laying senescence. The cytokinin contents of rooted leaf blades rise substantially and cytokinins can par-
tially replace these roots [7]. Thimann [89] observed that when cut leaves of many species, including oats,
were floated in cytokinin solution in the dark, the light requirement for delaying senescence was effec-
tively replaced. He suggested that treatment with cytokinins maintained the integrity of the cell mem-
brane. Studies have also shown that cytokinin, auxin, and/or ABA influence stomatal movement [6] and
that its closure accelerates senescence [89].
Cytokinins are also known to act as a sink for the transport of solutes from older to younger part(s)
of a plant. Leopold and Kawase [90] demonstrated this very clearly. They painted the primary (oldest)
leaves of a bean plant with benzyladenine at 4-day intervals. These leaves started senescing as soon as the
trifoliate leaves above expanded and thus died off first. However, the treated leaves in their experiment
lived longer than the untreated first trifoliate leaves because cytokinins do not readily move except in the
xylem stream. Many variations in the experimental design and test species have indicated that they act as
a sink for solutes in the potential or actively growing parts of the plant [7].
V. ETHYLENE
The effect of ethylene on plant growth was noted as early as 1858 by the behavior of plants exposed to il-
luminating gas [91]. Nevertheless, the Russian scientist Neljubow [92] is credited with having identified
the active growth-regulating component of the illuminating gas as ethylene. In the presence of ethylene,
etiolated pea plants exhibit inhibition of elongation, an increase in diameter, and horizontal growth of
shoots. In the literature, these three responses are known as the triple responseand are still sometimes
used to identify and measure ethylene response. However, Cousins [93] was the first to observe that gases
released from oranges caused premature ripening of banana. But it was not until 1934 that Gane [94] pro-
vided evidence that ethylene was produced autocatalytically by ripening fruits.
A. Chemical Nature
Ethylene is the simplest organic compound.
Its structural simplicity and the fact that it is gaseous in nature make it a unique plant hormone. It is a sym-
metric molecule having one double bond; the biological activity seems to be related to its unsaturated
bond, which is attached to a terminal carbon atom.
B. Metabolism
The task of unraveling the biosynthesis of ethylene was not an easy one. Feeding experiments using var-
ious radioactive materials with ethylene-producing plant tissues were unsuccessful in identification of the
pathway. However, based on model nonenzymatic system, Lieberman and Mapson [95] proposed me-
thionine as the precursor. Subsequently, Lieberman et al. [96] demonstrated the in vivo conversion of
[^14 C]methionine to [^14 C]ethylene in apple (Malus domestica) tissues. Studies with [^14 C]methionine have
shown that the C-1 atom is converted to CO 2 , C-2 to formic acid, and C-3 and C-4 to ethylene and the sul-
fur atom is retained in the tissue. Because the ethylene production system is extremely labile and is com-
pletely lost by tissue disruption, the characterization has been made at the living tissue level (Figure 4)
[97]. In these studies, climactaric fruit slices or plugs and auxin-treated stem segments of etiolated pea
and mungbean (Phaseolus aureus) seedlings have been used extensively [98].
Earlier studies on the metabolism of ethylene, conducted with improper precautions, had led to the
conclusion that the compound was metabolically inert. However, Beyer [99], employing proper precau-
tionary measures, convincingly demonstrated that ethylene was metabolized by plants, and the metabolic
products of the dark-grown aseptic pea seedlings were identified as CO 2 and ethylene oxide. In addition
to these two gaseous metabolites, ethylene was metabolized to a number of nonvolatile soluble products,
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