Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

Chapter 4
Biochemistry of Flower Senescence
Ajay Arora
4.1 Introduction
It is often opined that death and taxes are the only two inescapable aspects of the human
existence, but Ernest Hemingway correctly noted that “the sun also rises.” Plant senescence
is the final event in the growth and development of a plant and ultimately leads to the death
of a particular organ or whole plant. The senescence in plants is highly regulated, genetically
programmed, and developmentally controlled process. This phenomenon involves
structural, biochemical, and molecular changes that in many cases bear the hallmarks of
programmed cell death. Plant hormones and environmental factors play an important regulatory
role in senescence. Flower senescence has been described as the last stage of floral
development, although in the life cycle of most plant species, it is not a final event, rather
an integral process that allows the removal of a metabolically costly tissue (i.e., petal), after
it has attracted pollinators for sexual reproduction, and signals the initiation of ovule development
and seed production. At the end of their life, petals may wilt, lose color or abscise,
or in some cases, remain on the flower stem, encasing and protecting the developing ovary.
It is an actively ordered process that involves the synthesis of new RNAs and proteins and
results in highly coordinated changes in metabolism and the programmed disassembly of
cells.
All cut flowers are destined to die, and the challenge for postharvest researchers is to
slow the processes controlling flower death to enable cut flowers to reach distant markets
with a display life. Postharvest performance of cut flowers is affected by the developmental
stage of a flower at harvest, prosenescence signals that originate from specific tissues within
the flower (e.g., pollination-induced petal senescence), and stress-related metabolism (in
response to temperature, wounding, nutrient starvation). Cut flower stems are removed
from a source of nutrients, undergo water restrictions, and may be held at undesirable
temperatures in the dark for days prior to sale. Plant hormones, membrane stability, water
availability, cellular proteolysis, and carbohydrate metabolism act in concert to determine
the differential rate of senescence for each floral organ. Currently, flowers can be grouped
into several categories based on postharvest technologies that can extend their vase life
(e.g., sensitivity to ethylene, chilling sensitivity, leafy stems, multiple/single flowers per
stem, and woody stems).
Flower petals are ideal tissues for cell death studies as they are short lived, the tissue is
relatively homogenous, chemical manipulation can be applied without substantial wounding
(i.e., feeding through the vascular tissue), and the process of flower senescence has been
shown to be a genetically programmed event (Xu and Hanson, 2000; Eason et al., 2002;
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