3. Introduction &
Literature Review

Yellowing leaves, a symptom of
leaf aging and senescence, is unsightly and reduces the quality, value and vase
life of cut flowers such as alstroemeria, cut chrysanthemum, and lilies.
Senescence of the leaves is also a significant problem in a wide range of potted
plants, including miniature roses, poinsettias, and chrysanthemums (Tjosvold, et
al., 1994; Staby and Erwin, 1977; Hibma, 1988). Loss of leaf function in potted
plants not only reduces display quality, but also reduces photosynthetic
activity that is crucial to maintaining and extending flower life and
development.

Many years ago researchers
demonstrated that application of the synthetic cytokinin, benzylaminopurine (BAP)
to leaves would prevent leaf yellowing and senescence (Richmond and Lang, 1957),
and this material (or gibberellic acid (GA), another plant hormone with
anti-senescence activity) is now in commercial use to prevent leaf yellowing in
cut chrysanthemum flowers and potted roses, among others (Tjosvold, et al.,
1994; Han, 1997; Funnell and Heins, 1998). These materials are relatively
effective, but they are expensive chemicals, and their effect is short-lived
presumably because the plant tissues metabolize them.

The substituted phenylureas,
including thidiazuron (TDZ), carbanilide (1,3-diphenylurea, DPU),
N-(2-chloro-4-pyridyl)-N’-phenylurea (CPPU) are highly active cytokinin
analogs that are not metabolized by plants, and we reason that they might
provide an inexpensive and long-lasting treatment that could be used to prevent
leaf yellowing and extend the life of cut flowers and potted plants. Preliminary
experiments were performed using TDZ [1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea], a
compound that is widely used by cotton growers prior to mechanical harvesting of
cotton and whose potent cytokinin activity has been exploited in plant tissue
culture (Capelle et al., 1983; Genkov and lordanka, 1995).

In a preliminary experiment with
cut flowers of Alstroemeria (cv. Diamond), we placed the flowers in deionized
water (Dl) or a solution containing 1 uM TDZ. As can be seen from the figure,
the chlorophyll content of the leaves of the TDZ-treated plants remained high
throughout the experiment and by 10 days was significantly higher than the
concentration in leaves of flowers held in Dl. By 20 days, there was no
detectable chlorophyll in the yellowed,
senescent leaves of the control flowers while the green leaves of the treated
flowers had as much chlorophyll as at the beginning of the experiment.

In a subsequent experiment,
we treated potted poinsettia plants (cv.‘Red Velvet’) with two spray
applications of 10 uM TDZ, and observed their postharvest performance under
standard conditions. After one month in the
interior environment, leaves on the control plants were yellowing. Leaves on the
TDZ-treated plants were still dark green. Leaf fall in the control plants
averaged 23 leaves per plant, and in the TDZ-treated plants, averaged less than
1 leaf (from 8 replicate plants).

Even more intriguing was the
observation that while the cyathea (the true flowers)
of the control plants had all abscised
after a month, those on the TDZtreated plants were still intact and fresh.

These spectacular preliminary
results encourage us to pursue the possible uses of this compound and its
analogues in a wide range of ornamental crops susceptible to premature leaf
yellowing. We hypothesize that the long-lived cytokinins effect provided by TDZ
treatment not only prevents leaf yellowing, but also reduces ethylene
sensitivity, and maintains potted plant quality through maintenance of
photosynthesis under low light conditions.