|dc.description.abstract||Senescence was examined in the economic weed Bidens pilosa, with the objectives to
a) determine the effects of deflowering and defruiting on growth, chlorophyll content,
photosynthesis and transpiration; b) to identify the stage of development of the head
at which the flowers, seeds/fruit produce senescence signals; and c) to test for
senescence activity in plant extracts made from the receptacles and leaves of Bidens
Total chlorophyll content in the controls, in association with the development of fruit,
was lower in the final harvests when compared with earlier harvests in both pot and
field-grown plant experiments. Deflowered Bidens pilosa plants had a higher chlorophyll
concentration than both defruited and control plants in both pot and field-grown plants.
Stem death of the control plants was higher than that of deflowered plants in both field
and pot experiments. The present results suggest that deflowering is essential if the
leaves are to be harvested commercially because it retards senescence and maintains
growth. Fruit and flower heads were responsible for the reduction in leaf and stem
growth after flowering in Bidens pilosa. Removing these organs slowed plant decline,
suggesting that the flower head and especially the fruit are responsible for senescence.
In contrast, the fruit were the main organs responsible for the decline in leaf chlorophyll
In pot-grown plants in full sunlight, photosynthesis and transpiration were low in
deflowered plants compared with the control and defruited plants 45 days after
treatment, and it coincided with a low stomatal conductance. These results suggest that
stomatal conductance played a role in lowering photosynthesis in deflowered plants. In
contrast, the control plants had a higher stomatal conductance than deflowered plants 75 days after treatment, yet photosynthesis and transpiration rates were the same in
both treatments. Thus stomatal conductance alone does not successfully explain
differences in photosynthesis in these treatments.
The dry weight of head with mature dry fruit was higher in plants grown at high light
intensities than at medium or low light intensities. It coincided with a greater decline in
chlorophyll concentration in the leaf nearest to the head and fruit. In contrast,
photosynthesis was the same at all light intensities in the leaf nearest to the head and
fruit. This suggests that high light accelerated the process of fruit maturation of the fruit
which then influenced senescence in the leaf nearest to the flower head.
Ethanolic and water extracts of senescent receptacles purified using paper
chromatography, induced senescence of leaves in light but not in the dark. In ethanolic
extracts, activity was detected in R[f]s 0.1, 0.2 and 0.3. In water extracts, activity was
detected in R[f] 0.1.
Senescent leaf extracts purified using column chromatography also induced
senescence in light under greenhouse conditions. At high concentrations, activity was
detected in fraction 10 eluted with ethyl acetate: methanol (55:45); fraction 11 eluted
with ethyl acetate: methanol (50:50); fraction 12 eluted with methanol (100%) and in
fraction 13 eluted with ethylacetate : isopropanol: water: acetic acid (52:28:28:4).
Under growth room conditions, activity was detected in fractions 12, eluted with
methanol (100%) and 13, eluted with ethyl acetate: isopropanol: water: acetic acid
(52:28:28:4) in the presence of light.
Fraction 1 (R[f] 0.00-0.10) from senescent receptacles, non-senescent and senescent
leaves, obtained following thin layer chromatography of ethanolic extracts induced
senescence under light. Fraction 1 was eluted with methanol. This fraction lacked
activity when eluted with ethyl acetate. Fraction 4 (R[f] 0.25 - 0.35) from non-senescent leaf extracts, which co-chromatographed
with 4-chloroindole acetic acid, gave activity in bean cuttings kept under continuous low
light. Senescent leaf extracts showed no activity.
Fraction 7 (R[f] 0.9 - 1.0) from non-senescent leaf extracts, also induced senescence in
bean cuttings under light. The same Fraction from senescent leaf extracts lacked