membrane-based changes
that culminate in loss of cellular compartmentation and
increased water loss. Decreases in membrane phospholipids
and increases in oxidation products, such as free fatty acids,
lead to decreased fluidity, increased presence of gel phase
lipid, increased membrane permeability and decreased
activity of membrane proteins (Mayak 1987; Paliyath &
Thompson 1990; Borochov
et al
. 1997; Thompson
et al
.
1997; Figure 19.5). The loss of protective antioxidants and
membrane attack by free radicals and lipoxygenases are
important parts of these processes (Behera
et al
. 1987;
Droillard
et al
. 1987; Fobel
et al
. 1987; Rubenstein 2000).
Ethylene causes some of the changes leading to increased
permeability in carnation and rose petals (Thompson
et al
.
1982; Borochov & Faragher 1983; Faragher & Mayak
1984; Faragher
et al
. 1987). However, some changes in
membrane properties have been observed before any rise in
