DISCUSSION
Callistemon citrinus
is a fast-growing plant, which frequently grows taller than the best
marketable height. So far, no research has been conducted about
controlling the height of Callistemon, which made the decision of
choosing the concentrations of the growth retardants and the number of
applications difficult. This experiment was a first approach to try to
find the proper concentrations and the best growth retardants in order
to succeed in the above aim. Cycocel and Paclobutrazol were used because
these two growth retardants are most commonly used and their use so far
in ornamentals has very good results in producing compact plants.
Plant height
In this research all the
treatments with Cycocel and Paclobutrazol, in all concentrations
significantly reduced the final height of the plants. The treatment with
20 ppm paclobutrazol caused the biggest reduction in height compared to
the other treatments. Although this height reduction represents a positive
effect on growth control, one also has to evaluate if this result remains
during time. At the end of the experiment all the plants had a compact
size with no side effects (except the treatment with 20 ppm
paclobutrazol), which means that these concentrations are a good starting
point for further research.
The fact that all the treatments did not differ
significantly can lead to a conclusion that one can choose to apply the
lower-concentration treatment (20 ppm Cycocel or 5 ppm Paclobutrazol) in
order to reduce the cost of applying growth retardants. That also implies
that even lower concentrations can be tested, to find the best combination
of height control and economical value of application, which is also
acceptable from an ecological point of view.
One can argue that four
applications at two weekly intervals can be characterized as excessive.
Taking in to account, however, that Callistemon citrinus is a fast
growing woody plant and the concentrations that were used were not very
high, four applications seems a logical choice.
Internode length
All the treatments reduced the
final internode length, which agrees with the fact that growth retardants
affect internodes length by the inhibition of gibberellin biosynthesis
which cause less cell division and elongation (Paspatis, 1989).
Number of shoots
Since Callistemon’s
ornamental value is its flowers, the number of shoots per plant is very
important considering that flowers can be formed only at the end of each
shoot. Cycocel treatments did not differ significantly from the control
plants. However although the statistical analysis showed no significant
difference in shoot number, the fact that plants after cycocel treatments
had an average of 3 more shoots than the control plants, will be important
in determining the final number of flowers and thus the market value of
the plant.
Paclobutrazol treatments increased
the number of shoots per plant while the treatment of 5 ppm Paclobutrazol
increased the average number of shoots by 84% (5 more shoots), which will
have an immediate impact on the final number of flowers. On the other
hand, while more flowers are desirable, it is still necessary to
investigate if this has a negative effect on the quality of the flowers or
the size of the flowers.
Other characteristics
Though the reduction of leaf
length by all treatments has a positive aesthetic effect on the general
appearance of the plants, it can also have a negative effect on
photosynthesis. It has to be mentioned that according to the literature
(Paspatis, 1998) growth retardants can affect the leaf size but leaves can
finally reach their normal size after some time. Although all treatments
caused a reduction on leaf length, the total leaf area did not change
significantly. This can be explained by the fact that the increase in the
number of shoots caused an increase on the total number of leaves, which
compensated for the decrease in average leaf length.
Although the effects of Cycocel
and Paclobutrazol on leaf colour were not evaluated quantitatively, it was
obvious that all the treatments produced greener leaves (more darker
green), giving them a similar colour the leaves of Callistemon
viminalis ‘Little John’ have. (C. viminalis ‘Little
John’ is the only dwarf variety of Callistemon). This change in
color is common after applying growth retardants (Papadakis, 1970). This
greening effect is probably due to an increase in the concentration of the
chlorophyll per unit leaf area as a consequence of the reduced leaf
expansion, rather than increase in the levels of chloroplast pigment
(Khalil & Rahman, 1995). Other experiments (Netherland and Lembi, 1992;
Kane and Smiley, 1983) also showed that chlorophyll content is not
statistically affected by growth retardants, although treated plants may
appear greener than the controls.
Root growth
Although there is evidence that
growth retardants can sometimes influence root growth (Paspatis, 1989) in
this experiment growth retardants had no effect on the root growth of
Callistemon citrinus (except the excess application of 20 ppm
paclobutrazol) probably because there was no inhibition of gibberellin
biosynthesis in the root tips.
Market Quality
For the market quality evaluation,
none of the treatments received market quality ratings greater than 3.0,
because plants had not yet developed flowering shoots, and therefore were
not sufficiently attractive to warrant higher ratings. But in general most
of the plants that received growth retardants had the potential to receive
a higher quality rating than the control plants, since they had more
compact shape and more shoots per plant, which implies the possibility of
more flowers per plant. The control plants received a low rating because
they already required pruning to maintain a compact shape. Plants that
were treated with 20 ppm Paclobutrazol received the lowest rate because of
the side effects that this treatment caused and the immediate impact this
had on the general aesthetic appearance of the plants.
Drawbacks
Although that the treatment with
20 ppm Paclobutrazol caused the biggest reduction in height and internode
length, and a large increase in number of shoots, it is not suitable for
controlling the height of Callistemon, since it caused side
effects, which could make the plants unmarketable. These plants had the
lowest score in market quality evaluation, supporting the above
conclusion. There was a deformation of the leaves, the leaf size (length
and width) was extremely small, with likely direct effects on
photosynthesis, and some branches only had leaves on their tip (Appendix
I).
