Biochemistry of Flower Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 65

stimulating the metabolism of cytokinin bases and ribosides, thereby reducing cytokinin
activity (Taverner et al., 1999).

4.8 Abscisic acid

Abscisic acid (ABA) is generally known as a strong growth inhibitor and a senescence-
stimulating factor, but it also controls stomata closure in certain plants. In vegetative tis-
sues, ABA appears to be involved in the response and adaptation of plants to environmental
stresses, especially in drought, salinity, and cold conditions (as may occur in storage con-
ditions of cut flowers). It has also been proposed that under water stress, turgor pressure
declines and it results in an increase in cytosolic and apoplastic ABA levels (Patterson,
2001). This increase leads to (a) the closure of stomata to avoid further water stress and
(b) the induction and accumulation of compatible solutes, such as proline, for water stress
tolerance (Shen and Ho, 1997). Exogenous applications of ABA can serve also to increase
the cold hardiness of plants. Provision of exogenous ABA in the vase solution effectively
reduces vase solution usage and extends flower life. The expression of ABA genes results in
the formation of, among other gene products, LEAs (late embryogenesis abundant), which
are neither enzymes nor storage proteins but rather serve to protect proteins and mem-
branes from damages during water loss in the cytoplasm due to desiccation. Reid (1985)
hypothesized that a gradient of auxin from the subtended organ to the plant axis maintains
the abscission zone in a nonsensitive state. A reduction or reversal of the auxin gradient
causes the abscission zone to become sensitive to ethylene. Consequently, senescence and
abscission could be altered by factors like shading, low irradiance, high temperature or
water stress, and poor nutrition, which alter auxin gradients by exposure to ethylene, and
by stresses that enhance ethylene production.

4.9 Gibberellins and other plant growth regulators

Gibberellins (GAs) are a large family of diterpenoid compounds, some of which are bioac-
tive growth regulators, controlling such diverse processes as germination, stem elongation,
and flowering. Cytokinins, and in some cases, GAs (such as GA3, GA4, and GA7 in Al-
stroemeria cut flowers; Jordi et al., 1996) delay the loss of Chl, whereas ethylene and
ABA enhance the rate of Chl loss; the application of BA and GAs together improved the
postharvest quality of cut Asiatic and Oriental lilies. Jasmonic acid can induce senescence,
while polyamines delay foliar senescence (Singh et al., 2005a, b). In waxflower, the foliage
of cut flowers often desiccates before flowers on the same sprig become senescent, since
leaves have relatively higher turgor and lower osmotic potentials than flowers, and are less
elastic (Joyce et al., 2000). Brassinosteroids induce PCD and the formation of secondary
walls (Kuriyama and Fukuda, 2002). Growth retardants (e.g., uniconazole, ancymidol, pa-
clobutrazol) reduce shoot elongation normally through inhibition of GA biosynthesis. GA
effectively retains Chl, while the addition of indole-3-acetic acid (IAA) in vase water has
little effect on leaf yellowing, while kinetin delays it (van Doorn et al., 1992). Benzylade-
nine (BA) improved the vase life of anthurium, heliconia, and ginger ( Alpinia purpurata )
when applied as a spray or a dip, but inhibited that of bird-of-paradise ( Strelizia regi-
nae ), beehive ginger ( Zinziber spectabilis ) and Uluhe fern curls ( Dicranopteris linearis ),
bamboo orchid ( Arundina bambusifolia ), and cut leaves of the fern lycopodium