Proceedings
of The World Avocado Congress III, 1995 37-41
EFFECT
OF GIBBERELLIC ACID ON INFLORESCENCE PHENOLOGY OF THE ‘HASS’ AVOCADO (PERSEA
AMERICANA MILL.)
S. Salazar-Garcia and C.J.
Lovatt
Department of Botany and Plant
Sciences, University of California, Riverside, CA 92521-0124, USA
Additional
index words
Flowering,
inflorescence development, resting buds, vegetative shoots
Abstract
'Hass' avocado shoots (Persea
americana Mill.) produced during the fall were observed to flower earlier
than older shoots produced in the summer, but summer shoots flowered more
intensively due to the production of a greater number of axillary
inflorescences. Under California conditions, vegetative shoot development
follows anthesis. To examine the effects of GA3 on floral expression
and inflorescence phenology, branches of avocado trees, on which summer and
fall shoots were present, were sprayed with 0, 50, 100 or 1000 mg GA3/liter
in November, December or January. All treatments were prior to budbreak. GA3
stimulated apical growth of all shoots. Thus, if a floral shoot was already
differentiated, the inflorescence developed in advance of inflorescences on
branches not treated with GA3 . In addition GA3, caused
precocious development of the leaves relative to the flowers of indeterminate
inflorescences and relative to the leaves of indeterminate inflorescences from
untreated branches. November GA3 treatments stimulated vegetative
shoot growth and expansion of partially formed inflorescences with fewer
secondary axes resulting in reduced floral intensity. Axillary growth was
inhibited with increased GA3 concentrations. Untreated branches
flowered later than GA3-treated branches.
1.
Introduction
Gibberellins have been reported to inhibit flower
initiation in deciduous perennial fruit crops (Sedgley, 1990) and some
subtropical and tropical fruit trees such as citrus (Davenport, 1990) and mango
(Kachru et al., 1972). The inhibition of flowering by GA3 is
normally associated with stimulation of vegetative growth. A delay in flowering
of more than 4 weeks was obtained when deblossomed branches of mango were
treated with a single spray of GA3 at either 10 or 50 mg/liter
(Nunez-Elisea and Davenport, 1991). On the other hand, gibberellins caused
early anthesis in strawberry (Porlingis and Boynton, 1961) and coffee (Schuch
et al., 1990). In each case, the effect of GA3 was influenced by the
concentration used and the stage of floral development at the time of application.
Recent research
with container-grown 'Hass' avocado trees, 22 months from budding, provided
evidence that the stage of floral development within the resting bud influenced
its response to GA3 (Salazar-Garcia et al., manuscript in prep.).
Thus, the objective of this study was to quantitatively evaluate the effects of
applying GA3 to the foliage of 'Hass' avocado branches on different
calendar dates under field conditions on floral expression and inflorescence
development. Our goal was to ascertain whether GA3 applied to the
canopy might have potential utility as a management strategy in the production
of the 'Hass' avocado.
2. Material and methods
Forty 10-year-old 'Hass' on
Duke-7 avocado trees growing in an irrigated commercial grove in southern
California were used. The experimental trees were in an "off' year and in
a similar phenological stage. On the south side (physiologically the most
advanced) of each tree, four branches, I m long, were selected and their shoots
classified according to the vegetative flush in which their growth was
initiated (summer or fall). The branches were sprayed until run-off with a GA3
solution (at pH 5.5) prepared from Progibb 4 % (Abbott Laboratories) plus 1
ml/liter Triton X-100 in water. Treatments consisted of a single application of
one of four GA3 concentrations (0, 50, 100, and 1000 mg/liter) and
three application dates, 13 November or 13 December 1993, or 13 January 1994.
Control branches received only water plus Triton X-100. All treatments were
made prior to bud break. According to Davenports scale (Davenport, 1982) at the
time of the first treatment in November, apical buds were at "zero"
which corresponds to "bud in rest, bracts closed with no sign of
growth." For control branches, elongation of secondary axes of the
inflorescence was first observed on 27 January 1994; anthesis occurred during
the third week of March through April, 1994.
The type of
growth produced by both apical and axillary buds was recorded for both summer
and fall shoots at the end of the flowering season. Flowering was defined as
the time when the secondary axes of the inflorescence started to elongate. A
randomized complete block design with ten replicates per treatment was used.
Before statistical analysis, data were transformed by arcsin of the square root
of the percentage. For means comparison, the Duncan's multiple range test at P
= 0.05 was used.
3. Results
3.1. Type of
growth produced
By 26 March
1994, 100% of the apical buds home on untreated (control) summer shoots produced
inflorescences. A November application of either 100 or 1000 mg GA3
/liter to summer shoots significantly reduced inflorescence production (Table
1). A similar response was obtained for fall shoots treated in November with
1000 mg GA3/liter. In each case, the reduction in inflorescence
number was associated with an increase in vegetative shoots (Table 1). However,
the number of vegetative shoots produced by apical buds of summer shoots
decreased in response to progressively later
GA3 applications, e.g., December and January. For fall
shoots, there was always a small population of apical buds that responded to GA3
by producing vegetative shoots.
Regardless of
the treatment, determinate inflorescences were the only type of growth produced
by axillary buds on both summer and fall shoots. In general, summer shoots
flowered more intensely due to the production of a greater number of axillary
inflorescences. The number of axillary buds producing inflorescences decreased
with increasing GA3 concentrations. For some application dates,
flowering of axillary buds of fall shoots could be totally inhibited with any
GA3 concentration. However, for summer shoots, inhibition of
flowering only occurred when GA3 at 1000 mg/liter was applied in
either November or December. Inhibition of inflorescence development from
axillary buds resulted in a concomitant increase in number of resting buds
(data not shown).