Further research
The effects of growth retardants
on flowering should also be investigated. At the end of the experiment
there was no flowering and it was not possible to evaluate the effects of
growth retardants on flowering since more time was needed as
Callistemon only flowers after almost one year.
The evaluation of growth
retardants on flowering is maybe the most important aspect since
Callistemon citrinus is marketable after starting flowering. Right now
all the growers in Greece are using the same time schedule for
Callistemon production, so all of them have the plants ready for
selling almost at the same time, causing prices to drop due to the large
supply. It is obvious that earlier flowering with the use of growth
retardants could lead to a significant increase in price during periods
with low supply.
Since it was not possible to set
up this experiment in a place that Callistemon is used as a garden
plant (for example Greece), greenhouses ware used with artificial lighting
and heating, since Callistemon has high requirements for both light
and heat. In Greece (mostly in the South), outdoor cultivation has been
used for Callistemon production by all growers. That means that
this experiment needs to be repeated not in a greenhouse but under outdoor
conditions. Also plants need to be tested by planting out to evaluate for
how long the effects of growth retardants on plant growth remain.
Future and alternatives
Right now, the use of some growth
retardants is no longer permitted for use in vegetables (such as
paclobutrazol in Greece), and maybe this restriction will be extended to
ornamentals in the future. So, though growth retardants can be used in
Floriculture, alternatives to growth retardants should be considered.
Non-chemical alternatives are gaining popularity due to species
specificity of chemicals and increased restrictions on chemical usage.
But it had to be said, that since Callistemon cultivation is
outdoor and not in a greenhouse, some alternatives of growth retardants,
like DIF method or light manipulation, is not possible to be used.
But until this happens
paclobutrazol and cycocel have every possibility for being used for the
growth control of Callistemon citrinus.
Frost Resistance
The first important conclusion
from the frost resistance experiment, is the importance of cold
acclimation the time that the experiment was conducted (May-June), for the
plants to induce cold hardiness.
Comparing the % electrolyte
leakage of non- and cold-acclimated Callistemon plants, we can see
that below the temperature of -4 oC (-4 to -8 oC)
the non-acclimated plants had an average of 80% of electrolyte leakage,
while the plants that acclimated in the cold room had an average of 41 %
at the Day 1, which was even less during the next days. At the
temperatures of -12 and -16 oC, non acclimated plants (leaves)
had an average of 90 % of electrolyte leakage, while the cold-acclimated
plants at the temperature of -16 oC had a 67 % leakage and at
-12 oC the leakage was not more that 60%.
This agrees with the literature,
that many plant species increase frost hardiness when exposed to
environmental conditions such as lowering temperatures (Levitt, 1980), and
that the response to freezing stress may differ between acclimated and
non-acclimated plants, as clod acclimated cells may have changed their
cellular function and structure in such a way that they can sustain the
freezing-induced dehydration stresses (Fircks, 1994).
Growth Retardants on acclimated
Callistemon plants
In this experiment growth
retardants caused a significant increase in the frost resistance of
Callistemon on Day 4 and Day 5. The fact that during the first 3 days
from the beginning of the cold acclimation, growth retardants in all
concentrations did not have any effect on the frost resistance, can lead
us to the conclusion that under the specific conditions of this
experiment, 3 days are needed for growth retardants to affect the frost
resistance of Callistemon.
The fact that there was no
significant difference between the two growth retardants (Cycocel and
Paclobutrazol) and between treatments can lead us to the conclusion the
lowest concentrations can be used or even lower concentrations can be
tested.
The way that growth retardants had
an effect on frost resistance of Callistemon is not easy to be explained
without further experiments. The most possible explanation is that during
the first 3 days plants were cold acclimated with all the results that
this can have on the cell structure and function.
One can say these 3 days are
necessary for the transport of growth retardants to the leaves and for the
inhibition of the gibberellin biosynthesis which is important for the
increase in hardiness (Roberts and Smith, 1981).Moreover, the inhibition
of the enlargement of the cells and the rate of their mitotic division
that growth retardants may cause, can affect the frost resistance, since
smaller cells are more resistant to low temperatures than large and fast
growing cells (Holubowixz, 1982).
Growth Retardants on
non-acclimated Callistemon plants
The fact that growth retardants
did not increase frost hardiness of non-acclimated Callistemon
plants means that in the time of year that the experiment was conducted,
plants need first to be cold-acclimated in order to induce frost
resistance. It is possible that growth retardants affected the levels of
gibberellin in the leaves and the size of the cells, but that was not
enough for affecting the frost resistance. This can lead to the conclusion
that cold acclimation is the important factor for frost resistance of
Callistemon and without this plants can not tolerate frost, with or
without the application of growth retardants.
Further research
Although the results from the
experiment were shown that growth retardants can have a significant effect
on frost resistance of Callistemon, further research is needed for
more accurate conclusions.
It is better that the experiment
is repeated under natural conditions of cultivation in the time of the
year that plants are cold acclimated (e.g. winter) and they don’t have to
be acclimated artificially.
Also the recovery from the frost
resistance has to be evaluated in order to see if growth retardants can
also accelerate the recovery of the plants from frost.
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