3.2. Inflorescence
development
By 13 January, the control
had not started budbreak, whereas 8 and 18% of the buds on summer or fall
shoots treated with 50 mg GA3/liter in November or December
exhibited elongation of the secondary axes, respectively. Two weeks later, 23%
of the inflorescences on the untreated control shoots exhibited elongation of
the secondary axes. However, all concentrations of GA3, regardless
of application date, increased the proportion of inflorescences that had
reached this or a more advanced stage of development (data not shown). GA3
at 100 mg/liter applied in November or December stimulated the elongation of
secondary axes of inflorescences 4 and 2 weeks, respectively, earlier than the
control (Fig. 1). Advancement of inflorescence development was greatest in
response to 1000 mg GA3/liter applied in November or December (more than 75% of the
total inflorescences had initiated elongation of the secondary axes by January
13).
The degree of
floral advancement resulting from each GA3 treatment persisted such that maximum flower opening
was precocious to the same degree. Thus, maximum flower opening was 37 days
earlier than the control for shoots treated with 100 or 1000 mg GA3/liter in November or 1000 mg
GA3/liter in December and 23
days earlier in response to 50 or 100 mg GA3/liter applied in December or January.
3.4. Precocious
vegetative growth of indeterminate inflorescences
For 'Hass'
avocados under southern California conditions, vegetative shoot growth and leaf
expansion are delayed relative to elongation of the inflorescence axes of
indeterminate inflorescences. Thus, by 5 March, when 100% of total
inflorescences of the untreated control had reached elongation of secondary
axes or a later stage of development only 11% of total indeterminate
inflorescences from the control had initiated the first leaves of the
vegetative shoot. However, GA3 at 1000 mg/liter applied in November, December or January caused
precocious development of the vegetative shoot (up to 80% of inflorescences had
initiated vegetative growth 37 days earlier than the control). GA3 at 100 mg/liter applied in January
was equally effective (Fig. 2). Earlier application dates were less effective
with a maximum of 22% of the indeterminate inflorescences initiating vegetative
shoot growth 37 days before the control. GA3 at 50 mg/liter had a significant effect only when
applied in January. With this treatment, by 5 March, 68% of inflorescences
exhibited vegetative growth (data not shown).
4. Discussion
The inhibitory
and promotive effects of exogenous application of GA3 on flowering are well
documented to be dependent on plant species, concentration and time of
application (Porlingis and Boynton, 1961; Guardiola et al., 1977; Lord and
Eckard, 1987; Schuch et al., 1990; Salazar-Garcia et al., manuscript in prep.).
In the present study, application of GA3 resulted in early budbreak and growth of the shoot
apex at whatever stage of development it had reached at the time of
application. Applications made prior to floral initiation resulted in the
production of vegetative shoots. Application of GA3 during early inflorescence development
resulted in the growth of partially formed inflorescences lacking some
secondary axes and containing a reduced number of flowers per inflorescence.
Application of GA3 after the inflorescence was
fully formed resulted in early flowering of normal inflorescences.
Indeterminate inflorescences also exhibited precocious development of the
vegetative shoot apex. Precocity was increased with increasing concentrations
of GA3 (50 to 1000 mg/liter). GA3 at 50 and 100 mg/liter had
no negative morphological effects. However, GA3 at 1000 mg/liter applied at any time caused a
remarkable elongation of inflorescence axes which, in general, appeared too
weak to support setting fruit.
The results of
this research suggest possible ways to use GA3 to manipulate flowering in the 'Hass' avocado.
November or earlier applications, when most buds are in a vegetative stage,
would be expected to reduce flowering and might be used to increase the
proportion of vegetative to reproductive growth prior to an "on"
year. December application would be expected to have a dual effect. First, it
should reduce floral intensity by stimulating resting buds not yet committed to
flowering to produce vegetative shoots. Second, it would be expected to
increase the earliness of flowering. January application should result in early
flowering with full floral intensity. The fact that summer and fall shoots
responded similarly to GA3 suggests that a full canopy spray should produce uniform results.
As found by
Salazar-Garcia et al. (manuscript in prep.), GA3 caused precocious growth of the vegetative
apex of indeterminate inflorescences. This result should prove advantageous.
Competition between the
developing vegetative shoot and setting fruit of indeterminate inflorescences
has been cited as a cause of the low productivity of the avocado (Cutting and
Bower, 1990; Whiley, 1990). The possibility of advancing the development of the
vegetative shoot with GA3 so that leaves are sources of
photosynthate to setting fruit rather than competing sinks could have a
positive effect on yield.
The results of
this research provide evidence that GA3 can reduce inflorescence
number (inhibitory effect) or stimulate early flowering (promotive effect)
depending upon time of application and concentration. The results suggest
several possible strategies using GA3 that may prove beneficial to
avocado production.
5. Acknowledgments
The authors
thank Dr. Charlie W. Coggins, Jr., for his advice on the selection of GA3
concentrations and Abbott Laboratories for donating the Progibb. The senior
author acknowledges the financial support of CONACYT and INIFAP of Mexico and
the University of California. This work was funded by a grant from the
California Avocado Commission and by the Citrus Research Center and
Agricultural Experiment Station of the University of California, Riverside.
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